Coesite in Australasian microtektites.

Tektites and microtektites (i.e. very small tektites) are pieces of glass formed as ejecta from impact events. They are found in strewn fields which may extend thousands of kilometres from the original impact site. Current models of their formation suggest that they form from the material being impacted, as a spray of droplets of material melted by the rapid heating of the impact of an asteroid or comet fragment at speeds of over 3 km per second. However, while we have a general picture of how these events unfold, much of the detail is unclear. For example, many tektites from the Australasian Strewn Field contain inclusions of material, which have for a long time been accepted as fragments of the original rock trapped within a matrix of glass melt. This view has recently been challenged by an alternative hypothesis, that the matrix material may be a condensate from rock which was vapourised during the impact, and that the most distant microtektites, from Antarctica, may have formed from material which was vapourised by heat from the atmospheric shock wave before the impacting object touched down. This would help to explain why the Antarctic tektites lack inclusions, whereas in those from close to the presumed impact site, in Southeast Asia, they may make up as much as 5% of the mass of the tektite.

In a paper published in the journal Geology on 3 June 2025, Luigi Folco, Enrico Mugnaioli, and Matteo Masotta of the Dipartimento di Scienze della Terra and the Centro per la Integrazione della Strumentazione  at the Università di Pisa, and Billy Glass of the Department of Geosciences at the University of Delaware, present the results of a study of four microtektites from the Australasian Strewn Field.

The Australasian Strewn Field covers about 15% of the Earth's surface, and formed about 800 000 years ago through the hypervelocity impact of a chondritic body. It is the youngest and largest of five known Cainozoic strewn fields, with the others  being the North American Strewn Field, which is about 34.86 million years old (Eocene), the Central European Strewn Field, which is about 14.7 million years old (Miocene), the Côte d'Ivoire Strewn Field, which is 1.07 million years old (Pleistocene), and the Central American Strewn Field, which is about 820 000 years old (Pleistocene). No crater has been found which can be linked to the Australasian Strewn Field, although high pressure phases have been found in tektites and other ejecta which strongly indicate that the formation of the field was linked to a crater-forming event, probably in Southeast Asia or the surrounding seas, or possibly in northwest China.

Several previous studies have established that Australasian microtektites show Australasian microtektites with distance from the presumed impact site. For this reason Folco et al. selected three microtektites from deep-sea locations close to the putative impact site, SO95-17957-2,04 and ODP 1144A,01 from the South China Sea, and ODP 769A,15_26 from the Sulu Sea, and one, FRO 2.9-1, from a site in the Transantarctic Mountains. These samples were analysed using a combination of optical microscopy, microanalytical scanning electron microscopy, dual beam microscopy, and microanalytical transmission electron microscopy coupled with three-dimensional electron diffraction.

The Australasian Strewn Field microtektites from deep-sea settings are spheroid in shape and dark brown in colour, with a transluscent laustre and numerous inclusions. They range from 350 to 700 µm in maximum elongation, and dominated by silica phases, compositional bands (schlieren), and vesicles. SO95-17957-2,04 and ODP 1144A,01 have normal composition, whereas ODP 769A,15_26 has a high nickel content (232 µg/g). The Antarctic specimen is a pale-yellow transparent sphere 485 µm in diameter with normal composition,  devoid of vesicles, with only one microscopic silica-rich inclusion,  a few tens of micrometres across with diffuse boundaries. This is considered to be fairly typical of Australasian microtektites from Antarctica, although it is the only Antarctic Tektite known with a silica-rich inclusion.

Micrographs of sectioned Australasian microtektite SO95-17957-2,04. (A) Microtektite is pale brown with teardrop shape. It shows folded schlieren (Sch), microscopic vesicles (V), and mineral inclusions (arrowed). Thick white arrow points to inclusion studied in this work. Optical microscope image, plane polarized light. (B) Same petrographic features as in A in backscattered electron (BSE) image. White rectangle outlines field of view of image in panel C. (C) Close-up BSE image of a quartz (Qtz) + lechatelierite (L) + coesite (Coe) inclusion. Silica phases in the inclusion can be distinguished by their different electron density contrast, which increases from lechatelierite to quartz to coesite. A diffusive boundary layer (Dbl) discontinuously surrounds the inclusion. Dashed line traces location of the dual beam microscopy section. Folco et al. (2025).

Examined under the scanning electron microscope, the microtectites from deep-sea environments were found to contain a inclusions which comprise a matrix of vesiculated lechatelierite (shock-fused quartz glass), with variable proportions of microscopic quartz grains and submicroscopic coesite grains (coesite is a form of silica dioxide which only forms at very high pressures). These inclusions are surrounded by diffusive boundary layers a few microns thick. The quartz grains tend to be arranged around the edge of the inclusions, and themselves be surrounded by grains of coesite, while the interior of the inclusions tends to be dominated by lechatelierite. The quartz grains tend to be anhedral and heavily fractured, while the coesite grains comprise  polycrystalline aggregates of nanoscopic crystals set in silica glass. Towards the interior of the inclusions, the coesite grains become smaller, and comprise a higher proportion of silica-glass. The interior part of the inclusions, while dominated by vesiculated lechatelierite, contain many of these low-coesite 'coesite grains'. All three of these microtektites have essentially the same structure, although ODP 769A,15_26 and ODP 1144A,01 are more vesiculated than SO95-17957-2,04.

The Austrolasian microtektite ODP 769A,15_26. (A) Optical microscope, plain polarised image of the sectioned spherule, showing dark brown colour, prolate shape, schlieren, microscopic vesicles and transparent to partly opaque mineral inclusions. (B) The same textural and compositional features seen under scanning electron microscope, back scattered electron imaging mode. The white rectangle outlines the area of the next panel. (C) Back scattered electron view of a quartz + lechatelierite + coesite inclusion. Silica phases in the inclusion can be distinguished by their different contrast, which increases from lechaterlierite to quartz to coesite. The dashed line marks the position of the dual beam film featured in the next panel. (D) Transmission electron microscope image of a dual beam section showing tens of submicroscopic anhedral coesite grains dispensed in vesiculated lechatelierite. Inset: a close-up view of one coesite grain showing characteristic (010) polysnthetic twinning. The dashed line in panel (C) traces the location of the DB section seen in panel (D). Abbreviations: sch, schlieren; V, vesicle: Qtz, quartz; Coe, coesite; L,  lachatelierite. Thin white arrows indicate inclusions; the thick white arrow indicates the coesite bearing inclusion studied in detail. Folca et al. (2025). 

At the edge of the inclusion in microtektite SO95-17957-2,04 studied with dual beam microscopy, coesite could be seen forming euhedral crystals which overgrow the quartz grains. Towards the lechatelierite core of the inclusion, the coesite can be seen to be segmented in submicroscopict abular grains by a fine network of silica glass veinlets producing the polycrystalline aggregates. These coesite grains show a tartan-like texture similar to that seen in microcrystalline coesite aggregates in silica glass found in shocked porous sandstones. The proportion of silica glass between the segments of coesite within the grains increases towards the core of the inclusion, where anhedral nanoscopic grains of coesite can be found floating free within the lechatelierite core. 

Transmission electron microscopy images of electron transparent dual beam microscopy section of quartz + lechatelierite + coesite inclusions from Australasian deep-sea sediment microtektites. (A) Whole section of coesite-bearing inclusion in microtektite SO95-17957-2,04. White rectangle outlines area featured in panel (B). (B) Textural relationships between quartz (Qtz), coesite (Coe), and lechatelierite (L). Few microscopic quartz relicts at periphery of inclusion are overgrown by euhedral coesite grains with polysynthetic (010) twinning. Toward the core of the inclusion dominated by lechatelierite, coesite is segmented by a network of silica glass veinlets producing polycrystalline aggregates, which then disaggregate with increasing amount of silica glass. White rectangle traces area featured panel (C). (C) Close-up view of euhedral coesite (top) adjacent to polycrystalline aggregate with subhedral outline (bottom). (D) Reconstruction of reciprocal space sampled by three-dimensional electron diffraction from a twinned coesite grain. This picture displays a view of the diffraction volume along hh0 vector. Projections of 00l* and hh0* vectors are indicated. (E) Nanoscopic anhedral coesite grain with embayed crystal boundaries embedded in lechatelierite in microtektite ODP 769A,15_26. (F) Several nanoscopic anhedral coesite grains dispersed in an area of about 2 µm² of lechatelierite in microtektite ODP 1144A,01. Broken-apart grains are arrowed. Vesicle (V). Folco et al. (2025).

Inclusions in the Transantarctic Mountains tektite, FRO 2.9-1, could be seen to be featureless glass undr the scanning electron microscope, with diffuse contact with the glass matrix of the tektite. This host matrix was composed largely of silica, with smaller amounts of aluminium oxide, iron oxide, magnesium oxide, titanium oxide, calcium oxide, potassium oxide, and sodium oxide. The transmission electron microscope confirmed that this composition did not vary through the tektite.

The Australasian microtektite ODP1144A,01. (A) Optical microscope, plane polarized image of the sectioned particle. It is a brown glass broken tear drop. Few microscopic vesicles, and faint schlieren and few mineral inclusions are visible. (B) The same textural and compositional features seen under the scanning electron microscope, back scattered electron imaging mode. The white rectangle outline the area of the next panel. (C) Back scattered electron close-up view of a highly vesiculated quartz + lechatelierite + coesite inclusion. Silica phases in the inclusion can be distinguished by their different contrast, which increases from lechatelierite, to quartz to coesite. The dashed line marks the location of the dual beam film featured in the next panel. (D) Transmission electron microscope image of a dual beam section showing a polycrystalline aggregate of submicroscopіс anhedral coesite grains set in lechatelierite. The dashed line in panel (C) traces the location of the dual beam featured in panel (D). Abbreviations: Sch, schlieren; V, vesicle; Qtz, quartz; Coe, coesite; L, lechatelierite. Thin white arrows indicate inclusions; The thick white arrow indicates the coesite bearing inclusion studied in detail in this work. Folco et al. (2025).

Coesite is a fairly common mineral in settings where quartz-bearing rocks have been subjected to shock metamorphism. Whether it occurs as a metastable phase in shocked rocks that have experienced peak pressures and temperatures much beyond its stability field (i.e. pressures in excess of 10 gigapascals and temperatures in excess of 2700°C) has been debated since the 1960s. There are three current models of coesite formation. The first suggests that coesite may form in a silica melt as pressure decreases rapidly following an impact event. The second model suggests that coesite forms within silica glass at very high pressures, without any melting actually occurring. The third model also sees coesite forming within solid quartz, although this time in porous sandstones as the peak of the pressure wave passes through. 

In the Australasian microtektites, coesite appears to be overgrowing quartz crystals, which Folco et al. interpret as a sign that they formed while the matrix was under high pressure, but still in a solid state. However, the adjacent polycrystalline aggregates consisting of submicroscopic elongated grains of coesite pervaded by silica glass veinlets do indicate that some melting has occurred, possibly of the coesite itself, and the nanoscopic anhedral coesite grains dispersed in the surrounding lechatelierite as evidence to significant melting and dispersal of coesite aggregates. In this scenario the coesite nanocrystals re relicts formed by the melting and dispersal of larger, pre-existing coesite crystals during the shock-metamorphism process.

The presence of coesite in the Australasian microtektites provides information about the location of the putative Southeast Asian impact site. Coesite forms at very high temperatures, but is unstable unless cooled rapidly; it has been estimated that after 10 seconds at very high temperatures then all coesite will have transformed into cristobalite. Since there is no cristobalite in the Australasian microtektites, it can be inferred that quenching was much more rapid in this instance, possibly aided by reactions such as the transformation of quartz and coesite melt into lechatelierite, which is endothermic (absorbs heat).

The abundant quartz and lechatelierite inclusions found in Australasian tektites are generally accepted to be indicative of a quartz-bearing target rock which underwent melting and fusion during the impact event. The boundary between these inclusions and the glass matrix of the microtektites shows varying levels of diffuseness, suggesting that these particles underwent varying levels of digestion into the matrix. The presence of coesite in tektites from deep-sea environments off Southeast Asia supports the idea that these tektites are from close to the impact site, and represent melt spherules formed by compression and depression of the impacted rock during crater formation. The absence of coesite from the Transantarctic Mountains tektite, FRO 2.9-1, could imply that this remained at higher temperatures for longer, allowing all coesite to be reabsorbed, possibly implying this tektite was exposed not just the heat from the origianal impact, but also from deceleration in ambient air or atmospheric re-entry. This would explain the near-total absence of inclusions in these tektites, and also the homogenous distribution of elements and isotopes observed. There is no structure in the microtektite indicative exposure of high pressure, which supports the idea that these more distant microtektites formed by rapid heating of the target rock prior to the actual impact.

The presence of pressure-related minerals and structures in microtektites from the South China and Sulu seas strengthens the argument for a Southeast Asian impact event.

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Antarctanax shackletoni: A new species of Archosauromorph from the Early Triassic Fremouw Formation, of the central Transantarctic Mountains.

Triassic and Jurassic terrestrial Vertebrate fossils have been periodically collected from the central Transantarctic Mountains of Antarctica since the 1960s. The Early Triassic Graphite Mountain assemblage is of particular interest, as it produces Early Triassic fossils that clearly belong to the Lystrosaurus Assemblage of the Karoo Basin in South Africa, as well as other fossils unique to Antarctica, giving insights into the recovery of southern high latitude faunas following the End Permian Extinction.

In a paper published in the Journal of Vertebrate Paleontology on 31 January 2019, Brandon Peecook of the Integrative Research Center at the Field Museum of Natural History, and the Burke Museum and Department of Biology at the University of Washington, Roger Smith of the Evolutionary Studies Institute at the University of the Witwatersrand, and Karoo Palaeontology at the Iziko South African Museum, and Christian Sidor, also of the Burke Museum and Department of Biology at the University of Washington, describe a new species of Archosauromorph from the Early Triassic Fremouw Formation of Graphite Mountain.

The Permian-Triassic boundary at Graphite Mountain has been identified as lying within the Buckley Formation of the Permian-Jurassic Beacon Supergroup. The Buckley formation overlies earlier glacial deposits, and comprises a series of fluvial and lucastrine deposits with several coal beds. The upper part of this formation is made up of a series of palaeosol deposits (fossil soil beds), with the Permian-Triassic Boundary marked by a shift from carbonaceous shales thought to have been laid down on a swampy floodplain with seasonal snowfall, to beds with numerous white claystone-lined root moulds, thought to have been laid down in a warmer, drier climate. Seventeen metres above this boundary the palaeosols of the Buckley Formation are replaced by the sandstones of the Fremouw Formation, which are thought to indicative of a repeating fluvial sequence, with a sequence in which course sandstones are replaced by finer sandstones, then green-grey or red siltstones with root structures, then a return to coarse sandstones. The first members of the Lystrosaurus Assemblage appear eight metres above the base of the Fremouw Formation, with the new Archosauromorph appearing ten metres above this.

Measured section of the main fossil-bearing horizons at Graphite Peak. Vertebrate fossils collected at this locality include (from bottom to top) (1) UWBM95546, unidentified bone, andUWBM95526, a possible Temnospondyl; (2) UWBM 95523, fragmentary material of Lystrosaurus, and UWBM 95524, a large Tetrapod burrow cast; (3) UWBM 95525, articulated skull and anterior skeleton of Lystrosaurus; (4) UWBM 95527 and UWBM 95528, scattered bones of Lystrosaurus, and UWBM 95529, a semi-disarticulated skeleton of Prolacerta; (5) UWBM 95531, Antarctanax shackletoni, new Archosauromorph; (6) UWBM95530, Thrinaxodon liorhinus dentary and scattered postcranial elements; and (7) UWBM88572, Procolophon jaws, and UWBM95522, a partial skull of a small Lapillopsid. Abbreviations: Fe, iron-rich horizon; Fm., Formation; mid., middle; PTB, inferred position of Permo-Triassic boundary. Peecook et al. (2019). 

The Archosauromorphs are a group of Diapsids that appeared during the Permian and rose to dominate terrestrial ecosystems across the Earth in the aftermath of the Permian extinction. The group includes (amongst other things) the Rhynchosaurs, Chelonians (Turtles and Tortoises), Crocodylomorphs, Pterosaurs, and Dinosaurs (including Birds).

The new Archosauromorph is named Antarctanax shackletoni, where ‘Antarctanax’ means ‘Antarctic king’ and ‘shackletoni’ honours the British Antarctic explorer Ernest Shackleton. The species is described from a single specimen which comprises of eight presacral vertebrae, several ribs, a left humerus, left metatarsals I–V, phalanges, unguals, and unidentifiable flat bones on one slab and an almost complete right pes and a dorsal rib on a second, which was adjoining the first but separated during preparation. The specimen is thought to have been mature at the time of death because all neural arches present are completely fused to their centra, with no indications of sutures.

Antarctanax shackletoni, line drawings and photographs of sides 1 (A) and 2 (B) of block 1. Abbreviations: cdv, cervicodorsal vertebra; cv, cervical vertebra; dv, dorsal vertebra; h, humerus; I–V, pedal digits; r, rib. Scale bar equals 5 cm. Peecook et al. (2019).

A phylogenetic analysis carried out using a range of early Amniotes (Vertebrates capable of laying eggs with watertight membranes) strongly suggests that Antarctanax shackletoni is an Archosauroform, i.e. a member of the Archosauromorph group that includes Dinosaurs, Crocodylomorphs, and Pterosaurs, but could not suggest whether it was more closely related to any particular Archosauroform group.

Antarctanax shackletoni, photograph (A) and line drawing (B) of block 2. Abbreviations: I–IV, pedal digits; mt, metatarsal; ph, phalanx; r, rib; t, tarsals. Peecook et al. (2019).

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Molluscs from the Early Cambrian Shackleton Limestone of the Central Transantarctic Mountains.

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.

(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.

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.

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.

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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Microtektites from the Transantarctic Mountains

Microtektites are microscopic glass spherules produced by the vaporisation of material from the Earth's crust due to hypervelocity impacts by extraterrestrial bodies. They are spherical in shape because they are formed by the cooling of liquid silica within the atmosphere. These are found in many locations, but can be linked together into regions related to, but typically far from, impact sites, known as scattered fields. To date four such scattered fields have been found on Earth, known as the Australasian, Central European, Ivory Coast and North American fields, each of which is thought to relate to a different impact event.

In a paper published in the journal Geochemica et Cosmochemica Acta on 6 March 2018, Matthias Van Ginneken of the Department of Earth Science and Engineering at Imperial College London, the Department of Earth Science at The Natural History Museum, the Department of Analytical, Environmental and Geo-Chemistry at the Vrije Universiteit Brussel, and the Laboratoire G-Time, Université Libre de Bruxelles, Mathew Genge, also of the Department of Earth Science and Engineering at Imperial College London, and Ralph Harvey of the Department of Geological Sciences at Case Western Reserve University, describe the discovery of a new microtektite producing area in a glacial moraine near Larkman Nunatak in the Transantarctic Mountains of Antarctica.

The new site and comprises a moraine roughly 1.5 km in length and 700 m wide, orientated in an east-west direction. 250 g of material was removed from this site in 2006, and subsequently washed and sieved, producing 52 glass spherules between 107 and 388 μm in diameter and pale yellow in colour; one of these contains a bubble 10 μm in diameter.

Stereomicrograph of three Larkman Nunatak microtektites. The scalebar is 100 lm. A vesicle is arrowed in (c). Van Ginneken et al. (2018).

Chemically these spherules fall within the Australasian Scattered Field, which have previously been found in deep sea sediments of the Indian and Pacific Ocean. These are the youngest of the four scattered fields, thought to have originated about 800 000 years ago from an impact somewhere in Southeast Asia, probably Vietnam, from which they are spread out in a three-lobed shape. The Antarctic microtektites are the furthest yet known from the presumed location of the impact, about 11 000 km away. 

Sampling location of Larkman Nunatak microtektites. (a) Sketch map showing the sampling sites of Australasian microtektites and the current extension of the Australasian strewn field. (b) Regional map showing the locations where microtektites were found in the Transantarctic Mountains. (c) Panoramic view of Larkman Nunatak. Arrowed is the sampling area where glacial moraine was collected. (c) Detail of the sampling area. Van Ginneken et al. (2018).

The Antarctic microtektites are smaller than any previously found Austrolasian microtektites and more-or-less totally depleted in volatile elements. This fits with a consistent pattern within the Austrolasian Scattered Field, in which the further microtektites are found from the presumed impact site, the smaller and more depleted in volatiles they are. Van Ginneken et al. suggest that this is because the furthest material actually originates from closest to the impact site, and thus was thrown higher into the atmosphere, and therefore the microtektites are more altered the further they are found from this origin.

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