The Early Cambrian Shackleton Limestone outcrops at a number of locations in the Central Transantarctic Mountains. It is thought to be about 2 km thick at its maximum extent, and contains a variety of massive limestones, sandy carbonates, and bioherms (expand), laid down at a time when East Antarctica was located in the tropics. These deposits have produced a range of small shelly fossils assigned including Archaeocyaths (extinct, sessile, reef-building marine organisms of uncertain affinities), Brachiopods, Bradoriid Arthropods, Cambroclavids (phosphatised fossils of uncertain affinities), Chancelloriids (spines that may have come from Sponges or some more advanced group), Hyoliths (conical shelled lophophorate animals), Sponge spicules, and Tommotiids (shelly fossils thought to be related to Brachiopods and Phoronids), which show affinities to similar fossils from other Early Cambrian sites around the world, but in particular those from South Astralia and south China.
In a paper published in the Journal of Paleontology on 9 January 2019, Thomas Claybourn of the Department of Earth Sciences at Uppsala University, and the Department of Biological Sciences at Macquarie University, Sarah Jacquet of the Department of Geological Sciences at the University of Missouri, Christian Skovsted of the Department of Palaeobiology at the Swedish Museum of Natural History, Timothy Topper, also of the Department of Palaeobiology at the Swedish Museum of Natural History, and of the Shaanxi Key laboratory of Early Life and Environments, and the State Key Laboratory of Continental Dynamics at the Department of Geology at Northwest University, Lars Holmer, also of the Department of Earth Sciences at Uppsala University, and the Shaanxi Key laboratory of Early Life and Environments, the State Key Laboratory of Continental Dynamics, and the Department of Geology at Northwest University, and Glenn Brock, also of the Department of Biological Sciences at Macquarie University, describe a series of Mollusc fossils from the Shackleton Limestone.
The first fossil described is Pojetaia runnegari, a Bivalve previously described from the Early Cambrian of South Australia and Newfoundland. These are small Bivalves with equibivalved shells (shells in which the valves are the same), suboval to subcircular in shape, and 0.8-1.5 mm in length, with a small projection near the hinge.
(1) Map of Antarctica showing approximate extent of the Transantarctic Mountains and area shown in (2). (2) Map of Nimrod Glacier, Holyoake Range, and Churchill Mountains. (3) Generalized relationship of Cambrian (Byrd Group) and Neoproterozoic (Beardmore Group) rock units of the Holyoake Range. (4) Simplified geological map of the Holyoake Range. Claybourn et al. (2019).
The first fossil described is Pojetaia runnegari, a Bivalve previously described from the Early Cambrian of South Australia and Newfoundland. These are small Bivalves with equibivalved shells (shells in which the valves are the same), suboval to subcircular in shape, and 0.8-1.5 mm in length, with a small projection near the hinge.
Pojetaia runnegari from the Shackleton Limestone. (1)–(4) Specimen SMNH Mo185039 in (1) lateral view, (2) dorsal view, (3) magnification of the central margin, showing laminar crystalline imprints, (4) magnification of the cardinal teeth shown in (2). (5), (6) Specimen SMNH Mo185040, (5) lateral view, (6) magnification of lateral surface, showing laminar crystalline imprints. (7) Specimen SMNH Mo185041 in lateral view. (8) Specimen SMNH Mo185042 in lateral view. (9) Specimen SMNH Mo185043. (5), (6), (8) imaged under low vacuum settings. (1), (2), (6)–(9) Scale bars are 200 μm; (3)–(5) scale bars are 100 μm. Claybourn et al. (2019).
The majority of the fossils found belong to a group called the Helcionelloids, which are widespread in the Early Cambrian Small Shelly fossils, and are of uncertain affinities, having variously been considered to be Gastropods, Monoplacophorans, the Protoconchs of larger shells (the Protoconch is the first part of the shell to form in a marine Mollusc, typically while it is in a planktonic, larval form, so that the growth form of the protoconch is quite different from that of the adult shell), or a separate class of Molluscs, the view that Claybourn et al. take here.
The first of these Helcionelloids recorded from the Shackleton Formation is Davidonia rostrata, a laterally compressed, high shell, that coils through one third of a whirl, with a rugose bands on all but the smallest specimens, which reaches a length of 0.5-1.3 mm and a width of 0.2-0.8 mm, typically being 1.5 times as long as it is high. This species has previously been recorded in Early Cambrian deposits from Anhui Province, China, South Australia, northeast Greenland, New York State, Quebec, and northwestern Spain.
The first of these Helcionelloids recorded from the Shackleton Formation is Davidonia rostrata, a laterally compressed, high shell, that coils through one third of a whirl, with a rugose bands on all but the smallest specimens, which reaches a length of 0.5-1.3 mm and a width of 0.2-0.8 mm, typically being 1.5 times as long as it is high. This species has previously been recorded in Early Cambrian deposits from Anhui Province, China, South Australia, northeast Greenland, New York State, Quebec, and northwestern Spain.
Davidonia rostrata. (6), (7) Specimen SMNH Mo185047, (6) lateral view, (7) dorsal view of supra-apical field; (8)–(11) specimen SMNH Mo185048, (8) magnification of lateral view of parietal train, showing polygonal crystalline imprints on the side surface, (9) dorsal view of supra-apical field, (10) lateral view, (11) magnification of oblique lateral view of supra-apical field, showing polygonal crystalline imprints; (12) specimen SMNH Mo182501 in lateral view; (13) specimen SMNH Mo182502 in lateral view; (14) specimen SMNH Mo182503 in lateral view. (10), (11) Scale bars are 100 μm; all other scale bars are 200 μm. Claybourn et al. (2019).
The second Helcionelloid recorded is thought most likely to be Davidonia corrugata, a high-shelled form with a hooked apex, moderate lateral compression, and broad, flat rugose bands, that reaches 0.5-0.6 mm in length and 0.5-.06 mm in height. This species has previously been recorded from South Australia, although the Australian material showed much more variation in their expansion rate.
The fourth Helcionelloid recorded is placed in the genus Anuliconus, which has previously been recorded in South Australia, but not assigned to a specific species. The thirty-three specimens from Shackleton are 0.4–0.5 mm wide, 0.5–0.7 mm long, and 0.7–0.9 mm high, being high in form and somewhat laterally compressed, with concave lateral areas near their apexes.
The second Helcionelloid recorded is thought most likely to be Davidonia corrugata, a high-shelled form with a hooked apex, moderate lateral compression, and broad, flat rugose bands, that reaches 0.5-0.6 mm in length and 0.5-.06 mm in height. This species has previously been recorded from South Australia, although the Australian material showed much more variation in their expansion rate.
Davidonia corrugata. (1)–(3) SpecimenSMNH Mo185044 in (1) oblique lateral view, (2) apical view, (3) magnification of apical region in lateral view, showing protoconch and transition to teleoconch; (4) specimen SMNH Mo185045, oblique view of supra-apical field; (5) specimen SMNH Mo185046 lateral view. (3) Scale bars is 100 μm; all other scale bars are 200 μm. Claybourn et al. (2019).
The third Helcionelloid recorded is thought most likely to be Xianfengella yatesi, a species previously recorded from South Australia and Greenland. The Shackleton material produced a single specimen of this low, cup-shaped shell, measuring 0.9 mm in length, 0.6 mm in width and 0.6 mm in height, and coiling through a third of a whirl. The surface of this specimen has rugose correlations and is covered by polygonal imprints.
Xianfengella yatesi, specimen SMNH Mo185049, (15) dorsal view, (16) oblique apical view, (17) magnified view of supra-apical field showing crystalline imprints, (18) oblique lateral view. (17) Scale bars is 100 μm; all other scale bars are 200 μm. Claybourn et al. (2019).
The fourth Helcionelloid recorded is placed in the genus Anuliconus, which has previously been recorded in South Australia, but not assigned to a specific species. The thirty-three specimens from Shackleton are 0.4–0.5 mm wide, 0.5–0.7 mm long, and 0.7–0.9 mm high, being high in form and somewhat laterally compressed, with concave lateral areas near their apexes.
Anuliconus sp., (22)–(24) specimen SMNHMo185051, (23) lateral view, (22) magnification of apex in lateral view, (24) apertural view; (25), (26) specimen SMNHMo185052, (25) lateral view, (26) apical view; (27), (28) specimen SMNH Mo185053, (27) lateral view, (28) apical view. (22), (24) Scale bars are 100 μm; all other scale bars are 200 μm. Claybourn et al. (2019).
The fifth Helcionelloid recorded is tentatively placed in the genus Protowenella, which has previously been recorded in Queensland, the Northern Territory, south China, and Siberia, but not assigned to a specific species. The four poorly preserved specimens assigned to this genus have open coiled shells (shells in which the coils do not touch) that form three quarters of a whirl and measure about 1.5 mm in length and 0.9 mm in height.
Protowenella sp., (19) lateral view, (20) dorsal view, (21) apical view. Scale bars are 200 μm. Claybourn et al. (2019).
The sixth Helcionelloid recorded is placed in the genus Yochelcionella, which has previously been recorded in New South Wales, north Greenland and Newfoundland, but not assigned to a specific species. The five specimens placed in this genus have flattened shells with an extension from the aperture that appears to form a sort of snorkel.
Yochelcionella sp., (1)–(5) specimen SMNH Mo185063, (1) lateral view, (2) apertural view, (3) apical view of subapical field and broken snorkel, (4) magnified view of concentric structures within snorkel, (5) magnified view of pitted microstructure; (6), (7) specimen SMNH Mo185064, (6) magnification of lateral view, (7) lateral view; (8) specimen SMNH Mo185065, lateral view. All scale bars are 200 μm, except (5) and (6), which are 100 μm. Claybourn et al. (2019).
The seventh Helcionelloid recorded is placed in the genus Stenotheca, which has previously been recorded in Wales, South Australia and north China, but not assigned to a specific species. The twelve specimens assigned to this genus have flattened shells that coil through one quarter of a whorl, and range from 0.3 to 0.5 mm in length and 0.35 to 0.55 mm in width.
(9)–(16), (19)–(21) Stenotheca sp. (9), (10) specimen SMNH Mo185066, (9) oblique lateral view, (10) dorsal view; (11)–(13) specimen SMNH Mo185067, (11) oblique lateral view, (12) dorsal view of subapical field, (13) apical view; (14) specimen SMNH Mo185068, lateral view; (15), (16) specimen SMNH Mo185069, (15) oblique lateral view, (16) dorsal view; (19) specimen SMNH Mo185071 in lateral view; (20) specimen SMNH Mo185072 lateral view; (21) specimen SMNH Mo185073 lateral view. All scale bars are 200 μm. Claybourn et al. (2019).
The final Helcionelloid recorded is placed in the genus Anabarella, and referred to the species, which has previously been recorded from South Australia. The three specimens referred to this species are strongly laterally compressed. And expand rapidly, coiling through less than half a whorl ad they do so.
Anabarella cf. Anabarella australis (17), (18) specimen SMNH Mo185070 in (18) lateral view, (17) apertural view (22) specimen SMNH Mo185074 in lateral view. All scale bars are 200 μm. Claybourn et al. (2019).
The Shackleton Formation material also contains two species assigned to the Family Pelagiellida, considered to be stem-group Gastropods, i.e. organisms more closely related to Gastropods than to any other living group, but which are not descended from the last common ancestor of all living Gastropods, either because they lived before it, or because they form a separate, extinct, branch of the Gastropod family tree.
The fist of these Pelagiellid Gastropods is assigned to the genus Pelagiella, and referred to the species Pelagiella subangulata, which has previously been recorded from South Australia, the Great Basins of America, Germany and south China. About 30 specimens are referred to this species, these being 0.58-0.64 mm in length and 0.38-0.48 mm in height, with rapidly expanding shells that coil dextrally through three quarters of a whorl.
The second species of Pelagiellid Gastropod recorded is Xinjispira simplex, a species previously recorded in North China. Twelve specimens of this species are reported, These are globular shells that coil through almost a whorl, and have rounded apertures. They measure 0.5-0.6 mm in length and about 0.4 mm in height.
The final Mollusc recorded is Scenella, a Limpet-like fossil accepted as a Mollusc, but of uncertain affinities. Scenella has previously been recorded from British Colombia, Utah, Estonia, and South Australia. Two specimens are recorded from the Shackleton Formation, one of which is broken; the unbroken specimen is 9.8 mm in length and 8.5 mm in width.
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The fist of these Pelagiellid Gastropods is assigned to the genus Pelagiella, and referred to the species Pelagiella subangulata, which has previously been recorded from South Australia, the Great Basins of America, Germany and south China. About 30 specimens are referred to this species, these being 0.58-0.64 mm in length and 0.38-0.48 mm in height, with rapidly expanding shells that coil dextrally through three quarters of a whorl.
Pelagiella cf. Pelagiella subangulata Tate, 1892, (1–3) specimen SMNH Mo185054, (1) view of spiral side,
(2) dorsal view, (3) dorsal view of supra-apical field; (4–6) specimen SMNH Mo185055, (4) apical view, (5) apical view, (6) dorsal view; (7, 8) specimen SMNH
Mo185056, with possible hyolith operculum embedded in aperture, (7) oblique apertural view, (8) umbilical side; (9, 10) specimen SMNH Mo185057, (9) lateral
view of abapical side, (10) oblique apertural view, showing curved groove passing through the umbilicus; (11, 12) specimen SMNHMo185058, (11) apical view, (12)
apertural view; (13–16) specimen SMNH Mo185059, (13) view of supra-apical field, (14) apical view, (15) dorsal view, (16); magnified dorsal view of part of the
abapical side on the projecting wing, showing pustulose ornamentation. All scale bars are 200 μm, except (16) which is 100 μm. All images taken using secondary electrons. Claybourn et al. (2019).
The second species of Pelagiellid Gastropod recorded is Xinjispira simplex, a species previously recorded in North China. Twelve specimens of this species are reported, These are globular shells that coil through almost a whorl, and have rounded apertures. They measure 0.5-0.6 mm in length and about 0.4 mm in height.
Xinjispira simplex (17–21) specimen SMNH Mo185060, (17)
oblique dorsal view of supra-apical field, (18) lateral view of abapical side, (19) lateral view of apical side, (20) magnification of internal mold with transverse fibrillar
crystalline imprints, (21) view of supra-apical field; (22, 23), specimen SMNHMo185061, (22) dorsal view, (23) lateral view of apical side; (24–26) specimen SMNH
Mo185062, (24) apical view, (25) maginification of circumbilical channel on apical side, (26) magnification of transverse fibers on supra-apical surface of the steinkern.
(20), (24) Scale bars are 100 μm; all other scale bars are 200 μm. All images taken using secondary electrons except (24)–(26) taken using backscattered electrons.
Claybourn et al. (2019).
The final Mollusc recorded is Scenella, a Limpet-like fossil accepted as a Mollusc, but of uncertain affinities. Scenella has previously been recorded from British Colombia, Utah, Estonia, and South Australia. Two specimens are recorded from the Shackleton Formation, one of which is broken; the unbroken specimen is 9.8 mm in length and 8.5 mm in width.
Scenella? from the Shackleton Limestone. (1–3) Specimen SMNH Mo185075, (1) oblique view along supra-apical field, (2) lateral view, (3) apical
view. (4, 5) Specimen SMNH Mo185076, (4) apical view, (5) lateral view, angled obliquely toward subapical field. Scale bars are 2 mm.
Claybourn et al. (2019).
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