Fremouw Formation

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Fremouw Formation
Stratigraphic range: Upper Permian - Upper Triassic
Beardmore Glacier - Antarctica.JPG
The Beardmore Glacier, which overlies parts of the Fremouw Formation.
Type Geological formation
Unit of Victoria Group
Sub-units Lower, Middle, Upper
Underlies Falla Formation
Overlies Buckley Formation
Thickness up to 1,000 metres (3,300 ft)
Lithology
Primary Sandstone, siltstone, mudstone
Location
Region Transantarctic Mountains of Antarctica

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.

Stratigraphy[edit]

The Fremouw Formation is Triassic in age, although the oldest rocks date 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[edit]

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[edit]

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.

Amphibians[edit]

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 amphibians, a dicynodont, and other animals[5]

Cryobatrachus kitchingi[4]

Lower Fremouw

A partial skull and other bone fragments

A temnospondyl

Cryobatrachus kitchingi.jpg

Kryostega collinsoni[6]

Upper Fremouw

Large snout fragment

A temnospondyl

Parotosuchus sp.[5]

Upper Fremouw

Fragment of right side of snout

A temnospondyl

Parotosuchus BW.jpg

Rhytidosteidae indet.[3]

Lower Fremouw

A temnospondyl

Reptiles[edit]

Taxon Member Material Notes Images

Archosauriformes indet.[7]

Lower Fremouw

Partial presacral vertebra and left humerus

A large-bodied archosauriform reptile

Palacrodon broomi[7]

Lower Fremouw

An enigmatic diapsid reptile; initially named Fremouwsaurus geludens

Procolophon trigoniceps[7]

Lower Fremouw

A procolophonid reptile

Prolacerta broomi[7]

Lower Fremouw

A prolacertiform reptile

Synapsids[edit]

Taxon Member Material Notes Images

Cynognathus sp.[8]

Upper Fremouw

A cynodont

Diademodontidae indet.[8]

Upper Fremouw

A cynodont

Ericiolacerta parva[9]

Lower Fremouw

A therocephalian

Kannemeyeriidae indet.[8]

Upper Fremouw

A dicynodont

Kombuisia antarctica[10]

Lower Fremouw

A dicynodont

Lystrosaurus curvatus[11]

Lower Fremouw

A dicynodont

Lystrosaurus murrayi[11]

Lower Fremouw

A dicynodont

Lystrosaurus BW.jpg

Myosaurus gracilis[3]

Lower Fremouw

A dicynodont

Myosaurus.jpg

Pedaeosaurus parvus[9]

Lower Fremouw

A therocephalian

Rhigosaurus glacialis[9]

Lower Fremouw

A therocephalian

Thrinaxodon liorhinus[12]

Lower Fremouw

A cynodont

Thrinaxodon liorhinus AMNH 9516 cast.jpg

Paleoenvironment[edit]

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.[13] 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.[14] 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.[13]

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.[10]

References[edit]

  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. 
  2. ^ a b 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. 
  3. ^ a b c "Background". Transantarctic Vertebrate Paleontology Project. Augustana College. 2008. Retrieved 13 July 2011. 
  4. ^ a b c Colbert, E.H.; Cosgriff, J.W. (1974). "Labyrinthodont amphibians from Antarctica". American Museum Novitates 2552: 1–30. 
  5. ^ a b c 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. ^ a b c d 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". Journal of Vertebrate Paleontology 31 (4): 784–797. doi:10.1080/02724634.2011.586662. 
  8. ^ a b c 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. 
  9. ^ a b c Colbert, E.H.; Kitching, J.W. (1981). "Scaloposaurian reptiles from the Triassic of Antarctica". American Museum Novitates 2709: 1–22. 
  10. ^ a b 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. 
  11. ^ a b Colbert, E.H. (1974). "Lystrosaurus from Antarctica". American Museum Novitates 2535: 1–44. 
  12. ^ Colbert, E.H.; Kitching, J.W. (1977). "Triassic cynodont reptiles from Antarctica". American Museum Novitates 2611: 1–30. 
  13. ^ a b Taylor, E.L.; Taylor, T.N. (1993). "Fossil tree rings and paleoclimate from the Triassic of Antarctica". In Lucas, S.G. and Morales, M. (eds.). The Nonmarine Triassic. Albuquerque: The New Mexico Museum of Natural History and Science Bulletin. pp. 453–455. 
  14. ^ 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. doi:10.1016/s0031-0182(03)00468-1.