Abstract
Hiemalora Fedonkin, 1982 is based on cosmopolitan fossils of the late Vendian (Ediacaran) Neoproterozoic sequences. There is no consensus on the nature and systematic position of these organisms. Taphonomic and morphological observations of extensive collections of Hiemalora pleiomorphus from Khatyspyt Formation (Arctic Siberia) indicate that this fossil is just a part of an unknown organism. Most probably Hiemalora represents a form with a thick-walled body, possessing a cone-like attachment bearing radial processes that resemble sponge root offshoots or algal rhizoids. Originally these structures penetrated the soft sediment and served as anchors as well as a source of symbiotic nutrition. Various imprints of Hiemalora could represent different sections of conic-shaped bodies. Probably, there were several organisms referred to as Hiemalora that had a similar substrate attachment organ.
Well-preserved material from recently discovered localities have significantly increased our understanding of some Ediacaran taxa and changed views on their reconstructions. More reliable revisions of older reconstructions are obvious when sample size is large and especially when that taxon is from a single fossiliferous bed.
Research history of the genus Hiemalora
The genus Hiemalora Fedonkin, 1982 (type species: H. stellaris=Pinegia stellaris Fedonkin, 1980) was first described from the Vendian sequence of the White Sea, Russia. The author allied this new organism with cnidarian polyps or jellyfish with straight tentacles. Hiemalora and Evmiaksia were then united into the cnidarian Family Hiemaloriidae (Fedonkin 1985). H. pleiomorphus was discovered in limestones of the Khatyspyt Formation in the Khorbusuonka Group, Arctic Siberia (Vodanjuk 1989). Specimens in the Khatyspyt collection were arranged, depending on the form of the preservation and reconstructed as jellyfish that had fallen on a plastic clay substrate post-mortem (Vodanjuk 1989). In addition to systematic descriptions, there are numerous illustrations and descriptions of fossil forms similar to Hiemalora (Gehling et al. 2000; Jenkins 1995; De 2003). A star-like form from the Late Proterozoic of Newfoundland was thought to be either an attachment structure or a trace fossil or even the fragment of the body (Anderson & Conway Morris 1982). Several fossil impressions from the Innerelv Member, Late Proterozoic of the NE Finnmark were assigned to Hiemalora by Farmer et al. (1992). Hiemalora is known from the Mackenzie Mountains of NW Canada (Narbonne 1998). Radial structures of these forms have been interpreted as tentacles of a polyp or the root-like structures of a holdfast (Runnegar 1995; Narbonne 1998). The star-shaped fossils have been compared with modern Protozoan xenophyophores in which the protoplasm has left traces in sediment (Seilacher et al. 2003). Hiemalora from Arctic Siberia has been regarded as an attachment structure (Dzik 2003). Thus, Hiemalora and other star-like fossils are known from many localities of the world, but there is no generally accepted interpretation of the nature and taxonomy of this form.
Material
Numerous imprints of Hiemalora were collected by M. A. Fedonkin and A. Y. Ivantsov (PIN RAS) from Arctic Siberia in the 1980–1990s. Hiemalora fossils occur in dark grey, thin-bedded limestones of the Khatyspyt Formation in the Khorbusuonka Group, which crops out in the Olenek River Basin, along the Horbusuonka and Anabyl rivers (Fig. 1). The Khorbusuonka Group can be correlated with Yudomian (Vendian) sediments by use of stromatolites and microphytolites and is dated radiometrically (Sokolov & Fedonkin 1985). Fossils of Hiemalora in Siberia are flat to low relief imprints in most cases. Specimens from the sole surface are preserved in negative hyporelief (Fig. 2a), and those from the top surface occur in positive epirelief most often (Fig. 2b). Specimens of Hiemalora arranged in preservational rank (Vodanjuk 1989) demonstrate that such assemblages represent one form of a single species, Hiemalora pleiomorphus (Fig. 6a–e).
Generalized stratigraphic section of the Vendian sequence, the Olenek uplift, Arctic Siberia (Vodanjuk 1989).
Hiemalora pleiomorphus from the Olenek uplift, Arctic Siberia. (a) PIN, N 3995/251 (from the sole of fossiliferous stratum); (b) PIN, N 3995/251-1 (from the top of fossiliferous stratum layer); scale 1 cm.
Fossil structure
Hiemalora imprints have a rounded central part with rod-like radial elements (Fig. 3). The central sections leave a low relief imprint but some forms show higher relief. These higher relief forms could represent remains of: (1) a short column; (2) a small hemisphere; or (3) a broad short cone. Occasionally these fossils are surrounded with a narrow (2–3 mm) ring-shaped border resembling a roll or a furrow (Fig. 3(2)). Fine, sub-parallel grooves interrupt the surface of hemisphere-shaped forms. The space between these grooves is slightly greater in the centre of the casts (Fig. 6a–c) than on the periphery. Small (1–2 mm) impressions (or bumps) are situated randomly on the surface of some specimens (Fig. 3(3)).
Schematic drawing of Hiemalora pleiomorphus from the Olenek uplift, Arctic Siberia (PIN No. 3995/251): (1) central part; (2) peripheral border; (3) small structure found on central disk; (4) radial elements. See Fig. 4 and Table 1 for size range.
Radial elements resembling straight or smoothly curved rods are flat or slightly raised (Fig. 3(4)). In some cases, these radial elements lie in the shallow isometric depression. Outlines of these elements are smooth and without fissures. There are rare clavate nodules on the endings of these structures. Rod-like radial elements are distributed evenly on the surface of the layer intersecting each other centrally. Some specimens have radial elements which traverse borders of the central parts (Fig. 6d–e) and in other specimens these elements diverge from centres of imprints. The radial structures of the best-preserved specimens diverge from the centre and different points of the periphery resembling a radial network. Some three-dimensional forms resemble stubs with radial elements lying in a ring-like depression.
Hiemalora imprints may be divided into three morphological types (Table 1, Fig. 4): (1) elongate radial elements with a small central part; (2) radial elements approximately equal in length to the central part; and (3) short radial elements with an enlarged central part. There seems to be no relationship between the diameters of the central part and the number of the radial elements. It is doubtful that these morphological types represent different taxa. This distribution may be related to age differences and/or post-mortem modification in the sediment. Various imprints of Hiemalora could also be different sections of a cone-shaped organism.
Histogram of distribution of Hiemalora pleiomorphus specimens from the Olenek uplift, Arctic Siberia according to the ratio of D/d. D, diameter of central part (mm); d, length of radial elements (mm); Q, numbers of radical elements. See Table 1 for data.
Measurements of specimens of Hiemalora pleiomorphus
Interpretation
As there are no signs of transportation or deformation, it appears that this material was preserved in situ. The variety of forms of the central part (Figs 5, 6a–e) suggests that the original organism had three-dimensions and was not flat. Since low profile cylindrical-shaped forms suggest that the living organism possessed an internal cavity, sometimes likely filled with the sediment during lifetime or even post mortem. In this case the relief of the central part depends on sediment filling the cavity of the living organism and if so the quantity will determine the degree of relief. A specimen where it could be shown that the central part can be separated from the enclosing sediment would be an indisputable argument for the previous existence of such a hypothetical cavity. The relief can also be explained by collapse of a soft body without a cavity. As it was noted above, some specimens have a ridge or a furrow around the outer border of their central parts, which could be a remnant of a massive body wall. Small, rare, round structures on the surface of the central section could represent holes filled with the sediment, which then were preserved in relief. In collections at the Paleontological Institute there is a single specimen (PIN No. 3995/82) with a sub-oval contour of the central part; if this material were more common, one might suggest that Hiemalora could divide to form two individuals or clones.
Hiemalora pleiomorphus from the Olenek uplift, Arctic Siberia. (a) PIN, N 3995/287; (b) PIN, No 3995/84; (c) PIN, No 3995/289; scale bar 1 cm.
Hiemalora pleiomorphus from the Olenek uplift, Arctic Siberia. (a–e) schematic drawings of a variety of preservational forms, (after Vodanjuk 1989); (f) tentative reconstruction of preserved fossil of Hiemalora. See Fig. 4, Table 1 for size range.
Virtually none of the specimens shows deformation of the radial elements, which are spaced evenly occasionally crossing. This could signal their relative rigidity; moreover only one specimen (N 3995/289) has a broken radial element. There are rare forms where the radial elements are preserved in differing relief, possibly explained by heterogeneity of the tissue.
High-relief specimens can assist in understanding the spatial distribution of radial elements in living organisms. For instance, a low-conical specimen shows radial elements growing from different layers that overlap each other. One can conclude from this, that these radial elements penetrated the sediment randomly rather than lying on the same layer. In other cases, the central part possessed rod-like elements that diverged in many directions and must have had a shape like an upside-down cone. After this collapsed and was affected by sediment compaction, the radial elements from different layers could have been projected onto one surface. Therefore, these elements seem to be reacting as would the tentacles of a jellyfish or polyp.
A study of a large sample of Hiemalora demonstrates that the radial elements are static structures resembling root offshoots of sponges or rhizoids of algae rather than flexible tentacles. During a lifetime of the organisms these structures seem to have penetrated the liquid-rich sediment, and served for anchoring the organism probably, also for nutrition.
Reconstruction
Hiemalora may be reconstructed as an organism with a thick-walled body and a cone-like attachment disc with diverging radial offshoots (Fig. 6f). Reconstruction of Hiemalora from fossils is difficult. The fossils have disproportionately elongate offshoots when compared with a small body making it difficult to reconstruct its lifestyle. Hiemalora may be only part of an unknown organism, i.e. basal attachment structure in sediment. This would then explain why the imprints are so diverse, and the offshoots and the main body can be correlated. There may well have been different organisms that are referred to as Hiemalora with a similar way of attachment in the substrate. Unfortunately, there are no data on Hiemalora's hypothetical upper-most parts. Considering measurements of these attachment structures which may have anchored an organism to a soft substrate, one would predict that non-preserved upper parts of some specimens may have been of significant size.
Hiemalora's disc-like attachment was likely within the sediment (Fig. 7a) while the body towered above the seabed, therefore explaining the multiple modes of burial of these fragmentary ‘animals’ (Fig. 7b, c). Burial conditions above the bacterial film and under it would have differed sharply (Gehling 1999). That is the likely explanation why no Hiemalora's upper-most parts would be missing in the fossil record since they were likely to have been destroyed. Disc-like attachments would more likely to have been preserved. The mechanism of the formation of one-sided forms leaving ‘leaf-like’ imprint, with a negative imprint of the upper surface of the leaf preserved on bed soles and a positive imprint of the same surface on the top of the underlying layer, was explained by Kryshtofovich (1957). To interpret fossilization of attachments one can use Kryshtofovich's (1957) model, which likens Hiemalora structure to the hard capsule around roots of modern plants.
Suggested patterns of modes of fossilization of Hiemalora pleiomorphus from the Olenek uplift, Arctic Siberia. Stages (a) to (c) show gradual development of taphomorph, with (c) being the final stage.
If the fossilization events of Hiemalora are reconstructed correctly in this paper, one can suggest that these organisms had a rigid external tissue rather than soft bodies. And with this new reconstruction of Hiemalora, the diagnosis of this genus needs to be amended. Hiemalora should now be viewed as a problematical benthic organism with the attachments characterized by a monaxonic, heteropolar symmetry of indefinitely high order (terminology after Beklemishev 1964) preserved as low-relief, only rarely as high-relief imprints with a rounded central part associated with a marginal set of the rod-like radial elements.
Taxonomy
The current situation regarding the taxonomy of Ediacaran organisms, particularly rounded ones, is similar to the taxonomy in palaeobotany during the 1980's: ‘… more than 40% of the genera have to be abolished due to various nomenclature reasons, about 30% are to be revised and re-described and only 40% could more or less safely be used’ (Meyen 1990, p. 50). In order to overcome such a situation, it is reasonable to collect specimens of one type from one depositional surface and after that specimens could be viewed in mono-topical assemblages. Harris (1932) pioneered the use of this method of analysis based on his palaeobotanical studies for Triassic/Jurassic sequences of Greenland.
Conclusions
Taphonomic observations on a large sample of fossils from the Khatyspyt Formation of Arctic Siberia demonstrate that Hiemalora is a mixed group of fragments of benthic organisms, the basal parts of which were situated within the sediments.
The radial elements of Hiemalora were evidently static structures resembling the root offshoots of sponges or rhizoids of algae rather than flexible tentacles, as has been suggested in the past. These structures originally penetrated liquid sediment, served for anchoring within it and, could have been nutrient collectors. Hiemalora holdfasts seem to have had a thick body wall with a cone-like attachment disc and diverged radial offshoots.
Acknowledgments
The study was supported by the Russian Fund for Basic Research (RFBR 02-05-64658) and the Scientific School of Academician Boris S. Sokolov (Grant NSH-1790.3003.5). The author is grateful to M. A. Fedonkin and A. Y. Ivantsov for access to the collections (Hiemalora) and the assistance they gave on this study, D. Grazhdankin and M. Leonov for the consultations, O. Panin and D. Serezhnikov for the improving language of the manuscript, A.V. Mazin for the photographs, P. Vickers-Rich for assistance from IGCP Project 493 (UNESCO), of which this study is a part. Thanks are due to the two reviewers: J. Gehling and B. MacGabhann.
- © The Geological Society of London 2007
References
Vendian Hiemalora from Arctic Siberia reinterpreted as holdfasts of benthic organisms
