Early Cretaceous mesofossils from Portugal and eastern North America related to the Bennettitales-Erdtmanithecales-Gnetales group†
The authors thank J.-P. and M. Rioult for valuable information and help in retrieving the original specimen of Cycadeoidea morierei. They also thank P. von Knorring for preparing the line drawings, M. von Balthazar for preparing some of the Puddledock specimens, and M. Stampanoni, F. Marone, S. Bengtson, and T. Huldtgren for help with SRXTM and PCXTM analyses at the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland. This research project has been supported by the European Commission under FP6: Strengthening the European Research Area, Research Infrastructures: project no. 20070197 (to P. C. J. Donoghue, S. Bengtson, and E. M. Friis), by the Swedish Natural Science Research Council (E. M. Friis), the Carlsberg Foundation (K. R. Pedersen), and the U.S. National Science Foundation (P. R. Crane).
Abstract
Four new genera and six new species of fossil seed (Buarcospermum tetragonium, Lignierispermum maroneae, Lobospermum glabrum, L. rugosum, L. stampanonii, Rugonella trigonospermum) are described from five Early Cretaceous mesofossil floras from Portugal and eastern North America. The four genera are distinguished by differences in size, shape, and details of seed anatomy, but all are unusual in having an outer seed envelope with a distinctive anatomical structure that surrounds the nucellus and the integument. The integument is extended apically into a long, narrow micropylar tube. The four new genera are part of a diverse, but previously unrecognized, complex of extinct plants that was widespread in Early Cretaceous vegetation and that coexisted in similar habitats with early angiosperms. The distinctive structure of these seeds, and the strong similarities to other fossil seeds (Ephedra, Ephedripites, Erdtmanispermum, Raunsgaardispermum, and some Bennettitales) already known from the Early Cretaceous, suggests that this newly recognized complex of extinct plants, together with Bennettitales, Erdtmanithecales, and Gnetales (the BEG group), is phylogenetically closely related.
For more than a century, the origin and rise to dominance of angiosperms, the most diverse group of plants on Earth—often referred to as Darwin's abominable mystery (See 22, pp. 5–21)—has been a topic of great botanical interest, and over the last 30 years concerted efforts at several frontiers of plant science, have begun to yield an increasingly convincing and coherent picture of early angiosperm diversification. In systematics, the application of new molecular and computational tools, combined with advances in phylogenetic theory, have clarified phylogenetic patterns among living angiosperms and have established well-corroborated hypotheses of branching patterns at the base of the angiosperm phylogenetic tree (e.g., 4; 55; 68; 54). In comparative morphology renewed and more focused investigations have revealed unexpected diversity among presumed “primitive” angiosperms and have narrowed some of the gaps in structure and biology that separate angiosperms from related seed plants (e.g., 19; 21). And in palaeobotany, remarkable discoveries of well-preserved angiosperm reproductive structures have provided direct insights into the changing systematic relationships and biology of angiosperms as they increased in diversity and ecological prominence through the first 70 million years of their evolutionary history (e.g., 33).
Despite this progress, there are many aspects of angiosperm evolution that still remain to be understood, but the most significant remaining problem is to develop a well-corroborated hypothesis of the phylogenetic position of angiosperms in relation to other groups of seed plants. Until a consensus emerges on this issue that can account for both molecular and morphological data, our understanding of early angiosperm evolution will remain incomplete.
This key question and others relating to the origin of angiosperms are unlikely to be resolved satisfactorily by studies of living plants alone because the four living groups of nonangiosperm seed plants provide only poor representation of the diversity of seed plants that existed in the past. On the basis of the paleontological record, one or several extinct groups of seed plants seem very likely to ultimately be shown to be more closely related to angiosperms than to conifers, cycads, Ginkgo L., or Gnetales (7). Identification of these extinct groups is crucial to an improved understanding of how the unique reproductive structures of angiosperms should be properly compared to those other seed plants. And in turn, this is vital to understanding how angiosperm stamens, ovules, and carpels may have evolved. The current situation therefore provides new opportunities for palaeobotanical research to contribute to the resolution of an important evolutionary problem at a time when new techniques and newly discovered fossil material are also opening up new areas for research.
In this paper, we describe four new genera of fossil seeds from the Early Cretaceous of Europe and eastern North America that expand the diversity of seed plants known from the critical interval when angiosperms were undergoing their initial diversification. These seeds are of paleoecological interest because they co-occur with fossils of early angiosperms. However, they also exhibit a common set of unusual structural features that unite them as part of a previously unrecognized complex of extinct plants that was diverse and widespread in the Early Cretaceous. In addition, strong similarities between seeds of this complex and previously described seeds of Bennettitales, Erdtmanithecales, and fossil and living Gnetales (BEG) provide evidence of a probable close relationship (24). If this BEG group is supported by additional observations and analyses it may have important implications for understanding the origin of angiosperms because previous phylogenetic analyses (e.g., 5; 38) suggest that angiosperms may be closely related to Gnetales and Bennettitales. These ideas need to be tested by additional analyses and observations as further information accumulates on these different groups of extinct and living seed plants.
MATERIALS AND METHODS
The fossil seeds formally described here were recovered from Early Cretaceous mesofossil floras at four localities in Portugal (Buarcos, Famalicão, Catefica, Torres Vedras) and one in eastern North America (Puddledock). The Buarcos locality is located northwest of Figueira da Foz (Beira Litoral region), 40°09′54′N, 8°52′11′W in the town of Buarcos, Portugal (Carta Geológica de Portugal 19C Figueira da Foz, 61). Samples were collected from a partly overgrown road cut in 1992, 1994, 1995, 1997, 1999, 2000, and 2001 (by K.R.P., E.M.F.). The fossils described here were extracted from samples 157, 209, 210, and 211. The plant-bearing sequence from Buarcos is included in the Calvaria Member of the Figueira da Foz Formation, the lowermost member of the formation and is thought to be of late Aptian to early Albian (11) or early Albian (36) age. A small palynoflora has been reported from this locality by 50. Rich mesofossil assemblages from the Buarcos locality include many reproductive structures related to early-branching lineages of extant angiosperms (29, 31). Cheirolepidiaceous conifers and ephedroid seeds are also common. Taxa already described from the Buarcos locality include Anacostia lusitanica 23, Pennicarpus tenuis 30, Ephedra portugallica Rydin, Pedersen, Crane et Friis and Ephedrispermum lusitanicum 65. Many other plant fossils of diverse relationships still remain to be formally described.
The Famalicão locality is located at a large clay pit on the eastern outskirts of the small village of Famalicão, about 5 km SSE of Leiria, Portugal (39°42′16″N, 8°46′12″W) (Carta Geológica de Portugal 23-C Leiria, 75). The fossils were recovered from sample 25, collected in 1989 (by K.R.P., E.M.F., P.R.C.) from a layer of dark organic rich clay, more than 1 m thick. This layer was exposed in a small temporary excavation deep below the bottom of the pit and was not accessible on subsequent visits to the locality. On the basis of lithological/stratigraphical studies of the Lower Cretaceous sequence in Portugal, 11 divided the sedimentary sequence in the Famalicão region into a lower Calvaria Member and an upper Famalicão Member of the Figueira da Foz Formation. The Calvaria Member is described as conglomeratic and sandy. It is regarded as of late Aptian to early Albian age. The plant bearing clay, excavated from beneath the floor of the Famalicão pit, lay below the Calvaria Member and must be older. We consider it to be of probable late Aptian age. The rich mesofossil assemblage from Famalicão includes more than hundred different kind of angiosperm flowers, fruits, and seeds (29, 31; 20) as well as diverse other seed plants. Two species of angiosperm, Anacostia lusitanica Friis, Crane et Pedersen and Anacostia sp., have been formally described from the Famalicão locality (23).
The Catefica locality is a road cut exposure along the small road between the villages of Catefica and Mugideira about 4 km SSE of Torres Vedras (39°3′30″N, 9°14′30″W). Samples were collected in 1989 (K.R.P., E.M.F., P.R.C.) and 1992 and 1995 (K.R.P., E.M.F.) in fluviatile cross-bedded sands with intercalated clay beds and darker organic rich horizons. The Lower Cretaceous strata at the Catefica locality are deposited in the western margins of the Runa Basin (Carta Geológica de Portugal 30-D Alenquer, 85). The strata were studied by 57 who recognized a lower and an upper “couches d'Almargem.” The sediments at the Catefica locality correspond to the lower “couches d'Almargem” that in the Torres Vedras area range from late Barremian to late Aptian in age (58; 59). Fossil plants were extracted from samples 49, 154, 242 and 382 collected in 1989, 1992, 1997, and 2001, respectively (K.R.P., E.M.F. with P.R.C. in 1989). The mesoflora comprises several different kinds of gymnosperm seeds including ephedroid seeds and the seeds described here, as well as twigs of cheirolepidiaceous conifers. The Catefica locality has also yielded many angiosperm flowers, fruits, seeds, and stamens with pollen in situ including a variety of chloranthaceous reproductive structures that are similar to material from the Torres Vedras locality (27, 29, 31, 13).
The Torres Vedras locality is a large open-cast clay pit in the northeastern outskirts of Torres Vedras, about 1 km NE of Forte de Forca on the road toward Sarge, Portugal (39°06′13”N, 9°14′47”W) (Carta Geológica de Portugal Torres Vedras 30C, 84). The fossils were recovered from sample 45, collected in 1989 (K.R.P., E.M.F., P.R.C.) from a sandy lignitic horizon just below a prominent ferruginous crust. The locality is no longer accessible and has been overtaken by urban development. Sediments in this area belong to the Lugar d'Almen Formation, the Fonte Granda Formation, and the Almargem Formation. Samples from the lignitic horizon belong to the lower part of the Almargem Formation, which is regarded in this area as late Barremian-early Aptian in age (58; see also 32). Mayoa portugallica Friis, Pedersen et Crane, based on a clump of characteristic pollen (32), was recovered from sample 44 at the Torres Vedras locality and has been assigned to Araceae. The flora also includes several staminate and pistillate structures related to Hedyosmum (Chloranthaceae) (27).
The Puddledock locality is located south of Richmond, Virginia, USA, east of the Appomattox River, in Prince George County (37°15′45″N; 77°22′25″W). A series of mesofossil assemblages have been extracted from samples collected from the Potomac Group in 1988 (P.R.C., E.M.F., K.R.P. with A.N. Drinnan) at the Tarmac Lone Star Industries Puddledock sand and gravel pit. Samples 082 and 083 were collected from the uppermost part of an organic rich clay layer exposed in the northern wall of the Puddledock pit. Further quarrying has now removed this exposure. Palynological analyses (Christopher in 12) indicate that the Potomac Group sediments at this locality are of late Early Cretaceous age and can be assigned to the basal part of subzone IIB in the palynological zonation established by 2 and others (14; 16; 17; 37). Subzone IIB is of Middle Albian age but may extend down into the Early Albian (15). The diverse mesofossil assemblages at the Puddledock locality include a variety of angiosperm reproductive structures (see 25, 28; 77), as well as remains of conifers (69, 28) and other seed plants. Among the angiosperms that have been formally described from the Puddledock locality are angiosperm flowers and fruits of Virginianthus calycanthoides 25, Appomattoxia ancistrophora Friis, Pedersen et Crane, Anacostia virginiensis 28, Potomacanthus lobatus 77 and Carpestella lacunata 78. Other seed plants described from the Puddledock locality comprise conifer shoots of Glenrosa virginiensis V. Srinivasan, Glenrosa hopewellensis V. 69, Athrotaxis cf. ungeri (Halle) Florin, Pseudofrenelopsis nathorstiana V. Srinivasan, Watsoniocladus florinii V. Srinivasan, Watsoniocladus virginiensis V. Srinivasan, and Watsoniocladus sp. (70).
Sediment samples from all five localities were sufficiently soft to break down in water. Plant fossils were extracted by wet sieving. Adhering mineral matrix was removed with 40% HF followed by treatment in 10% HCl and thorough rinsing in water. Fossils were then air dried and sorted using reflected light microscopy. Measurements were made by stereomicroscope, using SEM or from tomographic images. For attenuation-based synchrotron-radiation X-ray tomographic microscopy (SRXTM) and phase-contrast X-ray tomographic microscopy (PCXTM) specimens were mounted on small brass stubs without further treatment. In a few cases, specimens were remounted after they had been studied in SEM.
SRXTM and PCXTM was performed at the TOMCAT beamline of the Swiss Light Source of the Paul Scherrer Institut, Switzerland, using the technique described by 13 and 24. One or more specimens for each of the new species described here were studied using microtomography. For comparison, we also used these same methods for several related mesofossils and mineralized material. Smaller, coalified seeds were imaged using a 10× objective and PCXTM at 20 keV or SRXTM at 10 keV. Larger, coalified seeds were imaged using a 4× objective and SRXTM at 10 keV. The micropylar area of Buarcospermum tetragonium and Lignierispermum maroneae was imaged using a 20× object and SRXTM at 10 keV to obtain higher resolution of a selected area. Mineralized material was imaged using a 4× objective and SRXTM at 25 keV. Slice data were analyzed and manipulated using the program AMIRA (Mercury Computer Systems, Merignac Cedex, France, http://www.tgs.com/products/amira.asp) for computed tomography. Specimens for SEM were mounted on aluminum stubs and coated with gold for 60 s and studied using a Hitachi S-4300 field emission scanning electron microscope at 2 kV with the exception of specimen S101535 (Holotype for Buarcospermum tetragonium) that was coated with gold for about 7 min and studied using a Philips 515 scanning electron microscope at 15 kV. Nail polish was used for mounting specimens for both X-ray and SEM studies with the exception of some Cycadeoidea morierei seeds that were mounted with wax. Specimens studied with SRXTM and PCXTM were studied (gold-coated) with SEM either before or after microtomography.
All the fossil seeds that we describe in this paper have two layers around the nucellus. To avoid confusion, we describe these seeds in as neutral terms as possible. The inner layer we refer to as integument, in conformity with standard descriptions of seeds of conifers, cycads, Ginkgo, Gnetales, and most fossil seed plants. The outer layer we refer to as the “seed envelope.” In some previous studies (e.g., 5; 51) this layer has been referred to as “cupule.” We avoid this term and its potential implications for homology with structures in other seed plants.
The fossil material formally described in this paper is housed in the palaeobotanical collections of the Swedish Museum of Natural History (S) and the geological collections of the Field Museum, Chicago (PP). In addition, we include information about material that we have collected from other localities of Early Cretaceous age. Also included are illustrations of seeds of Cycadeoidea morierei (Saporta et Marion) Seward from the Early Cretaceous of Vaches-Noires of Normandy, France (courtesy of the University of Caen).
SYSTEMATICS
Buarcospermum gen. nov.
Derivation of generic name
From the Buarcos locality, Portugal, where the type species was discovered.
Generic diagnosis
Seeds small, radially symmetrical, broadly ovoid, micropylar region pointed; four-angled in cross section. Integument thin, micropylar tube long, narrow. Integument enclosed by a seed envelope, except for the micropylar opening, and attached to the envelope only at the base. Nucellus thin, enclosed by the integument; nucellus free distally, otherwise fused to the integument. Micropyle open at the apex. Below, micropylar closure distinct, multicellular, formed mainly from a layer of larger cells that are expanded radially toward the center of the micropylar tube. Inner surface of seed envelope smooth, nonpapillate; outer surface smooth. Seed envelope with an inner and outer sclerenchyma layer. Inner sclerenchyma layer continuous around the integument and surrounding the micropylar tube; circular in transverse section around micropylar tube, but four-angled below with four narrow wing-like crests radiating through the outer sclerenchyma layer to the surface and dividing it into four valves. Inner sclerenchyma layer almost circular in transverse section in the middle of the seed. Cells long, fibrous arching upwards in a chevron pattern and forming longitudinal ridges toward the outside. Arches short, formed by the fibers. Cells of outer sclerenchyma layer short, irregularly arranged. Demarcation between inner and outer sclerenchyma layers irregular, with a transitional zone of open spaces.
Type species
Buarcospermum tetragonium sp. nov.
Buarcospermum tetragonium sp. nov. (Figs. 1–2810–1415–19, 2324–28)
Buarcospermum tetragonium from the Early Cretaceous (late Aptian or early Albian) Buarcos locality, Portugal; SEM images. Scale bars: Figs. 1–6 = 1 mm; Fig. 7 = 0.1 mm; Fig. 8 = 0.2 mm; Fig. 9 = 0.5 mm. Figs. 1, 2, 4, 9. Holotype (S101535, sample Buarcos 210). Figs. 3, 8. Paratype (S156000, sample Buarcos 157). Figs. 5–7. Paratype (S156004, sample Buarcos 157). 1, 2. Lateral views of seed showing seed envelope with valves (outer sclerenchyma layer) and projecting inner fibrous tissue (inner sclerenchyma layer). 3. Lateral view showing epidermis partly preserved. 4. Apical view showing four-angled outline and four valves of the seed envelope. 5. Broken seed showing the wing-like crests of the inner fibrous layer and the outer valves of the seed envelope. 6. Broken seed in apical view showing fibrous crests (inner sclerenchyma layer) and valves (outer sclerenchyma layer). 7. Inner epidermis of seed envelope; note transversely elongate fiber cells below the epidermis. 8. Apical view showing micropylar tube surrounded by the sclerenchyma layer of the seed envelope and intact outer epidermis. 9. Apical view showing inner fiber layer and valves of the seed envelope.
Buarcospermum tetragonium from the Early Cretaceous (late Aptian or early Albian) Buarcos locality, Portugal; SRXTM images of holotype (S101535 sample Buarcos 210) showing reconstructed slice data of internal structure. Figs. 10, 14. Longitudinal sections. Figs. 11–13. Transverse sections. Scale bars: Figs. 10–13 (vertical scale at Fig. 10) = 1 mm; Fig. 14 (horizontal scale) = 0.5 mm. Heavy gold coating seen as outer, light covering. 10. Longitudinal section showing micropylar tube, micropylar canal, closure tissue, integument with basal attachment, and the various layers of the seed envelope (inner fiber layer forming the crests, transitional zone, outer sclerenchyma layer forming the valves). 11. Transverse section around the middle of micropylar tube showing tips of the four valves of the seed envelope, micropylar tube filled by the closure tissue and strongly adpressed to seed envelope: four wing-like crests of the inner fibrous layer extend almost to the surface of seed envelope. 12. Transverse section in the upper part of seed body showing seed envelope and integument: micropylar tube solid, filled by the closure tissue. 13. Transverse section near the middle of seed body showing seed envelope, integument, nucellus, and internal tissues. 14. Longitudinal (tangential) section through the seed envelope showing arched fibers of inner sclerenchyma layer, transitional zone, and outer sclerenchyma layer.
Buarcospermum tetragonium from the Early Cretaceous (late Aptian or early Albian) Buarcos locality, Portugal, holotype (S101535 sample Buarcos 210). Figs. 20–22. Buarcospermum tetragonium from Early Cretaceous (late Barremian-Aptian) Catefica locality, Portugal (S156369, sample Catefica 382); SRXTM images showing reconstructed slice data of internal structure. Scale bars: Figs. 15, 23 (vertical scale at Fig. 15) = 0.5 mm; Figs. 16–19 (vertical scale at Fig. 19) = 0.5 mm. Figs. 20–22 (vertical scale at Fig. 20) = 0.5 mm. Heavy gold coating seen as outer, light covering in Figs. 15–19, 23. 15. Longitudinal section (LS) showing micropylar tube, micropylar canal, closure tissue, integument, and nucellus: note micropylar tube filled with closure tissue and strongly adpressed to outer envelope. 16. Transverse section (TS) through tip of seed showing central micropylar canal formed by the thin micropylar tube and surrounded by sclerenchyma of the envelope: at the apex, the micropylar tube consists of one cell layer (inner epidermis). 17. TS below Fig. 16 showing the micropylar canal formed by the micropylar tube surrounded by sclerenchyma of the envelope: micropylar tube consists of two cell layers (inner and outer epidermis). 18. TS below Fig. 17 (around the middle of the micropylar tube above the four valves of the seed envelope) showing micropylar canal occluded by closure tissue: closure tissue comprised of radially elongate cells of inner epidermis of micropylar tube with several additional outer cell layers (continuous with outer epidermis of Fig. 17, see also Fig. 15). 19. TS near base of the micropylar tube below Fig. 18 showing tips of the four valves of the seed envelope and closed micropylar canal. 20. LS of micropylar region showing seed envelope surrounding micropylar tube with distinct inner epidermis and open micropylar canal that extends for most of its length (see Figs. 21, 22). 21. TS showing narrow, central micropylar canal formed by the thin micropylar tube surrounded by sclerenchyma layer of the seed envelope: micropylar tube consisting of one cell layer (inner epidermis). 22. TS below Fig. 21 showing narrow, central micropylar canal surrounded by the micropylar tube consisting of two cell layers (inner and outer epidermis). 23. TS showing transition zone between inner and outer sclerenchyma layers of seed envelope.
Schematic line drawings through micropylar area of Buarcospermum tetragonium. Fig. 24. Longitudinal; Figs. 25–28, transverse sections. Yellow = integument; dark green = inner sclerenchyma layer of seed envelope; light green = outer sclerenchyma layer of the seed envelope. Figs. 25–28 at successively lower levels indicated with arrows on Fig. 24. Scale bar Fig. 24 = 1 mm; Figs. 25–28 (at fig. 28) = 0.5 mm. Compare with Figs. 95–98, 128–131.
Derivation of specific epithet
Referring to the four-angled shaped of the seeds.
Specific diagnosis
As for the genus.
Holotype
S101535 from sample Buarcos 210, illustrated Figs. 1, 2, 4, 9–19, 23.
Paratypes
S156000, S156004 (sample Buarcos 157), S156001 (sample Buarcos 209), S156002 (sample Buarcos 211), S156212–S156214 (sample Buarcos 371).
Other specimens examined
S156003 (sample Torres Vedras 45), S156215–S156216 (sample Catefica 49), S156225–S156227 (sample Catefica 154), S156369 (sample Catefica 382), PP53211, PP53340 (sample Puddledock 082), PP53695–53697 (sample Puddledock 083).
Type locality
Buarcos, Northwest of Figueira da Foz, Portugal (40°09′54′N, 8°52′11′W).
Stratigraphic position and age of type stratum
Calvaria Member, Figueira da Foz Formation. Early Cretaceous (late Aptian or early Albian).
Description and comments on the species
Buarcospermum tetragonium is based on isolated seeds from four Early Cretaceous localities, three in Portugal (Buarcos, Catefica, Torres Vedras), and one in eastern North America (Puddledock).
The seeds are small, ∼1.8–2.2 mm long and 1.4–1.5 mm broad, radially symmetrical, and broadly ovoid. They taper apically into a pointed micropylar region, about 0.25 mm long and 0.06 mm broad (Figs. 1–6, 8–9). In cross-section the seeds are four-angled (Figs. 4–6, 11–13). The outer surface of the seeds, where the epidermis is abraded, is characterized by a four-parted organization with four elongated and pointed segments that extend from the base of the seed to the base of the micropylar area and are separated from each other by four distinct longitudinally extended ribs.
The X-ray microtomography of the holotype (S101535) and an additional specimen (S156369) shows that the seeds are composed of a thin integument enclosed by a sclerenchymatous seed envelope (Figs. 10–23). The integument is extended apically and surrounds a very long and narrow micropylar tube, ∼0.6 mm long and 0.012 mm wide (Figs. 10, 15). In the apical region, the integument consists of a single layer of thin-walled cells, the inner epidermis, which forms a circular ring around the open micropylar tube (Figs. 15, 16, 20, 21). Further down, an addition cell layer, the outer epidermis, is present (Figs. 15, 17, 20, 22). This layer becomes multicellular with small isodiametric cells several layers thick toward the middle and base of the micropylar tube (Figs. 15, 18–19). In the holotype (S101535), the micropylar canal is closed in the middle and toward the base by the cells of the inner epidermis that are expanded radially toward the center of the micropylar tube (Figs. 15, 18, 19). In specimen S156369, which perhaps represents a younger developmental stage, the micropylar canal appears to be open for the full length of the micropylar tube (Figs. 20–22). In both specimens, the micropylar tube is closely adpressed to the seed envelope in the middle and basal part of the micropylar region (Figs. 10, 11, 15, 18, 19).
The integument is surrounded by a robust envelope that is open only at the apex (Figs. 10, 15–19, 24–28). Integument and seed envelope are free from each other except at the base where the integument is broadly attached (Fig. 10). The integument encloses the nucellus (Fig. 10). Nucellus appears to be free from the integument apically but fused to it in the lower part (Figs. 10, 13), but these inner tissues and the apex of the nucellus are not well preserved. A megaspore membrane has not been observed.
The seed envelope is formed from several layers of sclerenchyma cells covered by an inner and outer cutinized epidermis. The inner epidermis of the seed envelope consists of short, thin-walled cells that are slightly longitudinally elongate and have a smooth surface toward the inside of the seed (Fig. 7). Cells from the fibrous layer are seen as distinct transverse striations under the epidermis (Fig. 7). The inner epidermis of the seed envelope is also smooth in the micropylar region and lacks papillae. The outer surface of the seed envelope is almost smooth (Figs. 1–3, 8) with a thin cutinized epidermis of small, slightly longitudinally elongate cells. In some seeds, the outer epidermis is almost intact and conceals the underlying valvate organization of the outer seed envelope (Fig. 3).
The inner layer of the seed envelope is elongated at the apex and is continuous around the integument. It is circular in cross section near the apex where it surrounds the micropylar tube (Figs. 8, 16, 17, 25–28), but it is four-angled below (Figs. 4–6, 18–19, 27, 28). Close to the apex of the seed, the four corners of the inner layer are extended into sharp, narrow, wing-like crests (Fig. 5, 6, 11, 12, 19, 27, 28). In the body of the seed, they are surrounded by and embedded in the outer sclerenchyma layer. These crests can be traced from the base of the seed to the region of the micropylar tube. They also extend radially to the outer surface of the seed (Figs. 4–6, 11, 12). Each wing-like crest has a vascular bundle in its innermost part that extends from the base to the apex (Fig. 13). The sclerenchyma cells of the inner layer are fibrous and arch upward in a chevron pattern (Fig. 14). The innermost fibers are thin-walled, while the remaining fiber cells are thick-walled. The arches are short and the chevron pattern forms shallow longitudinal ridges on the outside of the inner layer. In cross section, these ridges are seen as a strongly wavy outer line of demarcation (Figs. 12, 23). The ridges on the outside of the inner layer correspond to grooves in the sclerenchyma of the outer layer (Figs. 12, 23). A transition zone of partly open spaces between the inner and outer layer may indicate an area of thin-walled cells that are not preserved. The outer sclerenchyma layer has an inner zone of larger, radially extended cells (Figs. 10, 11) and an outer zone of smaller, more or less isodiametric cells that are irregularly arranged (Figs. 12, 13).
The outer sclerenchyma layer of the seed envelope is segmented into four valves that are separated by the wing-like crests of the inner layer (Figs. 1, 2, 4–6, 9). The valves are elongated and pointed at the apex. They also narrow toward the base (fig. 1). The valves extend from the base of the seed to the base of the micropylar region. In their apical part, the valves formed by the outer sclerenchyma layer of the seed envelope are free from the inner sclerenchyma layer (Figs. 9, 19); otherwise, they are more or less fused with the inner layer only separated by the partly open spaces of the transition zone (Figs. 10–14, 19, 23). The sclerenchyma cells of the outer layer are shorter than those of the inner layer and are irregular in shape and arrangement (Figs. 12, 13).
Lobospermum gen. nov.
Derivation of generic name
From the four-lobed shaped of the seeds.
Generic diagnosis
Seeds small, radially symmetrical, ellipsoidal, micropylar region pointed; four-lobed in cross section. Integument thin, micropylar tube long, narrow. Integument enclosed by a seed envelope, except for the micropylar opening, and attached to the envelope only at the base. Micropyle open at the apex. Below, it is closed by a distinct, multicellular closure that fills the lower half of the micropyle. Micropylar closure formed mainly from a layer of larger cells that are expanded radially toward the center of the micropylar tube. Seed lobes separated by deep grooves; each with a narrow longitudinal median ridge. Inner surface of seed envelope smooth, nonpapillate; outer surface smooth or rugulate. Seed envelope with an inner and outer sclerenchyma layer. Cells of inner sclerenchyma layer long, fibrous arching upwards in a chevron pattern. Arches formed by the fibers are high and broad. Cells of outer sclerenchyma layer narrow, longitudinally aligned. Inner and outer sclerenchyma layers clearly delimited.
Type species
Lobospermum stampanonii sp. nov.
Lobospermum stampanonii sp. nov. (Figs. 29–4437–44)
Lobospermum stampanonii from the Early Cretaceous (early or middle Albian) Puddledock locality, Virginia, USA; SEM images. Scale bars: Figs. 29, 34 = 1 mm; Figs. 30, 32, 35 = 0.5 mm; Fig. 31, 36 = 0.2 mm; Fig. 33 = 0.2 mm. Figs. 29–33. Holotype (PP53375, sample Puddledock 082). Figs. 34–36. Paratype (PP53376, sample Puddledock 082). 29. Lateral view of seed showing strongly lobed seed envelope. 30. Apical view showing micropylar area with seed envelope surrounding the micropylar tube. 31. Detail of micropylar area showing seed envelope and micropylar tube. 32. Outer surface of seed envelope showing longitudinally aligned sclerenchyma and remains of thin cuticle of outer epidermis (white arrows). 33. Detail of outer part of seed envelope showing the median vascular bundle of one lobe and pitted sclerenchyma. 34. Broken seed showing the anatomy of the seed envelope with the arched fibrous inner layer and the longitudinally aligned sclerenchyma of outer layer. 35. Detail of inner and outer sclerenchyma layers of the seed envelope. 36. Detail of seed envelope showing outer pitted sclerenchyma.
Lobospermum stampanonii from the Early Cretaceous (early or middle Albian) Puddledock locality, Virginia, USA; SRXTM images of holotype (PP53375, sample Puddledock 082) showing reconstructed slice data of internal structure. Scale bars = 1 mm: Figs. 37, 38, 43, 44 (vertical scale at Fig. 37); Figs. 39–42 (vertical scale at Fig. 40). 37. Longitudinal section (LS) through micropylar area showing micropylar tube, open micropylar canal, and closure tissue, surrounded by apical part of seed envelope. 38. LS perpendicular to Fig. 37 showing details of the micropylar area and anatomy of seed envelope. 39. Transverse section (TS) at seed apex showing central micropylar canal formed by the thin micropylar tube surrounded by sclerenchyma of seed envelope. 40. TS through the micropylar area showing closure tissue comprised of integument cells partly expanded toward the center of the canal. 41. TS below Fig. 40 in the middle of micropylar tube showing the closure tissue mainly composed of radially extended cells. 42. TS below Fig. 41 near base of micropylar tube showing degenerated cells near the center. 43. TS below the micropylar region in upper part of seed body showing the four-lobed configuration of the seed envelope with inner fibrous and outer smaller sclerenchyma cells. 44. TS below Fig. 43 near the middle of seed showing four-lobed seed envelope comprised of two distinct layers of sclerenchyma.
Derivation of specific epithet
In honor of Marco Stampanoni for his contribution to the development of X-ray microtomography in paleontology.
Specific diagnosis
As for the genus with the following additions. Outer surface of seed almost smooth. Cells of outer sclerenchyma layer of seed envelope equiaxial in transverse section.
Holotype
PP53375 from sample Puddledock 082, illustrated Figs. 29–33, 37–44.
Paratype
PP53376 from sample Puddledock 082.
Type locality
Puddledock, Tarmac Lone Star Industries sand and gravel pit, south of Richmond and east of the Appomattox River, Prince George County, Virginia, USA (37°15′45″N, 77°22′25″W).
Stratigraphic position and age of type stratum
Basal part of Subzone IIB, Potomac Group. Early Cretaceous (early or middle Albian).
Description and comments on the species
The species is based on a well-preserved isolated seed, ∼3.8 mm long and 1.7 mm broad, as well as other seed fragments. The seed is radially symmetrical and ellipsoidal in shape with a pointed apical micropylar region (Figs. 29, 30). In cross section, the seeds are four-lobed with deep grooves separating each lobe (Figs. 29, 30, 32, 43, 44). Each lobe bears a median ridge (Figs. 29, 30) that extends for the full length of the seeds or from the micropylar area to about the middle of the seed (Fig. 29).
The seeds are composed of a thin integument enclosed by an outer sclerenchymatous envelope (Figs. 30, 31, 37, 38, 40). The integument extends apically into a long, narrow micropylar tube (Figs. 31, 37–42). In the apical region, the micropylar tube consists of a single layer of thin-walled cells, the inner epidermis (Figs. 39, 40). Further down, an additional cell layer, the outer epidermis, is developed. Toward the middle of the micropylar tube, the cells of the inner epidermis extend radially to the center of the micropylar canal, making the micropyle solid by completely closing the lower half of the canal (Figs. 37, 38, 41, 42). Below, the base of the micropyle the apex of the nucellus is poorly preserved.
The integument is enclosed by a robust envelope that is open only at the apex (Figs. 37, 38). Integument and seed envelope are free from each other except at the base where the integument is broadly attached. The remains of tissues internal to the integument are dense in this specimen, and their exact organization is not fully understood.
The seed envelope is formed from an inner and an outer sclerenchyma layer. The inner epidermis of the seed envelope is smooth and cutinized (Fig. 32). The inner epidermis is also smooth where it surrounds the micropylar tube, and it lacks papillae (Figs. 37, 38). The outer seed surface is almost smooth with a thin cutinized epidermis. Sclerenchyma cells of the inner layer are fibrous (Figs. 34–36, 38, 43, 44) and prominently arched forming a distinct chevron pattern (Figs. 34, 35). The arches are broad and high with only few arches over the entire width of the seed. The fibrous layer is ∼0.1 mm thick in most places, but thinner under the grooves. The line of delimitation between the inner fibrous layer and the outer layer of the envelope is sharp, and the fibrous layer is easily detached. The outer layer consists mostly of thin-walled, elongated sclerenchyma cells of equal width that are aligned in distinct longitudinal rows (Figs. 32–36). These cells are equiaxial in transverse section. Over most of the seed, the outer layer is three cells thick, but in the grooves there may be only one or two layers of cells, while on the ridges there are several layers of cells (Figs. 34, 35). Vascular bundles extend from base to apex along the line of the ridges (Fig. 33).
Lobospermum glabrum sp. nov. (Figs. 45–53)
Lobospermum glabrum from the Early Cretaceous (late Aptian?) Famalicão locality, Portugal; SEM images. Scale bars: Figs. 45–49, 51 = 1 mm; Fig. 50 = 0.5 mm; Fig. 52 = 0.1 mm; Fig. 53 = 0.2 mm. Figs. 47–48. Holotype (S154554, sample Famalicão 25). Figs. 45–46. Paratype (S154553, sample Famalicão 25). Figs. 49, 52. Paratype (S154555, sample Famalicão 25). Fig. 50. Paratype (S156005, sample Famalicão 25). Figs. 51, 53. Paratype (S156006, sample Famalicão 25). 45. Lateral view of seed showing the strongly lobed seed envelope, median ridges of the lobes, and remains of a secretion in the micropylar area. 46. Apical view of seed in Fig. 45 showing lobes and remains of apical secretion. 47. Lateral view showing lobes and ridges of seed envelope. 48. Apical view of seed in Fig. 47 showing lobes separated by deep grooves and apical micropylar region. 49. Broken seed with distinct grooves between lobes showing cellular structure of seed envelope. 50. Apical part of seed showing outer epidermis of seed envelope. 51. Inner fibrous layer of seed envelope showing chevron pattern formed by the arched fibers. 52. Detail of outer part of seed envelope showing the median vascular bundle of one lobe and pitted sclerenchyma. 53. Detail of the inner arched fibers.
Derivation of the specific epithet
Referring to the smooth surface of the seed.
Specific diagnosis
As for the genus with the following additions. Outer surface of seed almost smooth. Cells of outer sclerenchyma layer not equiaxial in transverse section: cells toward the inside distinctly taller, cells toward the outside equiaxial.
Holotype
S154554 from sample Famalicão 25, illustrated Figs. 47, 48.
Paratypes
S154553, S154555, S156005–S156012 (sample Famalicão 25).
Other specimens examined
S107690, S107691 (sample Catefica 154), S156217, S156218 (sample Catefica 242), S156220–S156222 (sample Catefica 49).
Type locality
Famalicão, about 5 km SSE of Leiria, Portugal (39°42′16″N, 8°46′12″W).
Stratigraphic position and age of type stratum
Below Calvaria Member, Figueira da Foz Formation. Early Cretaceous (late Aptian?).
Description and comments on the species
All seeds are isolated (Figs. 45–49). They are ∼2.3–4.8 mm long, and 1.5–2 mm broad. The longest specimen is incomplete, and the inferred total length is ∼5.3 mm. Several fragmentary specimens consist only of the inner fibrous layer shows the distinctive chevron pattern (Figs. 51, 53). This layer is easily detached from outer layer. The seeds are similar in general shape and organization to those of L. stampanonii with a four-lobed morphology in cross section, lobes separated by deep grooves, and an almost smooth to faintly rugose outer surface (Figs. 45–49). However, they differ in details of the seed envelope. In L. glabrum, the median ridge of the lobes is indistinct and extends for the full length of the seeds (Figs. 45–48). In L. stampanonii, this ridge is more distinct but may only extend part of the distance to the base of the seed. The organization of the seed envelope is also similar to that in L. stampanonii with the inner sclerenchyma layer formed from fibrous cells arranged in a broad chevron pattern (Figs. 51, 53) and cells of the outer sclerenchyma layer narrow, elongate, and arranged in longitudinal rows (Fig. 52). However, the cells of the outer sclerenchyma layer differ in being of unequal size in transverse section with cells toward the inside being taller than the outer cells.
In one specimen, a distinct shiny substance covers the micropylar opening and most of the apical region of the seed (Figs. 45, 46). A similar covering has been observed in the same position in other Lobospermum species, and the substance has the appearance of a secretion. It is most likely the hardened remains of a pollination droplet.
Lobospermum rugosum sp. nov. (Figs. 54–57)
Lobospermum rugosum from the Early Cretaceous (early or middle Albian) Puddledock locality, Virginia, U.S.A; SEM images. Scale bars: Figs. 54–56 = 2 mm; Fig. 57 = 0.1 mm; Fig. 55. Holotype (PP53374, sample Puddledock 083). Figs. 54, 57. Paratype (PP53380, sample Puddledock 082). Fig. 56. Paratype (PP53379, sample Puddledock 082). 54–56. Lateral view of three different seeds showing the lobed seed envelope, distinct median ridges on the lobes, and transverse rugulate surface ornamentation. 57. Apical view showing the micropylar area with the seed envelope surrounding the micropylar tube and micropylar canal.
Derivation of specific epithet
Referring to the rugulate surface of the seeds.
Specific diagnosis
As for the genus, with the following additions. Outer surface of seed rugulate. Cells of outer sclerenchyma layer not equiaxial in transverse section: cells toward the outside distinctly taller and forming the rugulate surface ornamentation.
Holotype
PP53374 from sample Puddledock 083, illustrated Fig. 55.
Paratypes
PP53377–PP53380, PP52705–PP53708 (sample Puddledock 082), PP53709–PP53710 (sample Puddledock 083).
Other specimens examined
S156223 (sample Catefica 154), S156370–S156371 (Catefica 358).
Type locality
Puddledock, Tarmac Lone Star Industries sand and gravel pit, south of Richmond and east of the Appomattox River, Prince George County, Virginia, USA (37°15′45″N, 77°22′25″W).
Stratigraphic position and age of type stratum
Basal part of Subzone IIB, Potomac Group. Early Cretaceous (early or middle Albian).
Description and comments on the species
All seeds are isolated (Figs. 54–56). They are ∼3.7–4.1 mm long and 1.15–1.6 mm broad. Several specimens are fragmentary and consist only of the inner fibrous layer with its distinct chevron pattern. This inner layer is easily detached from outer layer. The seeds are similar in general shape and organization to those of L. stampanonii and L. glabrum, with a robust outer seed envelope and a thin integument that is extended apically into a long, narrow micropylar tube. The micropylar canal is open apically (Fig. 57). Lobospermum rugosum differs from the other two species mainly in the rugulate external surface ornamentation of the envelope, which consists of irregular transverse ridges (Figs. 54–56). In addition, the sclerenchyma cells of the outer layer of the seed envelope are of unequal size in transverse sections. The cells toward the outside are distinctly taller and form the rugulate ridges on the seed surface.
Rugonella gen. nov.
Derivation of generic name
From the strongly rugulate surface ornamentation of the seed envelope.
Generic diagnosis
Seeds small, bilaterally symmetrical (with a single axis of symmetry) and two lateral wings; nearly circular in longitudinal outline in the broadest view, micropylar region slightly pointed; shallowly triangular, three-lobed in cross section with three grooves separating the lobes. Integument thin, micropylar tube long, narrow. Integument enclosed by a seed envelope, except for the micropylar opening, attached to the envelope only at the base. Micropyle open at the apex. Below, micropylar closure distinct, multicellular, with enlarged cells that extends toward the center of the micropyle filling the lower half of micropylar tube. Inner surface of seed envelope smooth, nonpapillate; outer surface deeply rugulate, except on the wings. Seed envelope with an inner and an outer layer of sclerenchyma. Cells of inner sclerenchyma layer long, narrow and thin-walled, arching in a distinct chevron pattern. Fiber arches broad and high. Cells of outer sclerenchyma layer longitudinally aligned, narrow. Demarcation between inner and outer sclerenchyma layers distinct.
Type species
Rugonella trigonospermum sp. nov.
Rugonella trigonospermum sp. nov. (Figs. 58–71)
Rugonella trigonospermum from the Early Cretaceous (early or middle Albian) Puddledock locality, Virginia, USA; SEM images. Scale bars: Figs. 58–61 = 1 mm; Fig. 62 = 0.2 mm; Fig. 63 = 0.5 mm; Fig. 64 = 0.05 mm; Fig. 65 = 0.025 mm. Figs. 58, 59, 61, 63, 64. Holotype (PP53373, sample Puddledock 083). Figs. 60, 62, 65. Paratype (PP53382, sample Puddledock 082). 58. Dorsiventral view of seed showing the lobed seed envelope, lateral wings, and transverse, rugulate surface ornamentation. 59. Lateral view. 60. Dorsiventral view showing seed envelope split at apex exposing micropylar tube. 61. Apical view showing the bilaterally symmetrical, triangular shape of the seed envelope and flat lateral wings. 62. Detail of seed in Fig. 60 showing micropylar tube. 63. Outer surface of seed envelope showing outer papillate epidermis and transverse ridges. 64. Detail of micropylar area showing outer epidermal cells of micropylar tube and inner closure tissue surrounded by sclerenchyma of seed envelope. 65. Apical view of micropyle in Figs. 60, 62, showing outer epidermis of micropylar tube and closing tissue.
Derivation of species epithet
Referring to the triangular shape of the seeds.
Specific diagnosis
As for the genus.
Holotype
PP53373 from sample Puddledock 083, illustrated Figs. 58, 59, 61, 63, 64, 66–71).
Rugonella trigonospermum from the Early Cretaceous (early or middle Albian) Puddledock locality, Virginia, USA; SRXTM images of holotype (PP53373, sample Puddledock 083) showing reconstructed slice data of internal structure. Scale bar Figs. 66–68 (vertical at Fig. 66) = 1 mm; 67–71 (horizontal at Fig. 70) = 1 mm. 66. Longitudinal section (LS) through lobes showing cellular structure of seed envelope and internal tissue. 67. LS perpendicular to section in Fig. 66 showing details of the micropylar area and seed envelope. 68. LS (tangential) of sclerenchyma of seed envelope showing fibrous inner layer and shorter outer cells. 69. Transverse section (TS) at apex of the seed showing central micropylar tube surrounded by the triangular seed envelope. 70. TS through seed below micropylar area showing integument free from the seed envelope. 71. TS near middle of the seed showing three-lobed seed envelope composed of two distinct layers of sclerenchyma.
Paratypes
PP53381-PP53383, PP53700–PP53704 (sample Puddledock 082), PP53698–PP53699 (sample Puddledock 083).
Type locality
Puddledock, Tarmac Lone Star Industries sand and gravel pit, south of Richmond and east of the Appomattox River, Prince George County, Virginia, USA (37°15′45″N, 77°22′25″W).
Stratigraphic position and age of type stratum
Basal part of Subzone IIB, Potomac Group. Early Cretaceous (early or middle Albian).
Description and comments on the species
The species is based on well-preserved isolated seeds, ∼2.5 mm long and 2.3 mm broad. The seeds are bilaterally symmetrical and almost circular in their broadest outline (Fig. 58) with a short stalk in some specimens. In cross section, the seeds are narrow triangular/three-lobed (Figs. 58, 61, 69–71). The three lobes are unequal in size and differ in shape. The median, probably abaxial, lobe is separated from the two lateral lobes by two deep grooves (Figs. 58–61). It has a distinct, rounded ridge extending longitudinally from the base to apex (Figs. 58, 61, 70, 71). The two lateral lobes are flattened on one side, probably the ventral side, and separated from each other by a shallow groove (Figs. 61, 70, 71). They are extended laterally into a flattened wing, ∼0.4 mm wide, that forms the margin of the seed on either side (Figs. 61, 63, 70, 71). The probable dorsal surface is strongly rugulate and ornamented by irregular and transversely aligned ridges (Figs. 58–60, 66–71). The probable ventral surface is smooth or shallowly rugulate (Figs. 61, 70, 71).
The seeds are composed of a thin integument enclosed by an outer sclerenchyma envelope. The integument extends apically into a long, narrow micropylar tube (Figs. 60, 62, 64, 65, 67). In the apical region of the micropyle, the integument consists of a single layer of thin-walled cells. Toward the middle of the micropylar tube, there are several cell layers, including a layer of enlarged cells that are radially extended toward the center of the micropylar canal. These cells completely close the micropylar canal (Figs. 64, 65).
The integument is enclosed by a robust envelope that is open only at the apex. Integument and seed envelope are free from each other (Figs. 67, 69–71) except at the base where the integument is broadly attached (Fig. 67). The integument encloses the nucellus. It is indistinct, but clearly free from the integument in the distal region (Fig. 67). The tissues inside the integument are otherwise densely opaque, and a megaspore membrane was not observed.
The seed envelope (Fig. 67) is formed from an inner and an outer sclerenchyma layer. The inner epidermis of the seed envelope is smooth, cutinized, and consists of thin-walled, narrow, longitudinally arranged cells. The inner epidermis is also smooth around the micropylar tube and lacks papillae. The outer seed surface has a thin cutinized epidermis (Fig. 62).
The sclerenchyma cells of the inner layer are fibrous and arched, forming a distinct chevron pattern (Figs. 66–68). The arches are broad and high with only a few arches over the entire width of the seed. The delimitation between the fibrous layer and the outer layer of the envelope is sharp, and the fibrous inner layer is easily detached from the outer layer. The outer layer is a few cell layers thick, consisting of small sclerenchyma cells of unequal size. The outer cells are larger than the inner and form the ridges on the surface of the envelope (Figs. 66–68, 70, 71). The outer epidermis is thin and consists of small isodiametric cells arranged in longitudinal files (Fig. 62), that sometimes have a finely papillate surface ornamentation (Fig. 63).
Lignierispermum gen.nov.
Derivation of generic name
In honor of the French palaeobotanist Octave Lignier for his contribution to the understanding of reproductive structures of Bennettitales.
Generic diagnosis
Seeds small, obovoid, micropylar region pointed; four-angled, slightly flattened, bisymmetrical (with two planes of symmetry) in cross section. Integument thin, micropylar tube long, narrow. Integument enclosed by a seed envelope, except for the micropylar opening, and attached to the envelope only at the base. Nucellus thin, enclosed by the integument; nucellus free distally, otherwise fused to the integument. Micropyle open at the apex. Below, micropylar closure distinct, multicellular, formed from several cell layers filling the lower half of micropylar tube. Inner surface of seed envelope smooth, nonpapillate; outer surface smooth. Seed envelope sclerenchymatic comprising two distinct layers. Inner layer composed of transversely aligned fiber cells; at the apical corners of the seed, the fiber cells radiate toward the outside to form four, narrow, wing-like, apically pronounced crests. The outer layer is composed of radially extended cells that increase in size toward the apex and shorter, thin-walled cells covered by a distinct epidermis of equiaxial cells. Four distinct longitudinal bundles extend in the outer layer of the seed envelope from the seed base to apex.
Type species
Lignierispermum maroneae sp. nov.
Lignierispermum maroneae sp. nov. (Figs. 72–9882–8788–9495–98)
Figs. 72–77. Lignierispermum maroneae from the Early Cretaceous (early or middle Albian) Puddledock locality, Virginia, USA; holotype (PP53214 sample Puddledock 082). Figs. 78–81. Lignierispermum maroneae from the Early Cretaceous (late Barremian-Aptian) Catefica locality, Portugal; (S156219 sample Catefica 49). SEM images; Scale bars: Figs. 72, 73, 78 = 1 mm; Fig. 74 = 0.5 mm; Fig. 75, 80 = 0.1 mm; Fig. 76, 79 = 0.25 mm; Fig. 77, 81 = 0.2 mm. 72, 73. Lateral views showing seed envelope with four longitudinal ribs, projecting micropylar area and ring of ruptured cells immediately below. 74. Apical view showing the four-angled outline, outer epidermis of seed envelope and ring of ruptured cells toward the apex. 75. Micropylar area in apical view showing the micropylar tube and open micropylar canal surrounded by the sclerenchyma cells of the seed envelope. 76. Lateral view of seed apex showing outer epidermis of seed envelope and ring of ruptured cells. 77. Outer surface of seed envelope showing epidermis with transversely elongate cells and scattered papillae/trichome bases. 78. Lateral view of seed showing longitudinal ribs, micropylar area, and transversely elongate epidermal cells. 79. Lateral view of seed apex showing epidermal features near micropyle. 80. Epidermis of seed envelope with broken papilla/trichome. 81. Outer surface of seed envelope showing rows of transversely elongate epidermal cells and scattered papillae/trichomes.
Lignierispermum maroneae from the Early Cretaceous (early or middle Albian) Puddledock locality, Virginia, USA; SRXTM images of holotype (PP53214, sample Puddledock 082) showing reconstructed slice data of internal structure. Figs. 82, 83, 86. Longitudinal sections. Figs. 84–85, 87). Transverse sections. Scale bars: Fig. 82, 83 = 1 mm; Figs. 84, 85, 87 (vertical scale at 85) = 0.5 mm; Fig. 86 = 0.25 mm. 82. Longitudinal section (LS) through micropylar area of seed showing micropylar tube, micropylar canal, closure tissue, integument with basal attachment, nucellus, and sclerenchyma of seed envelope. 83. Detail of micropylar region showing integument extended into micropylar tube that is open apically and closed in the middle: note radially expanded cells in the subapical region of the seed envelope. 84. Transverse section (TS) through the tip of seed showing central micropylar canal formed by the thin micropylar tube and surrounded by sclerenchyma of the seed envelope: micropylar tube consisting of one cell layer (inner epidermis), seed envelope with a distinct inner epidermis and smooth inner surface. 85. TS through the micropylar area below Fig. 84 showing micropylar tube with an additional cell layer (outer epidermis). 86. LS of seed envelope showing inner transverse fibers, outer sclerenchyma cells and outer epidermis. 87. TS near base of micropylar tube showing the micropylar closure with narrow star-shaped opening; seed envelope showing radially extended cells of outer sclerenchyma layer and four crest-like extensions of inner sclerenchyma layer.
Lignierispermum maroneae from the Early Cretaceous (early or middle Albian) Puddledock locality, Virginia, USA; SRXTM images of holotype (PP53214, sample Puddledock 082) showing reconstructed slice data of internal structure. Successive transverse sections from near apex to base. Scale bar = 1 mm. 88. Transverse section (TS) near base of micropylar tube showing wing-like crests and radiating sclerenchyma cells of the seed envelope: note thick-walled cells of the integument. 89. TS below the micropylar region showing seed envelope, integument and apex of nucellus. 90–94. Sections to base of seed showing decreasing size of radiating sclerenchyma cells: note four vascular bundles in the corners of seed in Fig. 92, eight vascular bundles in Figs. 93, 94.
Schematic line drawings through micropylar area of Lignierispermum maroneae in longitudinal (95) and transverse sections (96–98). Yellow = integument; dark green = inner sclerenchyma layer of seed envelope, light green = outer sclerenchyma layer of seed envelope. Figs. 96–98 at successively lower levels indicated with arrows on Fig. 95. Scale bar Fig. 95 = 1 mm; Figs. 96–98 (at Fig. 98) = 0.5 mm. Compare with Figs. 24–28, 128–131.
Derivation of specific epithet
In honor of Federica Marone in appreciation of her support in the X-ray microtomographic studies.
Specific diagnosis
As for the genus.
Holotype
PP53214 from sample Puddledock 082, illustrated Figs. 72–77, 82–94.
Other specimens examined
S156219 (sample Catefica 49).
Type locality
Puddledock, Tarmac Lone Star Industries sand and gravel pit, south of Richmond and east of the Appomattox River, Prince George County, Virginia, USA (37°15′45″N, 77°22′25″W).
Stratigraphic position and age of type stratum
Basal part of Subzone IIB, Potomac Group. Early Cretaceous (early or middle Albian).
Description and comments on the species
Lignierispermum maroneae is based on a single, well-preserved, isolated seed from the Puddledock locality, USA (Figs. 72–75, 82–94), and one well-preserved isolated seed from the Catefica locality, Portugal (Figs. 78–81). The Catefica seed is slightly larger than the Puddledock specimen and broader in shape. They are treated here as one species because of the close similarity in general shape and structure and identical epidermal features.
The seeds are small. The Puddledock specimen is ∼2.7 mm long and up to 1.3 mm broad. The Catefica specimen is ∼3.1 mm long and 1.7 mm in maximum width but is broken proximally, and the complete length is not known. Both the Puddledock and the Catefica seeds are bisymmetrical and obovoid (Figs. 72–74, 78, 82). The Puddledock specimen is narrower than the Catefica specimen. It tapers proximally into a pointed base and is truncate near the apex with a central projecting micropylar area (Figs. 72, 73, 82, 83). In transverse section, the seeds are four-angled for most of their length (Figs. 87–92). Close to the base, four additional low ridges give the seed an octangular transverse section (Fig. 93, 94).
The X-ray microtomography of the holotype shows that the seed is composed of a thin integument enclosed by a sclerenchymatous seed envelope (Figs. 82–94). The integument is extended apically into a long, narrow micropylar tube, ∼0.36 mm long and 0.07 mm wide (Figs. 82, 83). In the apical region, the micropylar tube consists of a single layer of thin-walled cells, the inner epidermis, that forms a circular ring around the open micropylar tube (Figs. 75, 83, 84). Further down, an additional cell layer, the outer epidermis, is formed (Figs. 83, 85). Toward the middle of the micropylar tube, the inner epidermal cells expand and radiate to the center of the micropylar canal, closing the canal completely (Figs. 82, 83, 87). In the closure area, the central cells become indistinct, and at this level the micropylar tube is strongly adpressed to the seed envelope so that the two tissues are difficult to distinguish from each other (Figs. 82, 83, 87). At the base of the micropylar tube, the integument and seed envelope are again distinct from each other as in the apex. Also at the base of the micropylar tube, the cells of the integument become larger (Fig. 83), and the micropylar canal is at this level distinguished as a narrow, star-shaped opening (Fig. 87). The different tissues of the micropylar region are illustrated diagrammatically (Figs. 95–98).
The integument is surrounded by a robust envelope that is open only at the apex (Figs. 72–76, 78, 79, 82–94). Integument and seed envelope are free from each other except at the base where the integument is broadly attached (Fig. 82). The integument encloses the nucellus (Figs. 82, 83, 88–92). The nucellus membrane is poorly developed. It is fused to the integument except apically where it is free (Fig. 82). Tissues internal to the integument are generally crushed and not easy to interpret. A megaspore membrane was not observed.
The seed envelope has a distinct outer and inner epidermis. The inner epidermis consists of short, thin-walled cells that have a smooth surface toward the inside of the seed. It is distinct in the micropylar region where it forms a ring of small, nonpapillate and equiaxial cells (Fig. 84). The outer seed surface is almost smooth (Figs. 72, 73, 78, 82) with a thick cutinized epidermis of small, slightly transversely elongate cells and with scattered swellings (Figs. 77, 80, 81). These swellings are most abundant in the upper part of the seed. They are either cushion-shaped or more elongated. The elongated structures are always broken, and the rounded structures are often broken. The bases appear to be formed from several cells. These elongated or cushion-shaped structures are probably short trichomes or papillae and are similar to epidermal papillae or trichome bases recorded for Bennettitales (35; 79; 53). Just below the micropylar projection, the outer epidermis has a ring of regularly spaced hollows in the cuticle of the seed envelope (Figs. 72–74, 76) that may be damaged stomata or burst secretory cells.
The seed envelope consists of two layers. The inner layer of the seed envelope is continuous around the integument and also surrounds the micropylar tube. It consists of transversely aligned fiber cells. Near the apex, the fiber layer projects into the corners of seed envelope to form four, narrow, wing-like crests that extend almost to the surface of the envelope (Figs. 87–92). The outer layer consists of one layer of thick-walled and strongly pitted sclerenchyma cells that are arranged in a radiating pattern. Subapically, these cells are strongly expanded and make up most of the wall of the seed envelope (Figs. 82, 83, 87). Toward the outside, the outer layer of the seed envelope comprises a thin layer of smaller, thin-walled cells that are covered by the outer epidermis (Figs. 83, 86). Four vascular bundles extend immediately below this layer from the base to apex (Figs. 92, 94) and are seen as four longitudinal ridges on the seed surface (Figs. 72–74). Four additional vascular bundles are present in the outer layer at the base of the seed. These bundles occur between the four major bundles and extend for ∼0.5 mm and give the base of the seed an octangular shape in transverse section (Figs. 93, 94).
Recognition of new genera and species
The four new genera of fossil seeds described here are linked by the common organizational ground plan of having the integument enclosed by an angular, sclerenchymatous seed envelope (Table 1). Buarcospermum, Lobospermum, and Rugonella are further linked by having fiber cells arranged in a distinct chevron pattern in the inner part of the seed envelope. In Lignierispermum, the inner fibers are straight, transversely aligned and do not form a chevron pattern or arches. Buarcospermum and Lobospermum both have radially symmetrical four-angled seeds. Seeds of Rugonella are bilaterally symmetrical (single plane of symmetry) and three-angled, and Lignierispermum is four-angled but slightly flattened and bisymmetrical (two planes of symmetry). In Lobospermum and Rugonella, the fibers of the inner sclerenchyma layer form only a few broad arches over the entire width of the seed, while in Buarcospermum the arches are many, short and closely spaced. Seeds of Buarcospermum are clearly distinguished from those of both Lobospermum and Rugonella by division of the seed envelope into an inner winged part and an outer four-valved part.
| Species | Locality | Size (L × W) (mm) | Envelope shape in cross section | Outer surface of envelope | Outer sclerenchyma cells of seed envelope | Radially extended cells in outer layer of seed envelope | Fiber arches in inner layer of envelope |
|---|---|---|---|---|---|---|---|
| Buarcospermum tetragonium | Buarcos, Torres-Vedras, Catefica, Puddledock | 1.8–2.2 × 1.4–1.5 | 4-angled, radially symmetrical | Smooth | Isodiametric, irregular shape | Present | Many short, closely spaced |
| Lobospermum stampanonii | Puddledock | 3.8 × 1.7 | 4-angled, radially symmetrical | Smooth to slightly rugulate | Longitudinally elongate, equiaxial in TS | Absent | Few broad |
| Lobospermum glabrum | Famalicão, Catefica | 2.3–4.8 × 1.5–2 | 4-angled, radially symmetrical | Smooth to slightly rugulate | Longitudinally elongate, cells toward inside taller in TS | Absent | Few broad |
| Lobospermum rugosum | Puddledock, Catefica | 3.7–4.1 × 1.15–1.6 | 4-angled, radially symmetrical | Rugulate | Longitudinally elongate, cells toward outside taller in TS | Absent | Few broad |
| Rugonella trigonospermum | Puddledock | 2.5 × 2.3 | 3-angled, bilaterally symmetrical | Rugulate | Longitudinally elongate, cells toward outside taller in TS | Absent | Few broad |
| Lignierispermum maroneae | Puddledock, Catefica | 2.7–>3.1 × 1.3–1.7 | 4-angled, bisymmetrical | Smooth | Longitudinally elongate, equiaxial in TS | Present | Fibers not arched |
- a Note: TS = transverse section
Buarcospermum and Lignierispermum both have four apical wing-like crests that project from the fibrous inner layer of the seed envelope. They also both have radiating sclerenchyma cells toward the outside of the seed envelope, but in Lignierispermum these cells are strongly extended near the apex and form most of the wall at this level of the seed envelope. Lignierispermum and Buarcospermum differ from the two other genera in having much longer micropylar tubes. Closure of the micropyle in all genera is mainly by the enlarged cells of the inner epidermis of the integument that extend radially into the center of the micropylar canal (Table 1).
The different species of Lobospermum are distinguished by differences in size, shape and cellular structure as well as ornamentation of the envelope (Table 1). Seeds of L. rugosum have a rugulate outer surface and are generally more slender than those of L. stampanonii and L. glabrum. Lobospermum rugosum is similar to L. glabrum in having a median ridge on the lobes that is indistinct and extends for the full length of the seeds. The seed envelope is generally thinner than in L. stampanonii.
DISCUSSION
Phylogenetic position of Buarcospermum, Lignierispermum, Lobospermum, and Rugonella
Seeds that are organized in the same way as those of Buarcospermum, Lignierispermum, Lobospermum, and Rugonella are known only for members of two extinct orders of seed plants, Bennettitales and Erdtmanithecales, as well as for extinct and extant Gnetales. The same organization is also seen in several isolated seeds recognized in Early Cretaceous mesofossil floras from Denmark (Bornholm), Portugal, and eastern North America that remain to be described in detail (E. M. Friis, K. R. Pedersen and P. R. Crane, work in progress). These isolated mesofossils also include the small four-angled seeds (“square seeds”) discussed recently by 24 and other similar forms.
The seeds of these various groups of plants differ in certain structural details, but they are all linked by strong similarities in their unusual organization and anatomy. All have a thin membranous integument extended into a long micropylar tube, and the integument is tightly enclosed by one or two seed envelopes, which are open only apically where there is access to the micropylar tube. Most of these fossil seeds are three- or four-angled in cross section, but more rarely there are also two-angled forms, as well as five- and six-angled forms in some Bennettitales (72). A further consistent feature of the seeds in the groups listed is that the apical part of the micropyle consists of only a single layer of integument cells that form a tube around the open micropylar canal. At the apex, the micropylar canal is hollow and circular in cross section, but below it is closed by a distinct closure mechanism involving either expansion of the integument into the micropylar canal or a mucilaginous secretion. Details of the cellular closure mechanism of the micropylar canal differ among the different seed types, but the closure is mainly by the cells of the inner epidermis of the integument that extend radially into the center of the micropylar canal. The nucellus is free from the integument apically, but fused to the integument farther down. The nucellus is often somewhat crushed, and none of the seeds exhibit a distinct pollen chamber. A megaspore membrane has currently not been observed in any of the studied seeds. It was probably either poorly developed or lacking as in extant Gnetales.
A phylogenetic analysis that included the small “square seeds” and Erdtmanithecales in the seed plant matrix of 38 placed both taxa in a clade that also includes Bennettitales and Gnetales (24). Buarcospermum, Lignierispermum, Lobospermum, and Rugonella, as well as the other isolated seeds considered here, have the same combination of characters as the “square seeds” of 24 and are resolved as part of the same group. In these analyses this clade, referred to as the Bennettitales-Erdtmanithecales-Gnetales (BEG) group, is united by the common ovule/seed organization outlined above, as well as by paracytic stomata and tectate pollen grains with a granular infratectal layer (24). While this hypothesis needs to be tested by a new generation of more rigorous phylogenetic analyses of living and fossil seed plants, the evidence presented here provides further support for this grouping by further documenting clear structural similarities between the seeds of Buarcospermum, Lignierispermum, Lobospermum, and Rugonella, and those of Bennettitales, Erdtmanithecales, and Gnetales.
Buarcospermum, Lignierispermum, Lobospermum, Rugonella, and other isolated seeds
Currently, more than 20 different seed types with the same common organizational ground plan seen in Buarcospermum, Lignierispermum, Lobospermum, and Rugonella have been recognized among Early Cretaceous mesofossils (Figs. 99–121). Some have been assigned to Gnetales (species of Ephedra and Ephedrispermum, 65; discussed later) or to Erdtmanithecales (species of Erdtmanispermum, 52, 46; discussed later), but most are currently unassigned within the BEG group and remain to be formally named. Especially common and widespread are small four-angled seeds (“square seeds”) very similar to those described and illustrated recently (24). Seeds of this general kind occur in most of the Early Cretaceous floras that we have sampled in Portugal and eastern North America, which range in age from late Barremian/early Aptian to early/middle Albian. They all exhibit the same basic morphology with four distinct angles and a surface ornamentation of irregular transverse ridges. However, they also vary considerably in certain details. Some have pointed structures that extend upward from the apical corners of the seeds and superficially resemble tepals of epigynous angiosperm flowers (Figs. 109, 110, 122–124, 126, 127), while others apparently lack these structures (Fig. 125). There is also variation in structural details of the seed envelope. Some have radially extended cells in the apical region of the seeds. The magnitude of variation indicates that these small “square seeds” comprise several different species, but they perhaps should also be separated into more than one genus. Further detailed studies of specimens from different localities of different ages, are needed before the limits of genera and species become clear and before these seeds can be formally named.
Reconstructions of selected isolated seeds from the Early Cretaceous of Denmark, Portugal and eastern North America. All have the same ground plan with a thin membranous integument extended into a long micropylar tube and surrounded by a partly sclerenchymatous two-, three- or four-angled seed envelope: note variation in size, shape, and surface ornamentation. Schematic line drawings. 99, 100. Lateral and apical view of Raunsgaardispermum lusitanicum, Juncal locality, Portugal. 101, 102. Lateral and apical view of Erdtmanispermum balticum, C. Nielsen locality, Bornholm, Denmark. 103, 104. Lateral and apical view of Buarcospermum tetragonium, Buarcos locality, Portugal. 105, 106. Lateral and apical view of Ephedra drewriensis, Drewry's Bluff locality, Virginia, USA 107, 108. Lateral and apical view of Ephedrispermum lusitanicum, Torres Vedras locality, Portugal. 109, 110. Lateral and apical view of “square seeds,” Torres Vedras locality, Portugal. 111–113. Lateral and apical view of undescribed triangular seed, Puddledock locality, Virginia, USA 114, 115. Lateral and apical view of Lignierispermum maroneae, Puddledock locality, Virginia, USA 116, 117. Lateral and apical view of Rugonella trigonospermum, Puddledock locality, Virginia, USA 118, 119. Lateral and apical view of Lobospermum stampanonii, Puddledock locality, Virginia, USA 120, 121. Lateral and apical view of Lobospermum glabrum, Famalicão locality, Portugal. See text for references.
Selection of “square seeds” from the Early Cretaceous of Portugal and eastern North America. SEM images; scale bar for all, shown in Fig. 122, = 1 mm. Figs. 122, 123. S154562, sample Torres Vedras 43. Figs. 124, 126. PP53366, sample Puddledock 082. Fig. 125. S154564, sample Torres Vedras 43. Fig. 127. PP53364, sample Puddledock 082. 122, 123. Lateral and apical view of seed, Torres Vedras locality, Portugal: note long apical projection and central pointed micropylar area. 124, 126. Lateral and apical view of seed, Puddledock locality, Virginia, USA: note long apical, straight projections and central, pointed micropylar area. 125. Lateral view of seed without apical projections, Torres Vedras locality, Portugal. 127. Lateral view of seed, Puddledock locality, Virginia, USA: note recurved apical projections and strongly rugulate surface.
The “square seeds” are similar to those of Lobospermum and Rugonella in having a rugulate surface pattern of irregular, transverse ridges. In Lobospermum, these ridges are only pronounced in L. rugosum. They are faintly developed in L. stampanonii and very faint in L. glabrum. Lobospermum stampanonii, Rugonella, and the “square seeds” also have a similar closure of the micropylar canal by radially expanded cells of the integument that block the micropylar canal.
Lobospermum and Rugonella differ from the “square seeds” in having arched fibers forming a chevron pattern in the inner sclerenchyma of the seed envelope. Rugonella is also distinguished by its bilaterally symmetrical, triangular shape. Seeds of Buarcospermum are distinguished from the “square seeds” in having a smooth outer surface of the seed envelope and arched fiber cells. In the anatomy of the seed wall, the “square seeds” are more similar to Lignierispermum in having transversely aligned inner fibers. Otherwise, the “square seeds” are distinguished from all four genera by their tepal-like apical extensions (Figs. 122–124, 126, 127).
Also part of this same complex of isolated gymnosperm seeds is Raunsgaardispermum lusitanicum Mendes, Pais et Friis (Figs. 99, 100), recently described from the earliest Cretaceous of Portugal (47). Raunsgaardispermum Mendes, Pais et Friis is clearly similar to Buarcospermum, Lignierispermum, Lobospermum, and Rugonella in its overall organization, with a thin inner integument extended into a long micropylar tube surrounded by a sclerenchymatous envelope. However, Raunsgaardispermum differs from the other seeds in having an envelope with two valves, rather than three or four, and in the presence of narrow longitudinal ribs with distinct bifurcations. Raunsgaardispermum is distinguished from Buarcospermum, Lignierispermum, Lobospermum, and Rugonella, but linked to Ephedra (Gnetales), by the presence of papillae on the inner surface of the seed envelope surrounding the micropylar tube. However, it is distinguished from Ephedra by its nonephedroid pollen. The pollen grains of Raunsgaardispermum are monocolpate and psilate-punctate; more similar to those of Bennettitales (47). Unfortunately, the pollen grains of Buarcospermum, Lignierispermum, Lobospermum, Rugonella, and the “square seeds” are not yet known.
Buarcospermum, Lignierispermum, Lobospermum, Rugonella, and seeds of Gnetales
Extant Gnetales include three genera, Ephedra L., Welwitschia Hook. f., and Gnetum L., which are relicts of a formerly much more diverse and widely distributed group (e.g., 65). In the fossil record, Gnetales are particularly well represented and diverse in Early Cretaceous floras. Until recently, the group was mainly represented by distinctive polyplicate Ephedra-type pollen (8), but over the past few years the macro- and mesofossil record of Gnetales has expanded considerably. Especially important are compressions/impressions of Ephedra-like shoots and branching systems with attached leaves and reproductive structures from the Early Cretaceous Yixian Formation. These include species assigned to Liaoxia Cao et S. Q. Wu (e.g., 66), Ephedrites Göppert et Berendt (e.g., 73) or Ephedra (e.g., 83). Another probable gnetalean fossil from the Yixian Formation is Gurvanella dictyoptera Krassilov. Gurvanella Krassilov was first described from the Early Cretaceous of Mongolia based on isolated winged seeds (40), but was later recognized also in the Yixian Formation where it is known from branching systems bearing both leaves and seeds (first described as Chaoyangia 18; 81; 73). Details of the ovulate structures are unclear and are preserved only as impressions/compressions. However, the branching systems on which the seeds are borne are similar to those of other ephedroid plants from the same strata.
Compression/impression ephedroid fossils are also known from the Early Cretaceous of the Lake Baikal area (Eoantha zherikhinii 41), the Potomac Group sequence (Drewria potomacensis 9) and from the Early Cretaceous Crato Formation of Brazil (48). The Crato Formation has also yielded macrofossils related to Welwitschia (64; 10; 48). Eoantha Krassilov is especially significant because the seeds contain ephedroid pollen in the micropyle. Drewria Crane et Upchurch is important because it is a small plant with opposite and decussate laminar leaves that have venation similar to that in cotyledons of Welwitschia.
All extant genera of Gnetales have seeds consisting of a nucellus enclosed by a thin, membranous integument and one (Ephedra, Welwitschia) or two (Gnetum) additional seed envelopes (45). The nucellus is fused to the integument for part of its length, but the integument is free from the seed envelope except at the base where it is broadly attached. The apical cells of the nucellus disintegrate to form an indistinct pollen chamber (45), and a megaspore membrane is thin (Ephedra, Welwitschia) or lacking (Gnetum) (5) At the apex, the integument is elongated into a long, narrow micropylar tube that projects well beyond the seed envelope. The apex of the micropylar tube is irregular and lobed to various degrees. It is strongly lobed in Gnetum (e.g., 74), less so in Ephedra (e.g., 82), and modified into a prominent funnel in pollen-producing “flowers” of Welwitschia. In Gnetum, at the time of pollination, the outer cells of the micropylar tube divide to form a distinct umbrella-like flange (1; 45). After pollen grains have entered the ovule, the micropylar canal is closed, either by a hardened mucilaginous secretion (Ephedra, 76) or by periclinal divisions and radial extensions of the cells in the middle and proximal part of the micropylar tube (Gnetum and Welwitschia, 1). Toward the base, the central cells of the closure tissue in Welwitschia and Gnetum degenerate to form an irregular central cavity (1).
In extant Ephedra, the single seed envelope is usually bilaterally symmetrical or triangular. More rarely, it is four-angled. In Early Cretaceous Ephedra, the seed envelope is usually four-angled (65). Internally, the envelope is distinctly sclerenchymatous, although sometimes fleshy externally. The outer surface is usually smooth, but in some species such as E. rhytidosperma (e.g., 82), it is ornamented by irregular transverse ridges. The inner surface of the seed envelope is smooth except around the micropylar tube where it is distinctly papillate.
In seeds of Gnetum, the inner (first) seed envelope is usually free from the integument except at the base where it is broadly attached. Sometimes the two layers are fused for part of their length (74). The inner seed envelope has an inner sclerenchymatous layer of longitudinally elongate fiber cells, followed by a layer of sclerenchyma cells that are elongated radially and an outermost layer of thin-walled, more or less isodiametric cells (56). The middle layer of radiating cells is particularly well developed in the subapical region (Fig. 128) and is sometimes missing farther down (e.g., Gnetum montanum Markgraf described by 62). The outer (second) seed envelope is fleshy, with abundant lactifers as well as scattered sclereids and fibers (62).
Schematic line drawings comparing seeds of Gnetales and Bennettitales. Fig. 128. Gnetum gnemon L; line drawing adapted from fig. 114 in 45 and fig. 1 in 1. Figs. 129–131. Cycadeoidea morierei; line drawings adapted from fig. 28 in 43 and new observations. Yellow = integument; dark green = inner sclerenchyma of seed envelope, light green = outer sclerenchyma of seed envelope. 128. Longitudinal section (LS) of Gnetum gnemon showing nucellus, integument, and seed envelope with radiating cells in the subapical region and epidermis; outermost seed envelope not shown. 129. LS of Cycadeoidea morierei showing nucellus, integument, and seed envelope with radiating cells in the subapical region. 130. Transverse section (TS) of seed near base of micropyle showing the different layers of the seed envelope: inner fibrous layer with wing-like crests, middle layer of thin-walled cells, outer layer of radiating cells, and epidermis. 131. TS of ovulate structure near the surface showing apical part of four- or five-angled seeds at various levels and tips of micropyles with open micropylar canal; adapted from fig. 6 in 43. Compare with Buarcospermum tetragonium (Figs. 24–28) and Lignierispermum maroneae (Figs. 95–98).
Seeds of Welwitschia differ from those of Ephedra and Gnetum (and other taxa in the BEG group) in lacking the robust sclerenchyma layer in the seed envelope. Instead, the seed envelope forms a pronounced flattened, membranous, papery wing (45). The fibers of the wing are arranged in a distinct chevron-like pattern reminiscent of that in Buarcospermum, Lobospermum, and Rugonella (45). This feature is not seen in the seed envelope of Ephedra and Gnetum.
Recent investigations of mesofossil floras from the Early Cretaceous have recognized small fossil seeds very similar to those of extant Ephedra. In Ephedra portugallica Rydin, Pedersen, Crane et Friis from the Early Cretaceous (late Aptian or early Albian) of Buarcos, Portugal, and Ephedra drewriensis Rydin, Pedersen, Crane et Friis (Figs. 107, 108) from the Early Cretaceous (early Aptian) of Drewry's Bluff, Virginia, USA, the seed envelope is four-angled (65). As in some species of extant Ephedra (e.g., E. rhytidosperma), the surface of the sclerenchymatous tissue in the outer seed envelope is sometimes ornamented by irregular, transverse ridges. In both species, distinctive polyplicate pollen grains, very similar to those of extant Ephedra, occur in the micropyles. Ephedra drewriensis occurs at the same locality as Drewria potomacensis, although at a slightly different stratigraphic level.
Dispersed fossil seeds assigned to Ephedrispermum lusitanicum (Figs. 107, 108) from the Early Cretaceous (late Aptian or early Albian) of Buarcos, Portugal, differ from those assigned to Ephedra in the absence of papillae on the inner surface of the envelope and the crosswise pattern of sclerenchyma cells that comprise the hard inner layer of the envelope. However, these seeds also have distinctive Ephedra-like pollen in the micropyle (65).
In addition to the three species of fossil ephedroid seed already formally described, there are also a variety of other seeds from the Early Cretaceous of eastern North America and Portugal that are probably related to Ephedra (e.g., Fig. 7 in 65). These are all of similar construction to the fossil seeds of Ephedra and Ephedrispermum with a more or less angular sclerenchymatous envelope. At least one of these seeds has ephedroid pollen in the micropyle.
Buarcospermum, Lignierispermum, Lobospermum, and Rugonella are similar to seeds of Gnetales in their general organization (see above), but they differ in structural details. Lobospermum rugosum and Rugonella are particularly similar to the seeds of some Ephedra species (e.g., E. rhytidosperma) in having a rugulate surface pattern of irregular transverse ridges. This configuration is also reminiscent of that in seeds of Eoantha zherikhinii (41), which contain ephedroid pollen.
Seeds of Ephedra are similar to those of Buarcospermum, Lignierispermum, Lobospermum, and Rugonella in having four-angled and three-angled forms in different taxa. However, the seeds of all four fossil genera are clearly distinguished from those of Ephedra in having cellular closure of the micropylar canal. This contrasts with the hardened mucilaginous secretion that closes the micropyle of Ephedra. In details of the micropylar closure, the new fossil genera are more similar to both Welwitschia and Gnetum. The four new genera are further distinguished from Ephedra by the absence of a papillate lining on the seed envelope surrounding the micropylar tube. Seeds of Ephedra (and Gnetum) also lack the arched chevron arrangement of fibers seen in Buarcospermum, Lobospermum, Rugonella, and extant Welwitschia.
The outer fleshy (second) seed envelope, seen in seeds of Gnetum, has not so far been observed for any of the isolated seeds. This could perhaps be due to the lower fossilization potential of parenchymatous tissue, but in the absence of any remains of an outer tissue we assume that the fossils had only a single seed envelope. Gnetum is also distinguished from other members of the BEG group by the unusual umbrella-like flange formed by proliferation of the tissues of the micropyle (1; 45). The development of such a structure may only be possible given the protection afforded by the outer seed envelope. In all the seeds described here, the tip of the micropyle is missing, but there is no indication of such a flange in any of the fossil seeds.
The seed of Lignierispermum shares several characters with seeds of Gnetum. Both have cellular closure of the micropyle, and both have a seed envelope with an inner fibrous layer, followed by a layer of radiating sclerenchyma cells and an outermost layer of thin-walled cells.
Buarcospermum, Lignierispermum, Lobospermum, Rugonella, and seeds of Erdtmanithecales
Erdtmanithecales are an extinct group of seed plants established by 26 to accommodate fossil plants that produced the dispersed pollen grains of Eucommiidites Erdtman. Pollen of Eucommiidites is characterized by a distinct distal colpus that is flanked by two lateral colpi or a subequatorial ring-colpus. The pollen wall is tectate, sometimes finely perforate, and has a granular infratectal layer. Eucommiidites ranges from the Early Jurassic to the Late Cretaceous.
Several different microsporangiate structures with Eucommiidites pollen in situ have been identified from the Cretaceous including Erdtmanitheca texensis 52, Eucommiitheca hirsuta 26, and Bayeritheca hughesii 42. Eucommiidites pollen grains have also been discovered in situ in the micropyles of several different kinds of seeds from the Early Cretaceous. These seeds are all isolated and include Erdtmanispermum balticum Pedersen, Crane et Friis from Bornholm, Denmark (52) (Figs. 101, 102), Erdtmanispermum juncalensis 46, Spermatites pattensis Hughes from southern England (39), and Spermatites patuxensis Brenner from eastern North America (3).
Seeds of Erdtmanithecales assigned to Spermatites, studied as macerations, clearly show an integument with a long, narrow micropylar tube containing Eucommiidites pollen, and a seed envelope surrounding the integument. Seeds assigned to Erdtmanispermum are lignitic and have cuticles as well as other tissues preserved. They consist of a robust seed envelope enclosing a membranous integument that extends apically into a long narrow micropylar tube. The tube extends beyond the seed envelope. The seed envelope is sclerenchymatous, distinctly three-parted, and sometimes splits into three valves. The inner surface of the seed envelope is smooth, including where it surrounds the micropylar tube. The outer surface is smooth or sometimes slightly rugulate with irregular transverse ridges (46).
Seeds of Erdtmanithecales are similar in general structure to those of Buarcospermum, Lignierispermum, Lobospermum, and Rugonella. Like Erdtmanispermum, seeds of these four genera lack the papillate lining around the micropylar tube. However, Erdtmanispermum seeds are distinguished from Buarcospermum, Lignierispermum, and Lobospermum in their constant triangular form and from the triangular seeds of Rugonella by their radial (rather than bilateral) symmetry and lack of wings (Figs. 101, 112). Seeds of Erdtmanithecales are further distinguished from Buarcospermum, Lobospermum, and Rugonella in the lack of long fibrous, arched cells in the inner layer of the seed envelope. Due to the lignitic preservation, X-ray studies of Erdtmanispermum have so far failed to provide details of the micropylar closure.
Buarcospermum, Lignierispermum, Lobospermum, Rugonella, and seeds of Bennettitales
Bennettitales are an important but entirely extinct group of Mesozoic plants that are diverse and widespread from the mid-Triassic through the Jurassic and into the Late Cretaceous. Bennettitales are known mostly from their pinnate, cycad-like leaves, but there are also diverse flower-like reproductive structures preserved as impressions, compressions, or permineralizations. Seeds are typically small and borne in dense aggregations interspersed with interseminal scales on a domed or conical receptacle (e.g., 43; 80; 35). In some Bennettitales, the reproductive structures are bisexual with microsporangiate organs surrounding the ovulate structures. Other reproductive structures are unisexual with microsporangiate organs and seeds borne separately (35; 79). Ovulate reproductive structures are often surrounded by large tepal-like structures that result in a variety of different “floral” morphologies (79). The structure of the pollen-producing organs is less clear, but they often appear fused to the tepal-like structures to form a shallow cup (35; 79).
The organization of the seeds in Bennettitales has been a matter of much discussion. Seeds preserved as compressions such as Vardekloeftia sulcata Harris from the Late Triassic of Greenland (34; 51) often show well-preserved cuticles from the various tissues. In Vardekloeftia Harris, there is an inner indistinct layer that is partly fused to and surrounded by another, well-cutinized layer that is extended apically into a long tube. This tube contains monocolpate pollen grains. The tube projects beyond an additional outer seed layer in a similar way to the micropylar tube of Gnetales, Erdtmanithecales, and the fossil seeds described here. The additional outer layer consists of sclerenchymatous tissue covered by an outer cuticle. The corresponding inner cuticle of this outer layer is free from the cuticle of the integument except at the base. The nature of the various cuticles and the remains of the tissues that they enclose in Vardekloeftia strongly suggest that the innermost, thin layer is nucellus, the layer that forms the tube is integument, and the additional outer sclerenchymatous layer is equivalent to the seed envelope of Gnetales, Erdtmanithecales, and the fossil seeds described here. The elongated tube containing pollen grains is therefore the micropylar tube rather than a nucellar plug as suggested by 63. It is interesting that this structure is so clearly seen in Vardekloeftia, which is one of the earliest Bennettitales and is distinguished from Jurassic and Cretaceous taxa by the much smaller number of seeds per ovulate head (about 20 in contrast to hundreds in younger taxa).
Further support for the interpretation that the ovules of Bennettitales share a common ground plan with seeds of Buarcospermum, Lignierispermum, Lobospermum, Raunsgaardispermum, Rugonella, Gnetales, and Erdtmanithecales comes from other bennettitalean ovulate structures preserved as compressions (e.g., Bennettites crossospermus Harris, Williamsonia himas Harris) and permineralizations (24). In Bennettites crossospermus 34, fig. 14H, pl. 11), described and illustrated a “micropylar plate” through which the micropylar tube projects. We think that this “micropylar plate” probably corresponds to the envelope seen in Cycadeoidea morierei as well as the seeds described here. A similar situation is illustrated in a specimen of Williamsonia himas from the Middle Jurassic of Yorkshire, UK (35, fig. 60K).
The permineralized reproductive structures of Bennettitales, that are usually described as species of Cycadeoidea Buckland or Williamsonia Carruthers, are in many respects more informative than the compressions fossils. However, because of the dense packing of the various organs in the ovulate structures, organizational details for the various tissues are not always straightforward to interpret.
Among the permineralized fossils is a well-preserved bennettitalean ovulate structure from Normandy variously referred to as Williamsonia morierei Saporta et Marion, Bennettites morierei (Saporta et Marion) Lignier, or Cycadeoidea morierei. The fossil was discovered at the Vaches-Noires coastal exposures near Villiers-sur-Mer and was described in detail by 43. The exposure at Vaches-Noires consists mainly of Jurassic sediments, but above there are soft Early Cretaceous sediments of Albian age. According to 60; personal communication, 2008), the fossil corresponds in preservation to other fossils washed out of the Early Cretaceous sediments. In general, the morphology and organization the specimen corresponds closely to other Early Cretaceous ovulate structures assigned to Cycadeoidea, such as C. albiana (Stopes) Wieland (72) and C. gibsoniana (Carruthers) Seward (67). Here we refer to the Normandy specimen as C. morierei.
Cycadeoidea morierei is a small ovulate cone ∼55 mm long and 35 mm in diameter (43). Numerous small ovules and interseminal scales are densely arranged on a conical receptacle. The ovules are borne on long stalks and are surrounded by interseminal scales that are flattened proximally, but expanded distally. The seeds are ∼6–7 mm long and 2.5–3 mm in diameter, elongate elliptical in lateral view (Figs. 132–134, 136, 137) and four- to five-angled in cross section (Figs. 130, 131, 134). The ovulate structure has been sectioned, and some of the sections as well as Lignier's original illustrations are still available for study. In addition, several fragments of the cone that were left after sectioning can still be examined. Among these fragments are isolated seeds and interseminal scales, which permit their morphology and structure to be examined separately. Both seeds and interseminal scales appear to separate easily indicating that they were discreet units in the reproductive structure. This is also supported by the presence of well-preserved cutinized epidermis with stomata on both organ types (Fig. 138). The separation may also be facilitated by the relatively weak mineralization of Cycadeoidea morierei. In other permineralized bennettitalean reproductive structures, the precise limits of seeds and interseminal scales are sometimes difficult to determine.
Cycadeoidea morierei from the Early Cretaceous (Albian) of Vaches-Noires near Villiers-sur-Mer, France; fragments from holotype. Figs. 132–134. Tomographic reconstructions of seed. Figs. 135–138. SEM images. Scale bars: Figs. 132–134, 136, 137 = 2 mm; Fig. 135 = 1 mm; Fig. 138 = 0.5 mm. 132–134. Seed showing the inner, fibrous sclerenchyma layer with its four subapical crests. 135. Broken seed showing integument, part of micropylar tube, and seed envelope consisting of layer of inner, fibrous cells, layer of thin-walled cells, and a subapical, outer layer of radially extended cells. 136, 137. Seed with part of seed envelope missing showing inner, fibrous layer, apical crests, and remains of radiating cells and epidermis of seed envelope. 138. Surface of outer epidermis showing elongate cells and scattered paracytic stomata.
The seeds of C. morierei consist of a distinct outer sclerenchymatous tissue with several different zones. This outer sclerenchymatous tissue surrounds a thin inner tissue, which is extended into a long, narrow tube (Figs. 129, 135). Sections show that this tube is hollow distally similar to the micropylar tube seen in Gnetales and other seeds of the BEG group. In the middle and proximally, the micropylar tube is closed by several layers of small equiaxial cells. At the base of the micropyle, the central part shows an irregularly defined cavity that is very similar to the central, apparently lysigenous cavity of Gnetum (1). The most straightforward interpretation is that the thin inner layer that forms the tube is the integument. In this case, the mechanism of closure is similar to that in Gnetum, as originally pointed out by 1, but without the umbrella-like closing structure. Internal to the integument and adhering closely to it is another tissue that we interpret as the nucellus.
The outer sclerenchymatous envelope of Cycadeoidea morierei enclosing the integument has three different layers. The inner layer is thin over most of the seed and consists of elongated and longitudinally aligned fibers (couche fibreuse of 43). Subapically, this fibrous layer is thicker and extended radially toward the corners of the seeds to form four or five wing-like crests that project through the middle zone of the seed envelope in a star-shaped pattern (Figs. 130, 131, 34). The wing-like crests become less prominent below the micropylar region and disappear at about the middle of the seed (Figs. 132–134, 136, 137). The middle layer consists of thin-walled and almost equiaxial and thin-walled cells (tissu charnu of 43). Over most of the seed, this middle zone of C. morierei is one to two cell layers thick, but it thickens toward the micropylar region and fills out the space between the wing-like crests (Figs. 129–131, 135, 136). Subapically, the seed envelope of C. morierei has an outer layer of strongly enlarged and radially extended cells (l'assise plissée, ou rayonnante of 43) (Figs. 129–131, 135). The outer epidermis of the seed envelope is well developed (Figs. 135, 137, 138) with distinct paracytic stomata (Fig. 138).
We interpret the outer envelope with its three layers as corresponding to the seed envelope of Ephedra, the inner seed envelope of Gnetum, and the seed envelope of Buarcospermum, Ephedrispermum, Erdtmanispermum, Lignierispermum, Lobospermum, Raunsgaardispermum, and Rugonella. It is particularly similar to the inner seed envelope of Gnetum. Both have subapically enlarged and radiating sclerenchyma cells. This similarity was also pointed out by 1. A similar subapical zone of strongly extended radiating cells is also found in Lignierispermum (Figs. 82, 83, 87).
Based on the similarities between Cycadeoidea morierei and seed architecture of extant and fossil Gnetales, Erdtmanithecales, and the four genera of dispersed seeds described in this paper, we think it very likely that when suitably preserved, and when studied in sufficient detail, the seeds of all previously described Bennettitales will be shown to have a nucellus surrounded by two coverings: an integument, fused to a greater or less extent to the nucellus, and an outer seed envelope. This interpretation, which has been raised in the past (e.g., 67; 1), was rejected in the early 20th century (e.g., 44; 72). Since then, it has rarely been discussed. We believe that the similarities between the seeds described here and those of Cycadeoidea morierei warrant its revival.
In addition to similarities in the anatomy of the seed envelopes discussed here, the similarity between the elongated micropylar tube of fossil seeds such as Buarcospermum and Lignierispermum and the so-called nucellar plug described in permineralized bennettitalean material (e.g., 72; 63; 71) is especially important. SRXTM and PCXTM images of Buarcospermum and Lignierispermum clearly show a thin integument, which is largely fused to the nucellus and that extends apically into a long, slender micropylar tube (e.g., Figs. 10, 15, 82, 83). At the tip, the tube is open, but below it is solid: the micropylar canal having been closed by growth of the surrounding cells of the integument that comprise the micropylar tube. A similar structure occurs in Cycadeoidea morierei (see above) and also appears to be present in other bennettitalean seeds (see below). Below the level of the micropylar canal in at least some specimens (Fig. 83), there is evidence of a depression in the nucellus that could be interpreted as a pollen chamber.
If the reinterpretation of bennettitalean seed architecture presented here is borne out by future research, then it will require revision of most current interpretations of seeds attributed to genera such as Cycadeoidea and Williamsonia. For example, in C. albianus (Stopes) Wieland tissues referred to as “(1) the inner thin-walled cells; (2) the fibrous layer; (3) the stone layer;” which “form the seed coat proper”, and perhaps the “(4) deliquescent layer, and (5) the tubular cells of the “cupule” (72, p. 413 ) would all be part of the seed envelope. The structure interpreted as a plug of nucellar tissue in the micropyle (72, p. 413 ) would be the micropyle, formed by the integument and occluded by closure tissue. We therefore reject the previous interpretation by one of us (5) that the enlarged tubular cells (“cupule” of 72) derived from the cells of the seed stalk correspond to the envelope (“cupule”) of Vardekloeftia.
Other permineralized Bennettitales have the same structure as Cycadeoidea albianus and can be reinterpreted in the same way. For example, C. gibsoniaus (Carruthers) Seward from the Early Cretaceous of southern England (24). Similarly, in C. maccafferyi 63 and Williamsonia bockii 71, tissues described as sarcotesta and sclerotesta would be part of the seed envelope, while the structure interpreted as a nucellar plug would be the micropyle formed by the integument but occluded by the closure tissue. Given that such a “nucellar plug” has been reported for most permineralized Bennettitales (63), we think it very likely that all Bennettitales have an elongate, slender micropylar tube surrounded by a much more robust outer envelope. The micropylar tube will usually appear solid in both longitudinal and transverse section as a result of the closure tissue and will at some levels appear to be part of the seed envelope because it is tightly adpressed to it as a result of its expansion further down. However, if the tip of the micropyle is preserved, the micropylar tube should be open (e.g., 71, fig. 3B).
The outer envelope, which surrounds both the nucellus and integument, is conventionally interpreted as the single (only) integument. By the interpretation presented here, this layer (referred to in this paper as “envelope”) is in fact an additional (second) covering around the nucellus.
Among the isolated seeds described here, those of Buarcospermum and Lignierispermum are particularly similar to seeds of Cycadeoidea morierei and other Bennettitales, as well as to seeds of extant Gnetum. In C. morierei, as well as in Buarcospermum and Lignierispermum, the inner, fibrous sclerenchyma layer of the seed envelope expands in the apical part of the seed to form four or five narrow wing-like crests. The ridges are particularly pronounced in the subapical micropylar region and radiate toward the outside of the seed envelope in a star-shaped manner (compare Figs. 12, 87, 130, 131). A further character that unites Buarcospermum and Lignierispermum with C. morierei is the cellular closure of the micropylar canal. In this respect, they are also similar to seeds of Gnetum and Welwitschia, that have a cellular micropylar closure mechanism (45).
Phylogenetic and ecological implications
The isolated seeds of Buarcospermum, Lignierispermum, Lobospermum, and Rugonella correspond in all the major features of their structure and organization to seeds of Bennettitales, Erdtmanithecales, and Gnetales (the BEG group). Together with the “square seeds,” phylogenetic analyses placed them inside the BEG group as sister to Gnetales in a polychotomy with Bennettitales and Erdtmanithecales (24). More detailed phylogenetic analyses will require more information on the parent plants of Buarcospermum, Lignierispermum, Lobospermum, Rugonella, and similar fossils. We have not observed pollen in any of the isolated seeds described here, and there is no information on how the seeds were borne on the plant. There is also no information on other parts of the plants such as bracts, leaves, or axes that could be used for a more detailed evaluation of their relationship within the BEG group.
In modern Gnetales, pollen are sealed inside the integument either by cellular micropylar closure or by secretion. The presence of a similar micropylar closure in the fossil taxa described here suggests that they had similar reproduction and that the apparent absence of pollen inside the seeds is due to the currently limited observations and material. Each of the new taxa described here are represented only by a few specimens, none of the seeds have been macerated, and the resolution of the X-ray images may not be sufficient to recognize in situ pollen. However, another explanation for the lack of pollen inside the seeds could be that pollen germination took place outside the integument as known for some conifers (e.g., 49) and has been suggested for Bennettitales (71).
Notwithstanding the need for additional information, the distinctive architecture of Buarcospermum, Lignierispermum, Lobospermum, and Rugonella is of exceptional phylogenetic interest because of the strong similarities with seeds of Bennettitales, Erdtmanithecales, and Gnetales. Buarcospermum and Lignierispermum in particular share many features with both extinct Cycadeoidea and Gnetum. Buarcospermum, Lobospermum, and Rugonella have chevrons of fibers similar to those seen in the winged seeds of Welwitschia. Similarly, Raunsgaardispermum combines seed characters of Ephedra with pollen characters of Bennettitales.
Previous studies have already identified isolated seeds that can be assigned to Ephedra based on the papillate lining around the micropylar tube and in situ polyplicate Ephedra-type pollen (65). Isolated seeds of Erdtmanithecales have also been identified based on distinctive Eucommiidites type pollen inside the micropyles. Other fossil remains of Gnetales and Erdtmanithecales document that these lineages were well established and diverse in the Early Cretaceous, although they were not a dominant element of the Early Cretaceous vegetation (6; 26). In contrast, the Bennettitales have long been recognized as a prominent part of certain Early Cretaceous ecosystems and are known mainly from abundant leaf fossils (79).
Isolated seeds that can be assigned to the Bennettitales have only been identified from the Late Triassic (Vardekloeftia, 51). It has generally been assumed that the seeds of Bennettitales were perhaps not shed individually from the ovulate structures, and in his extensive studies of the Middle Jurassic floras of Yorkshire in which Bennettitales are abundant, 35 failed to identify any dispersed bennettitalean seeds. The cutinized epidermis and abundant stomata on outer surface of the Cycadeoidea morierei seeds and the interseminal scales may indicate that these organs could potentially survive exposure and dispersal as discrete units. The definitive recognition of dispersed bennettitalean seeds in Mesozoic floras would greatly facilitate comparison with the material described here.
Buarcospermum, Lignierispermum, Lobospermum, and Rugonella co-occur with various angiosperm fossils, including many angiosperm seeds that are comparable to seeds of the BEG group in their small size, and in having two layers around the nucellus. It is also interesting that the outer integument in many, apparently bitegmic, Early Cretaceous angiosperm seeds is sclerenchymatous (31). The full phylogenetic implications of these observations still need to be worked out, but they are important points of similarity. They also raise again the possibility that the structural similarities among angiosperms, Bennettitales, Gnetales, and related groups, perhaps reflect underlying biological similarities that resulted in similar ecological preferences.
