Fremouw Formation

The Fremouw Formation is a Triassic-age rock formation in the Transantarctic Mountains of Antarctica. Fossils of prehistoric reptiles and amphibians have been found in the formation. 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.

Fremouw Formation Stratigraphic range: Late Permian-Late Triassic
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	84.0000°S 165.0000°E
Approximate paleocoordinates	81.8°S 69.8°E
Region	Transantarctic Mountains of Antarctica
Fremouw Formation (Antarctica)

Stratigraphy

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.[1] It overlies the Permian Buckley Formation, which consists of coal and Glossopteris fossils.[2] 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.[3]

Biota

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.[2] The bone was found the previous year by a researcher from Ohio State University who was studying the geology of the Transantarctic Mountains.[4] 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.[5] These fossils are found around the Shackleton and Beardmore glaciers in places such as Gordon Valley and Fremouw Peak.

Tetrapods

Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon		Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited.

Temnospondyls

Taxon	Member	Material	Notes	Images
Austrobrachyops jenseni[4]	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[5]
Antarctosuchus polyodon	Upper Fremouw	A mostly complete skull	A capitosaur temnospondyl
Cryobatrachus kitchingi[4]	Lower Fremouw	A partial skull and other bone fragments	A lydekkerinid temnospondyl
Kryostega collinsoni[6]	Upper Fremouw	Large snout fragment	A stereospondyl temnospondyl
Parotosuchus sp.[5]	Upper Fremouw	Fragment of right side of snout	A temnospondyl
Rhytidosteidae indet.[3]	Lower Fremouw		A temnospondyl

Reptiles

Taxon	Member	Material	Notes	Images
Antarctanax shackletoni[7]	Lower Fremouw	Eight presacral vertebrae, left humerus, ribs, feet	Medium-sized archosauriform
Archosauriformes indet.[8]	Lower Fremouw	Partial presacral vertebra and left humerus	A large-bodied archosauriform reptile
Palacrodon browni[8]	Lower Fremouw		An enigmatic diapsid reptile; initially named Fremouwsaurus geludens
Procolophon trigoniceps[8]	Lower Fremouw		A procolophonid reptile
Prolacerta broomi[8]	Lower Fremouw		A prolacertiform reptile

Synapsids

Taxon	Member	Material	Notes	Images
Cynognathus sp.[9]	Upper Fremouw		A cynodont
Diademodontidae indet.[9]	Upper Fremouw		A cynodont
Ericiolacerta parva[10]	Lower Fremouw		A therocephalian
Kannemeyeriidae indet.[9]	Upper Fremouw		A dicynodont
Kombuisia antarctica[11]	Lower Fremouw		A dicynodont
Lystrosaurus curvatus[12]	Lower Fremouw		A dicynodont
Lystrosaurus murrayi[12]	Lower Fremouw		A dicynodont
Myosaurus gracilis[3]	Lower Fremouw		A dicynodont
Pedaeosaurus parvus[10]	Lower Fremouw		A therocephalian
Rhigosaurus glacialis[10]	Lower Fremouw		A therocephalian
Thrinaxodon liorhinus[13]	Lower Fremouw		A cynodont

Paleoenvironment

Cycads from the Fremouw Formation are similar to the living Bowenia from Australia

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.[14] 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.[15] 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.[14]

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 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 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.[11]

See also

• List of fossiliferous stratigraphic units in Antarctica

References

1. Elliot, D.H.; Colbert, E.H.; Breed, W.J.; Jensen, J.A.; Powell, J.S. (1970). "Triassic tetrapods from Antarctica: evidence for continental drift". Science. 169 (3951): 1197–1201. Bibcode:1970Sci...169.1197E. doi:10.1126/science.169.3951.1197. PMID 17815934.
2. Barrett, P.J.; Baillie, R.J.; Colbert, E.H. (1968). "Triassic amphibian from Antarctica". Science. 161 (3840): 460–462. Bibcode:1968Sci...161..460B. doi:10.1126/science.161.3840.460. PMID 5659679.
3. "Background". Transantarctic Vertebrate Paleontology Project. Augustana College. 2008. Archived from the original on 7 July 2011. Retrieved 13 July 2011.
4. Colbert, E.H.; Cosgriff, J.W. (1974). "Labyrinthodont amphibians from Antarctica". American Museum Novitates. 2552: 1–30. hdl:2246/2750.
5. Sidor, C.A.; Steyer, J.S.; Damiani, R. (2007). "Parotosuchus (Temnospondyli: Mastodonsauridae) from the Triassic of Antarctica". Journal of Vertebrate Paleontology. 27 (1): 232–235. doi:10.1671/0272-4634(2007)27[232:PTMFTT]2.0.CO;2.
6. Sidor, C.A.; Damiani, R.; Hammer, W.R. (2008). "A new Triassic temnospondyl from Antarctica and a review of Fremouw Formation biostratigraphy". Journal of Vertebrate Paleontology. 28 (3): 656–663. doi:10.1671/0272-4634(2008)28[656:ANTTFA]2.0.CO;2.
7. Peecook, Brandon R.; Smith, Roger M. H.; Sidor, Christian (2019). "A novel archosauromorph from Antarctica and an updated review of a high-latitude vertebrate assemblage in the wake of the end-Permian mass". Journal of Vertebrate Paleontology: 1–16. doi:10.1080/02724634.2018.1536664. ISSN 0272-4634.
8. Smith, N.D.; Crandall, J.R.; Hellert, S.M.; Hammer, W.R.; Makovicky, P.J. (2011). "Anatomy and affinities of large archosauromorphs from the lower Fremouw Formation (Early Triassic) of Antarctica" (PDF). Journal of Vertebrate Paleontology. 31 (4): 784–797. doi:10.1080/02724634.2011.586662.
9. Hammer, W.R. (1995). "New therapsids from the Upper Fremouw Formation (Triassic) of Antarctica". Journal of Vertebrate Paleontology. 15 (1): 105–112. doi:10.1080/02724634.1995.10011210.
10. Colbert, E.H.; Kitching, J.W. (1981). "Scaloposaurian reptiles from the Triassic of Antarctica". American Museum Novitates. 2709: 1–22. hdl:2246/5362.
11. Fröbisch, J.; Angielczyk, K.D.; Sidor, C.A. (2010). "The Triassic dicynodont Kombuisia (Synapsida, Anomodontia) from Antarctica, a refuge from the terrestrial Permian-Triassic mass extinction". Naturwissenschaften. 97 (2): 187–196. Bibcode:2010NW.....97..187F. doi:10.1007/s00114-009-0626-6. PMID 19956920.
12. Colbert, E.H. (1974). "Lystrosaurus from Antarctica" (PDF). American Museum Novitates. 2535: 1–44.
13. Colbert, E.H.; Kitching, J.W. (1977). "Triassic cynodont reptiles from Antarctica". American Museum Novitates. 2611: 1–30. hdl:2246/2011.
14. Taylor, E.L.; Taylor, T.N. (1993). "Fossil tree rings and paleoclimate from the Triassic of Antarctica" (PDF). In Lucas, S.G.; Morales, M. (eds.). The Nonmarine Triassic. Albuquerque: The New Mexico Museum of Natural History and Science Bulletin. pp. 453–455.
15. Cúneo, N.R.; Taylor, E.L.; Taylor, T.N.; Krings, M. (2003). "In situ fossil forest from the upper Fremouw Formation (Triassic) of Antarctica: paleoenvironmental setting and paleoclimate analysis". Palaeogeography, Palaeoclimatology, Palaeoecology. 197 (3–4): 239–261. Bibcode:2003PPP...197..239C. doi:10.1016/s0031-0182(03)00468-1.