Ediacaran fossils are abundant in the lower part of the Nama Supergroup (Fig. B), a Late Ediacaran to Early Cambrian foreland basin succession that crops out over much of southern Namibia (Germs ; Grotzinger & Miller ). The newly discovered fossils reported here occur in the Aar Member, a shallow sub-tidal succession of fine siliciclastics and carbonates that is approximately 550 million years old (Hall et al. ). The Aar Member is renowned for the occurrence of abundant specimens of Ediacaran megafossils, including Pteridinium (Pflug ; Grazhdankin & Seilacher ; Elliott et al. ; Meyer et al.,b) and the classic form of Ernietta (Pflug ; Elliott et al. ) preserved within beds of hummocky cross-stratified sandstone, as well as exceptionally preserved specimens of Rangea in gutter casts near the base of the Aar Member (Vickers-Rich et al. ).

The new Ernietta fossil locality herein reported occurs approximately 8 m below the top of the Aar Member. Strata in the uppermost 15 m of the Aar Member consist of nearly equal amounts of carbonate and fine siliciclastic deposits (Figs C, D, A). Carbonates comprise variably dolomitized, dark, lime mudstone with abundant small-scale slump structures and minor amounts of microbial laminite and flat-pebble conglomerate. Fine-grained siliciclastic sediments comprise mainly friable shales and siltstones, with abundant thin event beds of very fine- to fine-grained sub-arkosic sandstone exhibiting hummocky cross-stratification, wave-, and combined flow-ripple marks. Deposition probably occurred on a shallow-marine ramp within the range of both storm waves and the photic zone during a major transgression (Hall et al. ).

The fossils occur in a sandstone lens 8.5 cm thick with a preserved length of approximately one metre (incomplete on both ends) and a width of 25–35 cm (Fig. A–E). The lens is erosional into the underlying shale, with a floor that is flat to very gently convex (Figs E, A, B), walls that are steep to locally nearly vertical (Fig. B), and a fill consisting of event-bedded sandstone with abundant mudstone intraclasts. In contrast with its channel-like base, the top of the fossil lens is smooth and flat. Similar features are typical of Phanerozoic gutter casts formed as a result of sediment bypass (Myrow,b; Myrow & Southard ; Pérez-López ). Comparisons can also be drawn with concentrations of three-dimensional specimens of Rangea near the base of the Aar Member, which also occur in sandstone-filled gutter casts (Vickers-Rich et al. ).

The sediment fill of the gutter cast is bipartite, with the lower half consisting of a 4-cm-thick, hummocky cross-stratified, intraclast-rich, sub-arkosic arenite (see Dott for sandstone classification) that grades from fine- to medium-grained at the base to fine-grained at the top of the unit (Fig. A–D), a succession that implies deposition during the waning stages of a storm. Small, vertically oriented specimens of Ernietta occur sporadically at the base of the gutter (Fig. D), along with a probable specimen of Rangea which appears in section as a horizontally oriented, hollow, conical bulb surrounded by jarosite and its weathering products (Fig. A), similar in morphology and taphonomy to Rangea specimens preserved in gutter casts lower in the Aar Member (see Vickers-Rich et al. ; figs 6.3, 6.4, 7.3, 7.4). The upper 4.5 cm of the fossiliferous gutter cast (Fig. A–C) consists of very fine-grained, mud-rich, sub-arkosic wacke. Grain to grain contacts are sparse, and there are no sedimentary structures except for a few discontinuous wispy laminations, implying transport as a muddy mass flow following the storm. Specimens of Ernietta occur abundantly in the upper part of the gutter cast (Figs C–E, A–C) and comprise the subject of this study.

Ernietta specimens in the upper part of the gutter cast show a wide array of orientations in both a horizontal and vertical sense (Figs D, E, A–C). Considered as a group, specimens are preferentially oriented with their open end directed in a southerly or southeasterly direction relative to their base (mode 180–195° Az, median 165.5° Az; Fig. B), a direction consistent with the N-S orientation of the preserved part of the gutter cast. There is some dispersion in these measurements, including at least two individuals that are slightly inclined upstream to the NW (Figs E, B–C). This dispersion in specimen orientation may reflect turbulence in the flow, or more likely interference between adjacent individuals or the walls of the gutter during transport.

Previous reports have shown a nearly ubiquitous vertical orientation of Ernietta specimens, with their suture line at the base of the organism and their truncated opening directed upward, which most authors have taken to indicate their original life orientation (Jenkins et al. ; Jenkins ; Seilacher ; Seilacher et al. ; Bouougri et al. ; Seilacher & Gishlick ; Elliott et al. ). A few specimens in the gutter cast show similar orientations, but these specimens represent end members of a nearly continuous distribution from fully vertical, though a range of inclined angles, to specimens that are fully horizontal (Fig. C). Dominance of this assemblage by inclined and horizontal specimens is highly atypical of any previously described occurrences of Ernietta and may relate to the abrupt setting of the mass flow that was carrying the Ernietta specimens. An occurrence of inclined and horizontal specimens of the related erniettomorph Namalia was mentioned by Germs, and it may be fruitful to re-examine the sedimentology and taphonomy of Germs’ fossil site in this light.

Specimen orientation in the flow had a profound effect on preservation of Ernietta. The external morphology of Ernietta was moulded by the sediments that surrounded the organism at the time it was buried, and thus, the top of the gutter cast marks the top of preservation of any fossils in the gutter cast. It seems likely that any organic tissues that protruded above the sediment–water interface organism decomposed rapidly after the death of the organism and were not preserved. As a consequence, the basal parts of the body including the suture are faithfully preserved on all Ernietta specimens in the gutter cast, but maximum preserved length of any specimen is strictly dependent on its orientation within the 4.5-cm-thick bed in which the fossils occur. Vertically oriented specimens are a maximum of 4.5 cm long and invariably end at a truncated, open top (Fig. A) that corresponds with the top of the gutter cast, whereas Ernietta that are horizontal (prone) can preserve the full 10.5–12 cm length of the complete specimen (Fig. B). Inclined specimens preserved obliquely to regional bedding range from 4 to 10 cm in length depending on the angle at which they are preserved in the sediment, with low angles with respect to bedding invariably represented by long specimens and increasingly higher angles represented by increasingly shorter specimens (Figs C, D, A). These relationships suggest that the specimens in the gutter represent a population of Ernietta whose individuals were variably oriented during final burial, resulting in differential preservation of their distal morphology largely related to their position within the bed in which they were preserved.

Thirty-nine specimens of Ernietta are visible in the gutter cast (Fig. D, E) with an additional 60 specimens collected from local float weathered from the upper part of the gutter cast. The superb preservation of the specimens in the gutter cast permits enhanced description of the morphology of Ernietta, but, as with most or all other specimens of Ernietta, moulding of the originally soft and pliable bodies of Ernietta against each other (Elliott et al. ) makes obtaining meaningful biometric data of the fossils difficult. All specimens are cm-scale (length along the basal suture 23–55 mm, average 45 mm) and elliptical in transverse cross-sectional view, with their longer horizontal axis parallel with the basal suture and their shorter horizontal axis at right angles to this (Fig. A, D). Side views of specimens are referred to as being in ‘wide profile’ if they show the maximum width of the specimen (Figs A, B, A) or being in ‘narrow profile’ if they show its narrowest view (Fig. C, D).

As discussed above, specimens in the Aar Member gutter cast show a complete gradation from short, vertically oriented (upright) specimens, through a range of increasingly longer specimens that are also increasingly inclined to their original vertical orientation, to specimens that are nearly or completely horizontal (prone). Irrespective of orientation, the basal (proximal) portion of all specimens in the gutter cast appear to be broadly similar in both size and shape (Figs D, E, A–D, A) and consists of an elongate, sand-filled anchor that is characterized by a palisade-like wall structure of parallel tubes that run parallel to the vertical axis of the fossil and meet at a zigzag suture at the base of Ernietta.

Specimens that are oriented vertically in the gutter cast are short (<4.5 cm long) and exhibit only this basal anchor with its well-developed palisade structure connected to a short, smoother, trunk that terminates abruptly at or near the top of the gutter cast (Fig. A). Specimens that are oriented obliquely in the Aar Member gutter cast exhibit additional, more distal features of the Ernietta organism. These specimens show that the trunk flares distally into two fanlike structures, with the seam between these fans parallel with the basal zigzag suture of Ernietta (Figs C, D, A). The six specimens that are horizontal or nearly horizontal show the maximum length and include the only two specimens showing the full distal termination of the fans at the end of the Ernietta organism (Fig. B).

Ernietta is herein reconstructed as a baglike organism with an open central cavity surrounded by soft, highly pliable, organic walls that extended beyond the bag as two, facing, fanlike structures (Fig. A, B). The walls and their continuation into fans consisted of a palisade-like array of parallel tubes 1.4–3.6 mm (average 2.5 mm, n = 27) in diameter that met on both sides of the zigzag suture at the base of Ernietta and ran parallel to the vertical axis of the fossil to its distal fan apex. Thin sections and partially exfoliated specimens show that this tubular array was double-layered, a diagnostic feature of Ernietta (Elliott et al. ). Tubes in the basal part of Ernietta typically are filled with sand grains, but this fill is absent from tubes in the trunk and fans of Ernietta in which the open tubes collapsed during burial and are preserved as moulds on the sediment that surrounded the outside of the organism and that filled its central cavity. Thin sections and partially exfoliated specimens show variation in the number of sheets of tubes preserved in different parts of the two specimens illustrated in Figure , ranging from no preserved sheets of tubes (only moulds of the outside or inside walls) to one sheet of tubes to two sheets of tubes, depending on the completeness of the sand fill of these tubes. Local evidence of overprinting in the fan implies that the palisade of tubes in the fan was also double-layered (Fig. B). Complete specimens preserving the distal tips of the fan (Fig. B) show that it was splayed, crenelated or scalloped and that the moulding ends abruptly and is smoothly stepped up at their extreme distal ends, implying that these tubes were closed (blind) at their tips (Fig. A, B).

This palisade structure is ubiquitous in well-preserved specimens of Ernietta from the gutter cast, but extreme pliability of the Ernietta body in life and the variable preservation of the tubes, depending on whether they were sand-filled or collapsed, hinders precise counts of the number of tubes on a specimen and makes it difficult to trace individual tubes distally for more than a few centimetres. With these caveats in mind, the number of tubes (23–28) on either side of Ernietta is similar among all the specimens in this study, implying that growth over this cm-scale size range occurred by inflation rather than by insertion of new tubular elements. Strong similarity in the number of tubes in the base (mean 25.4, range 23–28, n = 28) and the fan (mean 25.5, range 22–28, n = 4) and long-distance continuity of tubes (Fig. A) in the best preserved specimens of Ernietta implies congruence between the tubes that make up the basal anchor, trunk, and fans of a complete specimen of Ernietta (Fig. A, B).

All sectioned Ernietta specimens in the gutter cast contain a succession of fills in their tubes and in their central cavities. Polished sections (Jenkins et al. ; fig. ; Elliott et al. ; fig. ; this paper, Fig. A, B, D) and thin sections (this paper, Figs C, B–F) can contribute to our understanding of the genesis of this fill.

Seilacher & Gishlick observed that basal parts of tubes typically are filled with sandstone, whereas higher parts of the tubes invariably are preserved only as moulds. Our thin-section analyses support their observation and show that the tubes of the proximal anchor of Ernietta in the gutter cast were filled with extremely well-rounded, essentially mud-free, medium- to coarse-grained quartz sand that was cemented by syntaxial quartz overgrowths to produce the quartz arenite fill of these basal tubes (Figs D, E, F). Sandstone-filled tubes can be locally discontinuous within specimens (Figs D, E, F) or nearly absent from some specimens (Fig B, C). The lithology and diagenesis of the sand-filled tubes in the anchor of Ernietta does not match the sediment in the gutter cast, implying that was incorporated into the tubes before their final deposition in the gutter cast. Tubes above this basal anchor presumably remained fluid-filled and were preserved only as moulds on the fossil surface (Figs A, D, D). Seilacher & Gishlick suggested that this sand was packed into the basal tubes during life to form a weight belt while more distal parts of the same tubes remained fluid-filled, a view that is consistent with the thin section evidence from the Ernietta specimens in the gutter cast in this study. Evidence for closure of the tips of tubes of Ernietta (Figs D, A, B) provides further evidence that the sand at the bottom of the tubes did not accumulate passively, but introduces similar uncertainty as to how the sand could be selected and packed by biological processes.

In contrast with evidence for closed tubes at the distal end of Ernietta, all evidence suggests that its central cavity was open during life (Fig. A, B). Previous studies have documented fills for Ernietta that range from simple (Jenkins et al. ; fig. B, D) to revealing a complex history of fill during reorientation of the Ernietta organism (Jenkins et al. ; fig. C), but these studies were from loose specimens collected from float that precluded comparison of the sediment fill in Ernietta and in the surrounding sediments in which the fossils were entombed. Our thin-section studies of oriented specimens from outcrop show that the basal fill of the central cavity of the Ernietta specimens consists of fine- to medium-grained sub-arkosic, intraclast-rich arenite that is lithologically indistinguishable from the sandstone in the basal part of the gutter cast (Figs A, C, B, C, E, F). This basal fill of specimens in the upper part of the gutter cast is not similar to the finer grained wackes in which the specimens are now entombed, implying that they began life during the accumulation of the earlier, coarser sediment, either in the gutter cast or more likely in the immediate source area of the gutter cast, and were later transported and deposited in the muddy mass flow that filled the top of the gutter cast. This sediment occurs as a spherical (Fig. B, C) or hemispherical (Fig. D, E) plug of sand at the base of specimens in the upper part of the gutter cast. The presence of a spherical ball of well-sorted sand at the base of Ernietta in the upper part of the gutter cast is similar to sands preserved in the basal discs of Ediacaran fronds from Newfoundland (Laflamme et al. ), in the basal bulbs of the Ediacaran fractal frond Rangea from Namibia (Vickers-Rich et al. ), and in some modern actinians (Haywick & Mueller ), and provides support for Seilacher's suggestion that these sandstone plugs acted as a basal weight ball that provided stability on the sea bottom.

The presence of this sandy plug at the base of Ernietta would also help to keep them mostly upright during transportation, and may account for the complex vertical succession of fills seen in some specimens of Ernietta (Jenkins et al. ; fig. C; this study, Fig. B, C). Sands immediately above the basal plug are laminated parallel with the basal suture of Ernietta, and, in vertical specimens, the lamination in layer 2 continues parallel to regional bedding to the truncated top of the fossil (Fig. E, F). Lamination is also parallel with the basal suture in inclined specimens, but these laminations are no longer parallel with regional bedding (Fig. B, C, layer 2), implying that these basal sands accumulated while the organism was upright but that it was then toppled from its original vertical orientation. This laminated sediment is overlain by arcuate wedges of very fine-grained sandstone that reflect the transition from a vertically oriented organism to one that was progressively more inclined in the sediment (Fig. B, C, layer 3). The final fill of the specimen was by horizontally laminated sediment that is indistinguishable from the final fill of the gutter cast in size and mineralogy (Fig. B, C, layer 4), implying that the body cavity of Ernietta was still open and in its final position, and was cast by sediment during the final stages of fill of the gutter cast.

It would be tempting to regard the progressive tilting in the fill of some specimens of Ernietta (Jenkins et al. ; fig. C; this study, Fig. B, C) as representing a biological response to gradual toppling of the organism, but it is more likely that it represents a natural consequence of transport of partly filled Ernietta in a muddy mass flow. The concentration of the fossils at the top of the flow implies that they were lighter than the sediment–water mixture that comprised the mass flow, probably due to the part of the organism that was unfilled with sediment, and that they were ‘rafted’ along on or near the top of the flow. An overriding turbulent flow, which typically travels more rapidly than the dense mass flow beneath it and thus produces downcurrent sheer on the top of the lower flow (Bagnold ; Sumner et al. ), would tilt the organisms at the top of the debris flow in the direction of movement and would also have provided a mechanism for finer suspended sediment to collect in the ‘sediment trap’ inside the organism. As the organism tilted progressively, the older layers of sediment would have become progressively more inclined, with the last ones being deposited horizontally (Fig. B, C).

The vertically oriented specimens of Ernietta in the gutter cast are indistinguishable from the type specimens of Ernietta as described by Pflug (, ), Jenkins et al., Seilacher & Gishlick, and Elliott et al. and include only the basal anchor and the lowermost part of the trunk of a complete specimen of Ernietta. A key exception is NESM-F-389, the holotype for ‘Erniograndis sandalix’ Pflug (refigured in Elliott et al. ; fig. and therein regarded as a subjective junior synonym of Ernietta plateauensis), which extends to just beyond the base of the split that marks the origin of the two opposing fronds at the top of the complete specimen of Ernietta. No previously figured specimens directly attributed to Ernietta illustrate any part of the distal fan. A similar fan occurs in two specimens of conical erniettomorphs (Fig. A, D), including the only known specimen of Kuibisia Hahn & Pflug (Fig. D), a worn specimen that has an erniettomorph pattern of parallel tubes, but a conical shape that differs from typical Ernietta. Kuibisia is currently of uncertain taxonomic status; Runnegar listed Kuibisia as a subjective junior synonym of Ernietta, whereas Elliott et al. maintained a distinction between these two genera, but neither provided detailed justification for their assignment of this taxon. Nonetheless, these two specimens show that distal fans were a key, albeit rarely preserved, element of conical erniettomorphs from Namibia.

Ernietta is an iconic Ediacaran fossil taxon that is figured in most major global reviews and reconstructions of the Ediacara biota. It was first described nearly 50 years ago, and many scores of specimens have been figured and described subsequently. Most specimens occur in graded, hummocky cross-stratified, or swaley cross-stratified event beds of sandstone (Bouougri et al. ), which represent the norm for Nama-style preservation (Narbonne ). However, like the gutter cast specimens of Rangea described by Vickers-Rich et al., the exceptionally preserved specimens from the gutter cast in the present study reveal that most or all previously described specimens of Ernietta are incomplete, and represent only part of larger and more complex fossils (Fig. ).

Discovery of complete specimens of Ernietta in this Ediacaran gutter cast on Farm Aar is analogous in some regards to discoveries of exceptionally preserved and complete fossils of Halkieria (Conway Morris & Peel ), Anomalocaris (see review in Collins ), Hallucigenia (Smith & Caron ), and other Cambrian fossils that had previously been known only from disarticulated parts of the organism. Like these Cambrian discoveries, discovery of complete specimens of Ernietta rules out some previous reconstructions of its morphology and life habit that considered Ernietta as an epifaunal disc that was loaded to produce a basal bulb passing up into an open cylinder (Dzik ) or as a fully infaunal organism that was closed at both ends (Crimes & Fedonkin ). However, discovery of complete specimens of Ernietta has also supported and enhanced some earlier views about the three-dimensional morphology and ecology of Ernietta and its place in the early evolution of complex life. Our new reconstruction of Ernietta (Fig. A, B) is similar to those of Jenkins et al., Seilacher et al., and Seilacher & Gishlick in reconstructing the distal end of Ernietta as an open structure marked by a crenellate margin. It is especially close to the reconstruction of Jenkins et al., which shows the split at the termination of the trunk as the base of two opposing fanlike structures, differing mainly in showing thinner walls and a double- rather than triple-walled palisade structure for Ernietta.

The type specimens of Ernietta figured in previous papers are herein interpreted as basal attachment structures that were buried in the substrate to provide stability for the main part of the Ernietta organism, two facing fans that extended above the substrate into the overlying water column (Fig. A, B). Fans are exceptionally common among Ediacaran megafossils, including the erniettomorph Swartpuntia, and most likely filled mainly respiratory and/or feeding functions for the organism (Laflamme & Narbonne ). Similarity and continuity of the tubular walls which line the anchor, trunk and fans imply that preferential preservation of the proximal portions of Ernietta is unlikely to relate to proximal to distal differences in the original composition of the tubular walls. More likely, the basal part was preferentially preserved because it was both moulded by the substrate and internally filled with sand during the life of the organism. Parts of the Ernietta organism that extended above the sea bottom decomposed soon after death and could not be preserved. This is taphonomically analogous to the preservation of unrelated Ediacaran fronds such as Charniodiscus, in which the infaunal attachment disc had a much higher preservation potential than the leaf-like petalodia at the distal end of the stem (Laflamme & Narbonne ), and as with Ernietta, these basal discs commonly occur in abundance in settings in which the leafy frond is not preserved (Gehling et al. ; Narbonne ). Grazhdankin & Seilacher (, ) regarded the classic Ediacaran taxa from Namibia as having lived as endobenthic organisms buried in the substrate, but subsequent studies of Rangea (Vickers-Rich et al. ) and Pteridinium (Elliott et al. ; Meyer et al. ) are consistent with the traditional interpretations of fully epibenthic lifestyles for these taxa. The new specimens of Ernietta herein described rule out interpretations of Ernietta that required it to be either fully epibenthic (Dzik ) or fully endobenthic (Crimes & Fedonkin ), and provide critical evidence supporting the semi-endobenthic models proposed by Seilacher, Seilacher et al. and Seilacher & Gishlick.

Specimens of Ernietta in the gutter cast at Farm Aar exhibit a wide range in external shape that reflects both folding during transport and mutual deformation of adjacent individuals during deposition, and lack any evidence of ripping or breakage of the body walls. These taphonomic features are similar to those observed in occurrences of Ernietta elsewhere in Namibia (Pflug, ; Bouougri et al. ; Elliott et al. ), and imply considerable flexibility in the organic tissues composing Ernietta.

In contrast with their highly variable external shape, the architecture of Ernietta invariably consists of a double-layered palisade of morphologically similar tubes. These tubular elements could be modified to fill all of the functional needs of Ernietta – ranging from sand-filled tubes that helped to anchor the buried base of the organism in the seafloor, to the support function fulfilled by fluid-filled tubes in the mid-trunk region, to the tubes in its distal fan that may have performed feeding and/or respiratory functions. Despite these very different functions for different parts of the organism, there is no obvious evidence of tissue differentiation in any known specimens of Ernietta. This modular construction employing tubular elements is diagnostic of the Erniettomorpha, a major clade of Late Ediacaran life (Pflug ; Xiao & Laflamme ; Erwin et al. ; Laflamme et al. ). The exclusive use of parallel arrays of tubular elements for all components of their architecture, construction, and function is unknown among modern organisms, supporting recognition of the Erniettomorpha as an extinct clade in the early evolution of multi-cellular life.

Specimens were discovered by AYI in June 2014 and were analysed by all of the authors of this paper. Fossil specimens were collected under permits from the National Heritage Council of Namibia and are reposited in the National Earth Science Museum (NESM) Geological Survey of Namibia, Ministry of Mines and Energy collections in Windhoek. We thank J. Alegado, Greg Burzynski, Leslie Kriesfeld, Steve Morton, Doris Seegets-Villiers, Robert Smith, Oliver Trusler, Pedro Viegas and Maria Zakrevskaya for assistance in the field, with preparation of specimens, and with manuscript preparation, and Gabi Schneider for facilitating fieldwork and access to the type specimens at the Geological Survey of Namibia. R.W. Dalrymple reviewed the manuscript, providing exceptionally helpful comments on the sedimentology of the strata and fossils. PVR acknowledges grants from the National Geographic Society (NGS 8467-08, 8883-11, 9208-12). GMN acknowledges funding from a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant and a Queen's University Research Chair. This paper is a contribution to the UNESCO-IUGS International Geoscience Program (IGCP Project 587). We dedicate this paper to Barbara Boehm-Erni and the Erni family for their judicious protection of the natural heritage on farms Aar and Plateau for more than half a century.