Fremouw Formation
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The Fremouw Formation 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 ...
-age
rock formation A rock formation is an isolated, scenic, or spectacular surface rock outcrop. Rock formations are usually the result of weathering and erosion sculpting the existing rock. The term ''rock formation'' can also refer to specific sediment ...
in the
Transantarctic Mountains The Transantarctic Mountains (abbreviated TAM) comprise a mountain range of uplifted (primarily sedimentary rock, sedimentary) rock in Antarctica which extend, with some interruptions, across the continent from Cape Adare in northern Victoria La ...
of
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
. It contains the oldest known fossils of tetrapods from Antarctica, including synapsids, reptiles and amphibians. Fossilized trees have also been found. The formation's beds were deposited along the banks of rivers and on floodplains. During the Triassic, the area would have been a
riparian A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the terrestrial biomes of the Earth. Plant habitats and communities along the river margins and banks ar ...
forest at 70–75°S latitude.


Stratigraphy

The Fremouw Formation is mostly Triassic in age, with the oldest rocks dating back to the latest
Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...
. Much of the formation is quartzose sandstone that was deposited in stream beds. It overlies the Permian Buckley Formation, which consists of coal and ''
Glossopteris ''Glossopteris'' tymology: from Ancient Greek γλῶσσα (glôssa, " tongue ") + πτερίς (pterís, " fern ")is the largest and best-known genus of the extinct Permian order of seed ferns known as Glossopteridales (also known as Arberia ...
'' fossils. The formation is informally divided into lower, middle, and upper units. Most fossils are found in the Lower Fremouw Formation. Here, bones are preserved in fine grained
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 ...
s and
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, coarse grained channel sandstones, and conglomerates.


Paleoenvironment

Well-preserved plants are common in the Fremouw Formation. Logs have been found in channel deposits and roots and stems have been found in
permineralized Permineralization is a process of fossilization of bones and tissues in which mineral deposits form internal casts of organisms. Carried by water, these minerals fill the spaces within organic tissue. Because of the nature of the casts, perminera ...
soil. Smaller fossils on Fremouw Peak include
cycad Cycads are seed plants that typically have a stout and woody (ligneous) trunk (botany), trunk with a crown (botany), crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants o ...
s,
horsetail ''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds. ''Equisetum'' is a "living fossil", the only living genus of the entire subclass Eq ...
s,
seed fern A seed is an Plant embryogenesis, embryonic plant enclosed in a testa (botany), protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, includ ...
s,
Osmundaceae Osmundaceae (royal fern family) is a family of ferns containing four to six extant genera and 18–25 known species. It is the only living family of the order Osmundales in the class Polypodiopsida (ferns) or in some classifications the only ord ...
ferns, and even
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
. One cycad called '' Antarcticycas'' is similar in appearance to the living ''
Bowenia The genus ''Bowenia'' includes two living and two fossil species of cycads in the family Stangeriaceae, sometimes placed in their own family Boweniaceae. They are entirely restricted to Australia. The two living species occur in Queensland. '' ...
'' of
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
. In 2003, 99 fossilized tree trunks were described from Gordon Valley. These trunks comprise an intact fossilized forest, allowing for an estimation of the distribution of plants and tree cover. ''
Dicroidium ''Dicroidium'' is an extinct genus of fork-leaved seed ferns that were widely distributed over Gondwana during the Triassic (). Their fossils are known from South Africa, the Arabian Peninsula, Australia, New Zealand, South America, Madagascar ...
'' fossils are present around the conifer-like stumps, suggesting that they were the leaves of these large trees. Based on the geology of the area, the trees grew alongside riverbanks and on floodplains. The structure of the plants show no adaptation toward cold tolerance, suggesting that the climate was much warmer in the Triassic. The Fremouw Formation preserves many tetrapod fossils that span the Permo-Triassic boundary, which marks the Permo-Triassic mass extinction. Around the world, the fossil record of many tetrapod groups is absent or very limited in
Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a un ...
rocks, implying a major decline in diversity after the extinction. The presence of many of these groups in
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...
strata indicate that long
ghost lineage A ghost lineage is a hypothesized ancestor in a species lineage that has left no fossil evidence yet can be inferred to exist because of gaps in the fossil record or genomic evidence. The process of determining a ghost lineage relies on fossilized ...
s must have extended back into the Early Triassic. Tetrapods such as temnospondyl amphibians, diapsid reptiles, and dicynodont therapsids are common in the Late Permian and seemed to have recovered by the Middle Triassic, but there is little record of their presence in the Early Triassic. All of these tetrapods are present in Early Triassic strata of the Fremouw Formation, suggesting that Antarctica served as a refugium for these animals. During the extinction, global temperatures rose and the supercontinent
Pangea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
moved northward, putting pressure on populations that could not adapt to the warming climate. Antarctica, while much warmer in the Early Triassic than it is today, was cooler than other parts of
Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
and may have been more hospitable to tetrapod populations. Antarctica's milder climate allowed many groups to take refuge in the region while other populations experienced decline. In the Early Triassic, many Fremouw Formation tetrapods had smaller body sizes than their Permian ancestors, and many were adapted for burrowing. Both of these characteristics are seen as adaptations to Antarctica's greater seasonal variability and protracted day-night cycles.


Biota

The first
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...
or land-living vertebrate from Antarctica was found in the Fremouw Formation and described in 1968. It was represented by a small bone fragment that is probably part of the left
mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
of a
temnospondyl Temnospondyli (from Greek language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order (biology), order of small to giant tetrapods—often considered Labyrinthodontia, primitive amphi ...
amphibian. The bone was found the previous year by a researcher from
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 ...
who was studying the geology of the Transantarctic Mountains. The animal was later named '' Austrobrachyops jenseni''. After its discovery, paleontological expeditions were launched to the area around the Beardmore Glacier to uncover more fossils. Since then, fragmentary remains of temnospondyls,
therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...
s, and archosauriform reptiles have all been found in the formation. These fossils are found around the
Shackleton Sir Ernest Henry Shackleton (15 February 1874 – 5 January 1922) was an Anglo-Irish Antarctic explorer who led three British expeditions to the Antarctic. He was one of the principal figures of the period known as the Heroic Age of An ...
and Beardmore glaciers in places such as Gordon Valley and Fremouw Peak.


Tetrapods


Temnospondyls


Reptiles


Synapsids


See also

*
List of fossiliferous stratigraphic units in Antarctica This is a list of fossiliferous stratigraphic units in Antarctica. __NOTOC__ List of fossiliferous stratigraphic units See also * Geology of Antarctica * List of fossiliferous stratigraphic units in Namibia * List of fossiliferous st ...


References

{{reflist, refs= {{Cite journal, doi = 10.1080/02724634.2021.1929265, issn = 0272-4634, pages = e1929265, last1 = Tolchard, first1 = Frederick, last2 = Kammerer, first2 = Christian F., last3 = Butler, first3 = Richard J., last4 = Hendrickx, first4 = Christophe, last5 = Benoit, first5 = Julien, last6 = Abdala, first6 = Fernando, last7 = Choiniere, first7 = Jonah N., title = A new large gomphodont from the Triassic of South Africa and its implications for Gondwanan biostratigraphy, journal = Journal of Vertebrate Paleontology, date = 2021-07-26, volume = 41, issue = 2, s2cid = 237517965, url = https://doi.org/10.1080/02724634.2021.1929265 Geologic formations of Antarctica Triassic Antarctica Permian System of Antarctica Triassic System of Antarctica Sandstone formations Siltstone formations Mudstone formations Fluvial deposits Paleontology in Antarctica Permian south paleopolar deposits Dufek Coast