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 per ...
-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 sedime ...
in the
Transantarctic Mountains 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 cont ...
. 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 a ...
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 Paleo ...
. Much of the formation is
quartzose sandstone
A quartz arenite or quartzarenite is a sandstone composed of greater than 90% detrital quartz. Quartz arenites are the most mature sedimentary rocks possible, and are often referred to as ultra- or super-mature, and are usually cemented by sili ...
that was deposited in stream beds.
It overlies the Permian
Buckley Formation, which consists of coal and ''
Glossopteris'' 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, ...
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
conglomerate
Conglomerate or conglomeration may refer to:
* Conglomerate (company)
* Conglomerate (geology)
* Conglomerate (mathematics)
In popular culture:
* The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes
** ...
s.
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 with a crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants of a species are either male 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 ...
s,
seed ferns,
Osmundaceae 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 fr ...
. 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 country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
.
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 ...
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 an ...
strata indicate that long
ghost lineages 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 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 sta ...
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 t ...
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 teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
of a
temnospondyl
Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carb ...
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 pub ...
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
The Beardmore Glacier in Antarctica is one of the largest valley glaciers in the world, being long and having a width of . It descends about from the Antarctic Plateau to the Ross Ice Shelf and is bordered by the Commonwealth Range of the Que ...
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
Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthier ...
reptiles have all been found in the formation. These fossils are found around the Shackleton and Beardmore glaciers in places such as Gordon Valley Gordon Valley () is a small valley, the western half of which is occupied by a lobe of ice from Walcott Neve, lying west of Mount Falla in the Queen Alexandra Range, Antarctica. It was named by the Advisory Committee on Antarctic Names after Mark A ...
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