Systematic paleontology
Euarthropoda Lankester, 1904
Decapoda Latreille, 1802
Brachyura Latreille, 1802
Eubrachyura Saint Laurent, 1980
Cretapsaridae Luque, fam. nov.
Included genus: Cretapsara Luque, gen. nov., by monotypy.
Diagnosis: As for type genus and species.
Remarks:
Cretapsara athanata n. gen. et sp. differs from any known crab family in its combination of plesiomorphic and apomorphic characters. Superficially, it shares some resemblance to some marine eubrachyuran families such as Eogeryonidae, from the Upper Cretaceous of Spain [
Eogeryon elegius, Cenomanian (
14)] (
Fig. 5H) and the Lower Cretaceous of Brazil [
Romualdocarcinus salesi, Albian (
15)], and Marocarcinidae, from the Upper Cretaceous of Morocco [
Marocarcinus pasinii, Cenomanian (
16)] (
Fig. 5E).
Cretapsara differs from Eogeryonidae and Marocarcinidae in the presence of a bilobate rostrum and lack of orbital fissures, compared to the bifid and acute rostrum and two well-developed orbital fissures diagnostic of these two families [Marocarcinidae was initially interpreted as having no orbital fissures due to the erroneous interpretation of the eyestalks as part of the orbit (
16) but is here recognized as having two orbital fissures and scored as such in our analysis].
The overall carapace outline, the broad front, long legs, and the lack of orbital fissures that characterize
C. athanata (
Figs. 1 and
2) resemble those of some modern Grapsoidea—a group of highly terrestrial thoracotreme crabs common in subtidal and supratidal settings worldwide. Despite this,
C. athanata differs from grapsoids in important respects. In most grapsoids, there is a conspicuous V-shaped notch between the merus and ischium of the third maxillipeds (mxp3), so that the mandibles are visible through a large, rhomboidal gap when the mxp3 are closed, a feature absent in
Cretapsara (
Figs. 2, C and F, and
3, B, E, and G). Moreover, the podomeres of the walking legs in grapsoids, as in most thoracotreme crabs, are subtriangular to flattened in cross section, whereas those in
Cretapsara are circular in cross section and more similar to those of other heterotremes (
Fig. 3, movie S1, and data file S1). Such similarities between
Cretapsara and some grapsoids are likely convergent adaptations to similar ecologies.
Cretapsara athanata Luque, gen. et sp. nov.
Etymology: The genus name combines Creta (Latin), for chalk as in Cretaceous, and Apsara, a spirit of the clouds and waters; gender is feminine. The specific epithet is based on athanatos (Greek), immortal, referring to its lifelike preservation. The family name is based on the only known genus, erected herein.
Holotype: Specimen LYAM-9 (
Figs. 1 and
2) deposited in the Longyin Amber Museum, Xishan District, Kunming 650228, Yunnan Province, China. Specimen collected by local miners in 2015.
Type locality and horizon: Angbamo site, Tanai Township, Myitkyina District, Kachin Province, Myanmar [~98.8 ± 0.6 Ma; lowermost Cenomanian (
17–
19)].
Diagnosis: The carapace is subhexagonal, nearly as long as it is wide; the orbitofrontal margin is nearly as wide as the body and bears wide, shallow orbits that lack supraorbital fissures or intraorbital spines. The rostrum is bilobate, short, and wide, measuring about half the width of the orbitofrontal margin. The anterolateral margin is one-third as long as the posterolateral margin, and it bears a small outer orbital spine and two short subtriangular anterolateral spines; the posterolateral margin is straight to slightly convex, and it bears at least four small and equidistant tubercles. The posterior margin is wide, nearly straight, rimmed, and apparently ornamented with a row of small tubercles. The cervical and branchiocardiac grooves are well developed, reach the lateral margins of the body, and are expressed ventrally; the dorsal regions are well developed and delimited by grooves. The buccal cavern is wide, covered by a pair of operculiform mxp3 that lack a V-shaped incision in the occlusal margin. The thoracic sternites 1 to 3 are visible ventrally and are separated from a subtrapezoidal sternite 4 by a shallow groove. Sternites 5 and 6 are similar in shape; sternite 5 is the widest of all, and sternites 6 to 8 reduce in size posteriorly. The sterno-abdominal cavity is shallow, and there is no evidence of a linea media traversing the sternites. The pleon has a small subtriangular telson and, based on the stereomicroscope images and the micro-CT scanning, apparently has six free and unfused pleonites, from which at least the first two are fully visible dorsally; there is no evidence of uropods or uropod remains. The eyes are as large as the orbits; the corneal eye is globular, as wide as it is long, and apparently covered by small hexagonal facets; the eyestalk is short and cylindrical. The antennulae and antennae are reduced. The claws or chelipeds are equal in shape and size, and the four pairs of walking legs are well developed, slender, and similar in size and shape; they lack chelate, subchelate, or flattened distal podomeres (e.g., propodi and dactyli); the podomeres are semicircular in cross section. Excurrent openings are well developed, small, and circular.
Description:
Dorsal carapace: The carapace is subhexagonal in outline and about as long as wide (~2 mm) (
Figs. 1D and
2B). The orbitofrontal margin is nearly as wide as the maximum width of the carapace, and each orbit is wide, concave, and shallow, with a sinuous supraorbital margin that lacks supraorbital fissures or spines (
Figs. 2, A to C, and
3), apart from a small, anterolaterally diverging outer orbital spine (
Fig. 3, F and G, thick arrow). The rostrum is short and half the width of the orbitofrontal margin, downturned, apparently with a shallow axial sulcus and raised subparallel lateral margins grading into the inner orbital margins (
Figs. 2B and
3, A and F, and movie S1). The anterolateral and posterolateral carapace margins are distinct and separated by a shallow lateral expression of the cervical groove (
Fig. 3, F and G, dotted line). The anterolateral margin is slightly convex, about one-third the length of the posterolateral margin, and it bears two anterolateral spines that are short and subtriangular (
Figs. 2 and
3F). The edge of the posterolateral margin is straight to slightly convex, and it bears at least four small tubercles that are somewhat equidistant from each other (
Fig. 3F, movie S1, and data file S1). The posterior margin of the carapace is about two-thirds of the maximum carapace width, lacks reentrants, and, as evidenced by the micro-CT scans, appears to be rimmed and bearing a row of small tubercles (movie S1 and data file S1).
The cervical groove becomes faint axially, deepens laterally reaching the anterolateral margin, and is expressed ventrally (
Fig. 3, C, F, and G); the groove is concave posterior to the mesobranchial region and sinuous toward the lateral margin. The branchiocardiac groove is well developed, somewhat deep, flanking the cardiac region and, apparently, the urogastric region mesially and the mesometabranchial region distally (
Fig. 3F).
The epigastric region is wide, lacking spines or tubercles, laterally inflated, and axially sulcate; the lateral lobes reach the frontal region and are nearly continuous posteriorly with the protogastric lobes (
Fig. 3, A and F). The protogastric region is wide, lacking spines or tubercles, laterally tumid, and forming an almost ovate longitudinal lobe flanked by the epigastric region; axially, the protogastric depression separates the protogastric lobes and opens to the narrow anterior mesogastric region. The mesogastric region is relatively small, lacking spines or tubercles, defined anteriorly by two grooves that separate the protogastric lobes and posteriorly by a concave cervical groove. The metagastric region is well developed, inflated, and about three times as wide as long; it is bounded anteriorly by the cervical groove, laterally by grooves that separate it from the epibranchial regions, and posteriorly by a narrow and depressed urogastric region. The urogastric region is considerably shorter and narrower than the metagastric region, about twice as wide as it is long, depressed relative to the metagastric and cardiac regions, and separated from them by faint grooves, delimited laterally by the branchiocardiac groove. The cardiac region is almost subtrapezoidal, lacking spines or tubercles, is wider posteriorly, and is delimited laterally by the branchiocardiac groove. The intestinal region appears to be wide, shorter mesially, and slightly wider laterally, with two lateral swellings parallel to the posterior margin (
Fig. 3, A and F, and movie S1).
The postorbital region is tumid, is separated from the protogastric lobes by a deep and wide groove, and is bounded posteriorly by the cervical groove and laterally by the hepatic region; the postorbital region lacks spines or tubercles. The hepatic region is small, is flat to slightly depressed, and is bounded laterally by the anterolateral margin and posteriorly by the cervical groove. The epibranchial region is wide and is delimited anteriorly by the cervical groove and posteriorly by a shallow groove that separates the epibranchial and mesometabranchial regions; the epibranchial region has a swollen lobe between the anterolateral margin and the metagastric region. The mesobranchial and metabranchial regions are undifferentiated, smooth, and unornamented (
Fig. 3, A and F, and movie S1).
Ventral carapace: The buccal cavern is wide and covered by a pair of operculiform mxp3. The coxae of the mxp3 separate the sternum from the pterygostome. The pterygostome is curved, bulged, finely granulated, with a row of coarser tubercles parallel to the linea brachyura or molting plane (
Figs. 2, C and F, and
3, and movie S1).
The thoracic sternum has fused sternites 1 to 3 that are triangular, about as long as wide, and are visible ventrally; sternite 4 appears to be subtrapezoidal in outline; sternite 5 is the widest and measures approximately two-thirds of the maximum carapace width; sternite 6 is narrower than sternite 5 but nearly as wide as sternite 4; sternites 7 and 8 are small and barely evident in the micro-CT scan. The sterno-abdominal cavity is shallow, and there is no evidence of a linea media traversing the sternites.
The pleon, based on the micro-CT scan reconstruction, appears to comprise six free, unfused pleonites and a small telson. The pleonites are subrectangular in outline. The first pleonite is smallest and visible dorsally, and the second is wider and, apparently, also visible dorsally. The third and fourth pleonites are the widest, with the anterior part of the third pleonite visible dorsally, but the fourth is concealed under the body. The fifth pleonite is slightly narrower than the fourth sternite, whereas the sixth pleonite is considerably narrower than the preceding pleonites. The telson is triangular and about as wide as long. There is no visible evidence of uropods.
Eyes and antennae: The eyes and eyestalk are large, with each eye approximately one-third the maximum carapace width. The corneal eye is globular, as wide as long, measuring about one-fifth the width of the carapace, and seems to be covered with small hexagonal facets; the eyestalk is shorter and cylindrical (
Figs. 2, A to C, and
3, B, E, and G). The antennula and antenna are very small (
Figs. 2, A to C, and
3) and, due to their small size and limitations in the resolution of the micro-CT scans, no details of the flagella can be discerned.
Mouthparts and thoracic appendages: The mxp3 consist of a long slender exopod and an endopod with a long semirectangular ischium, a squarish merus, and slender distal palp (
Figs. 2, A, C, and F, and
3, B, E, and G). The coxae of the mxp3 are small and do not meet axially; the ischiomerus has straight occlusal margins lined with fine setae (
Fig. 2, A, C, and F) and does not form a rhomboid gap with the other mxp3 that reveals the other mouthparts, neither does it bear a crista dentata (
Figs. 2, C and F, and
3, B, E, and G). The palp, which is formed by the propodus, carpus, and merus, is long, thin, and located in an inner-mesial position.
The first of the five pairs of pereopods (P1) constitutes the claws (or chelipeds), which are symmetrical or homochelous (
Figs. 1 and
3). Their mobile fingers (or dactyli) are long and slender, barely curving downward, and parallel to the fixed finger or pollex of the propodus, which is also long and slender and apparently bears six to eight very small teeth on the occlusal margin (
Fig. 2D, movie S1, and data file S1). The other four pairs of thoracic appendages are slender walking legs (P2 to P5), similar in size and shape, with slender, acute dactyli and lacking chelate or subchelate terminations (
Figs. 1 and
3). The podomeres are semicircular in transverse section rather than flattened (
Figs. 1 to
4, movie S1, and data file S1). The claws and the legs have several small, fine setae scattered over their surface (
Fig. 2D and E).
Gills and excurrent openings: The micro-CT scan of the holotype of
C. athanata revealed the presence of at least six pairs of phyllobranchiate gills, bearing apparently up to 20 series of well-developed, flattened branchial lamellae that are perpendicular to the main gill shaft (
Fig. 4, D to F, and movies S2 to S4). A pair of conspicuous, small, and subcircular excurrent openings is evident beneath the antennal sockets adjacent to the epistomial region (
Figs. 2, A, C, and F, and
3, B and G; movie S1; and data file S1).