Thrinaxodon Col () is a rock col 2 nautical miles (3.7 km) southeast of Rougier Hill. The col is along the ridge that trends southward from Rougier Hill in the

Cumulus Hills The Cumulus Hills () are several groups of largely barren hills in Antarctica. Divided by the Logie Glacier, they are bounded by Shackleton Glacier on the west, McGregor Glacier on the north and Zaneveld Glacier on the south. The exposed rock in ...

Queen Maud Mountains The Queen Maud Mountains are a major group of mountains, ranges and subordinate features of the Transantarctic Mountains, lying between the Beardmore and Reedy Glaciers and including the area from the head of the Ross Ice Shelf to the Antarctic P ...

. The name was proposed to

Advisory Committee on Antarctic Names The Advisory Committee on Antarctic Names (ACAN or US-ACAN) is an advisory committee of the United States Board on Geographic Names responsible for recommending commemorative names for features in Antarctica. History The committee was established ...

(US-ACAN) in 1971 by geologist David H. Elliot of the

Ohio State University The Ohio State University, commonly called Ohio State or OSU, is a public land-grant research university in Columbus, Ohio. A member of the University System of Ohio, it has been ranked by major institutional rankings among the best publ ...

Institute of Polar Studies.Stewart, J., 2011. ''Antarctica: An Encyclopedia,'' 2nd ed. Jefferson, North Carolina and London, McFarland & Company, Inc. 1771 pp.

Geology

This col is an important

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

locality at which at least a dozen specimens of the mammal-like reptile ''

Thrinaxodon ''Thrinaxodon'' is an extinct genus of cynodonts, most commonly regarded by its species ''T. liorhinus'' which lived in what are now South Africa and Antarctica during the Early Triassic. ''Thrinaxodon'' lived just after the Permian–Triassic m ...

'' were found. Colbert, E.H., 1986. ''Triassic vertebrates in the Transantarctic Mountains.'' In: Turner, M.D., and Splettstoesser, J.F., eds, pp. 11-35, ''Geology of the central Transantarctic Mountains.'' ''American Geophysical Research Series'', 36. Washington, DC, American Geophysical Union, 453 pp. The

fossiliferous A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

exposed at Thrinaxodon Col consist of alternating

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

s and

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

s of the lower

Fremouw Formation The Fremouw Formation is a Triassic-age rock formation in the Transantarctic Mountains of Antarctica. It contains the oldest known fossils of tetrapods from Antarctica, including synapsids, reptiles and amphibians. Fossilized trees have also been ...

. At this location, the base of the Fremouw Formation overlies a younger

diabase Diabase (), also called dolerite () or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-graine ...

sill. Diabase sills and

dikes Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes, ...

cross-cut the Fremouw Formation throughout the Thrinaxodon Col area and region.Collinson, J.W., and Elliot, D.H., 1984. ''Triassic stratigraphy of the Shackleton Glacier area.'' In Turner, M.C., and Splettstoesser, J.F., eds., ''Geology of the central Transantarctic Mountains.'' ''American Geophysical Union Antarctic Series'', 36, pp.103–117.Collinson, J.W., Hammer, W.R., Askin, R.A. and Elliot, D.H., 2006. ''Permian-Triassic boundary in the central Transantarctic Mountains, Antarctica.'' ''Geological Society of America Bulletin'', 118(5-6), pp.747-763. The sandstones and mudstones exposed at Thrinaxodon Col are coupled together as

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

ﬁning-upward cycles. An individual cycle consists of a basal fluvial sandstone that grades upward into an upper

overbank An overbank is an alluvial geological deposit consisting of sediment that has been deposited on the floodplain of a river or stream by flood waters that have broken through or overtopped the banks. The sediment is carried in suspension, and becaus ...

mudstone. An

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

separates the top of the upper mudstone from the overlying basal sandstone of the overlying cycle. For example, the lowermost sandstone-mudstone cycle consists of a basal -thick

coarse-grained Granularity (also called graininess), the condition of existing in granules or grains, refers to the extent to which a material or system is composed of distinguishable pieces. It can either refer to the extent to which a larger entity is subd ...

sandstone that grades upward into a -m-thick medium-grained sandstone. The medium-grained sandstone contains

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too sm ...

clast Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

pebble A pebble is a clast of rock with a particle size of based on the Udden-Wentworth scale of sedimentology. Pebbles are generally considered larger than granules ( in diameter) and smaller than cobbles ( in diameter). A rock made predominant ...

s, and

logs. Overlying and associated with the underlying sandstones is a 6 m of green-gray

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

and mudstone with abundant fossil rootlets near the top.Sidor, C.A., McIntosh, J.A., Gee, B.M., Hammer, W.R., Makovicky, P.J., Smith, N.D., Smith, R.M.H., Tabor, N.J., Whitney, M.R. and Woolley, C.H., 2023. ''The Fremouw Formation of Antarctica: Updated vertebrate fossil record and reevaluation of high-latitude Permian–Triassic paleoenvironments.'' ''Earth-Science Reviews'', no.104587.

Fossils

Thrinaxodon Col is a

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

fossil locality from which fossil jaws and articulated skeletons have been collected. This fossil locality has yielded a far richer and well-preserved collection of vertebrate fossils than fossil localities associated Coalsack Bluff. Twelve of 14 specimens of ''Thrinaxodon liorhinus'' were collected from Thrinaxodon Col along with part of a large

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typicall ...

tusk, specimens of ''

Prolacerta ''Prolacerta'' is a genus of archosauromorph from the lower Triassic of South Africa and Antarctica. The only known species is ''Prolacerta broomi''. The generic name ''Prolacerta'' is derived from Latin meaning “before lizard” and its speci ...

broomi'', and unidentified cynodontia.Colbert, E.H. 1987., ''The Triassic reptile Prolacerta in Antarctica.'' ''American Museum Novitates'', 2882: 1–19.Kitching, J.W., Collinson, J.W., Elliot, D.H. and Colbert, E.H., 1972. ''Lystrosaurus zone (Triassic) fauna from Antarctica.'' ''Science'', 175(4021), pp.524-527. At Thrinaxodon Col, the oldest vertebrate fossils occur in a sandstone bed about above the top of the basal log-bearing sandstone. The Permian-Triassic boundary lies within this -thick interval above the log-bearing sandstone and below the ﬁrst appearance of vertebrate fossils. The ''Thrinaxodon liorhinus'' remains occur in Triassic sandstone that fills the upper portion of a paleochannel."Retallack, G.J. and Hammer, W.R., 1996. ''Paleoenvironment of the Triassic therapsid Lystrosaurus in the central Transantarctic Mountains, Antarctica.'' ''Antarctic Journal of the United States'', 31, pp.33-34.

References

Paleontological sites of Antarctica Mountain passes of the Ross Dependency Dufek Coast {{DufekCoast-geo-stub

Geology

Fossils

See also

References