Fremouw Formation

Fremouw Formation
Stratigraphic range: latest Permian-early Late Triassic252–235 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Unit of	Beacon Supergroup
Sub-units	Lower, middle, upper
Underlies	Falla Formation
Overlies	Buckley Formation
Thickness	up to 1,000 m (3,300 ft)
Lithology
Primary	Sandstone, siltstone, mudstone
Location
Coordinates	84°00′00″S 165°00′00″E / 84.0000°S 165.0000°E / -84.0000; 165.0000
Approximate paleocoordinates	81°48′S 69°48′E / 81.8°S 69.8°E / -81.8; 69.8
Region	Transantarctic Mountains of Antarctica
Type section
Named for	Fremouw Peak, Queen Alexandra Range, Antarctica.[1]
Named by	P. J. Barrett[1]
Fremouw Formation (Antarctica)

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 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 forest at 70–75°S latitude.

The Fremouw Formation is mostly Triassic in age, with the oldest rocks dating back to the latest Permian. Much of the formation is quartzose sandstone that was deposited in stream beds.^[2] It overlies the Permian Buckley Formation, which consists of coal and Glossopteris fossils.^[3] 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 siltstones and mudstones, coarse-grained channel sandstones, and conglomerates.^[4]

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 soil. Smaller fossils on Fremouw Peak include cycads, horsetails, seed ferns, Osmundaceae ferns, and even fungi. One cycad called Antarcticycas is similar in appearance to the living Bowenia of Australia.^[5] 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 fossils are present around the conifer-like stumps, suggesting that they were the leaves of these large trees.^[6] Based on the geology of the area, the trees grew alongside riverbanks and on floodplains. The structure of the plants shows no adaptation towards cold tolerance, suggesting that the climate was much warmer in the Triassic.^[5]

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 rocks, implying a major decline in diversity after the extinction. The presence of many of these groups in Middle Triassic strata indicates that long ghost lineages must have extended back into the Early Triassic. Tetrapods such as temnospondyl amphibians, diapsid reptiles, and dicynodont therapsids were 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 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.^[7]

The first tetrapod, 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 of a temnospondyl amphibian.^[3] The bone was found the previous year by a researcher from Ohio State University who was studying the geology of the Transantarctic Mountains.^[8] 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, therapsids, and archosauriform reptiles have all been found in the formation.^[9] These fossils are found around the Shackleton and Beardmore glaciers, in places such as Gordon Valley and Fremouw Peak.

Taxon	Species	Member	Material	Notes	Images
Austrobrachyops	A. jenseni[8]	Lower Fremouw	Jaw fragment, pterygoid bone, and other small fragments	A nomen dubium based on a combination of material from brachyopid temnospondyls, a dicynodont, and other animals[9]
Antarctosuchus	A. polyodon	Upper Fremouw	A mostly complete skull	A capitosaur temnospondyl
Cryobatrachus	C. kitchingi[8]	Lower Fremouw	A partial skull and other bone fragments	A lydekkerinid temnospondyl
Kryostega	K. collinsoni[10]	Upper Fremouw	Large snout fragment	A stereospondyl temnospondyl
Micropholis	M. stowi[11]	Lower Fremouw	A block containing the skeletons of 4 individuals of the taxon, accounting for much of the skeletons.	A Euskelian temnopodyl
Parotosuchus	P. sp.[9]	Upper Fremouw	Fragment of the right side of the snout.	A temnospondyl
Rhigerpeton	R. isbelli[12]	Lower Fremouw	Partial skull	A lapillopsid temnospondyl
Rhytidosteidae	R. indet.[4]	Lower Fremouw		A temnospondyl

Taxon	Species	Member	Material	Notes	Images
Cynognathus	Cynognathus sp.[15]	Upper Fremouw		A cynodont
Impidens	I. hancoxi[16]	Upper Fremouw	Partial snout	A cynodont, originally described as Diademodontidae indet.[15]
Ericiolacerta	E. parva[17]	Lower Fremouw		A therocephalian
Kannemeyeriidae	Indeterminate.[15]	Upper Fremouw		A dicynodont
Kombuisia	K. antarctica[7]	Lower Fremouw		A dicynodont
Lystrosaurus	L. curvatus[18]	Lower Fremouw		A dicynodont
	L. murrayi[18]	Lower Fremouw		A dicynodont
Myosaurus	Myosaurus gracilis[4]	Lower Fremouw		A dicynodont
Notictoides	N. absens [19]			A therocephalian
Pedaeosaurus	P. parvus[17]	Lower Fremouw		A therocephalian
Rhigosaurus	R. glacialis[17]	Lower Fremouw		A therocephalian
Thrinaxodon	T. liorhinus[20]	Lower Fremouw		A cynodont

• List of fossiliferous stratigraphic units in Antarctica