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Fremouw Formation

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.

Fremouw Formation
Stratigraphic range: latest Permian-early Late Triassic252–235 Ma
TypeGeological formation
Unit ofBeacon Supergroup
Sub-unitsLower, middle, upper
UnderliesFalla Formation
OverliesBuckley Formation
Thicknessup to 1,000 m (3,300 ft)
Lithology
PrimarySandstone, siltstone, mudstone
Location
Coordinates84°00′00″S 165°00′00″E / 84.0000°S 165.0000°E / -84.0000; 165.0000
Approximate paleocoordinates81°48′S 69°48′E / 81.8°S 69.8°E / -81.8; 69.8
RegionTransantarctic Mountains of Antarctica
Type section
Named forFremouw Peak, Queen Alexandra Range, Antarctica.[1]
Named byP. J. Barrett[1]
Fremouw Formation is located in Antarctica
Fremouw Formation
Fremouw Formation (Antarctica)

Stratigraphy

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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]

Paleoenvironment

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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.[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]

Biota

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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.

Tetrapods

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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

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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

Reptiles

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Taxon Species Member Material Notes Images
Antarctanax

A. shackletoni[13]

Lower Fremouw

Eight presacral vertebrae, left humerus, ribs, feet

Medium-sized archosauriform

Archosauriformes Indeterminate.[14]

Lower Fremouw

Partial presacral vertebra and left humerus

A large-bodied archosauriform reptile

Palacrodon

P. browni[14]

Lower Fremouw

An enigmatic diapsid reptile; initially named Fremouwsaurus geludens

Procolophon

P. trigoniceps[14]

Lower Fremouw

A procolophonid reptile

 

Prolacerta

P. broomi[14]

Lower Fremouw

A prolacertiform reptile

 

Synapsids

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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

 

See also

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References

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  1. ^ a b Barrett, P.J., 1969, Stratigraphy and petrology of the mainly fluviatile Permian and Triassic Beacon rocks, Beardmore Glacier area, Antarctica. Ohio State University Institute of Polar Studies Report 34, pp1-132 .
  2. ^ 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. S2CID 10995542.
  3. ^ 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. PMID 5659679. S2CID 28006253.
  4. ^ a b c "Background". Transantarctic Vertebrate Paleontology Project. Augustana College. 2008. Archived from the original on 7 July 2011. Retrieved 13 July 2011.
  5. ^ a b 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. Archived from the original (PDF) on 2012-03-23. Retrieved 2011-07-13.
  6. ^ 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.
  7. ^ 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. S2CID 20557454.
  8. ^ a b c Colbert, E.H.; Cosgriff, J.W. (1974). "Labyrinthodont amphibians from Antarctica". American Museum Novitates (2552): 1–30. hdl:2246/2750.
  9. ^ 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.
  10. ^ 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.
  11. ^ Gee, Bryan M.; Sidor, Christian A. (2021-05-21). "First record of the amphibamiform Micropholis stowi from the lower Fremouw Formation (Lower Triassic) of Antarctica". Journal of Vertebrate Paleontology. 41 (1): e1904251. Bibcode:2021JVPal..41E4251G. doi:10.1080/02724634.2021.1904251. ISSN 0272-4634.
  12. ^ Gee, Bryan M.; Beightol, Charles V.; Sidor, Christian A. (2023-06-28). "A new lapillopsid from Antarctica and a reappraisal of the phylogenetic relationships of early diverging stereospondyls". Journal of Vertebrate Paleontology. doi:10.1080/02724634.2023.2216260. ISSN 0272-4634.
  13. ^ 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. S2CID 92116260.
  14. ^ 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" (PDF). Journal of Vertebrate Paleontology. 31 (4): 784–797. Bibcode:2011JVPal..31..784S. doi:10.1080/02724634.2011.586662. S2CID 129231144.
  15. ^ 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. Bibcode:1995JVPal..15..105H. doi:10.1080/02724634.1995.10011210.
  16. ^ Tolchard, Frederick; Kammerer, Christian F.; Butler, Richard J.; Hendrickx, Christophe; Benoit, Julien; Abdala, Fernando; Choiniere, Jonah N. (2021-07-26). "A new large gomphodont from the Triassic of South Africa and its implications for Gondwanan biostratigraphy". Journal of Vertebrate Paleontology. 41 (2): e1929265. Bibcode:2021JVPal..41E9265T. doi:10.1080/02724634.2021.1929265. ISSN 0272-4634. S2CID 237517965.
  17. ^ a b c Colbert, E.H.; Kitching, J.W. (1981). "Scaloposaurian reptiles from the Triassic of Antarctica". American Museum Novitates (2709): 1–22. hdl:2246/5362.
  18. ^ a b Colbert, E.H. (1974). "Lystrosaurus from Antarctica" (PDF). American Museum Novitates (2535): 1–44.
  19. ^ Sidor, Christian A.; Kulik, Zoe T.; Huttenlocker, Adam K. (2021-10-01). "A new bauriamorph therocephalian adds a novel component to the Lower Triassic tetrapod assemblage of the Fremouw Formation (Transantarctic Basin) of Antarctica". Journal of Vertebrate Paleontology. 41 (6): e2081510. Bibcode:2021JVPal..41E1510S. doi:10.1080/02724634.2021.2081510. ISSN 0272-4634.
  20. ^ Colbert, E.H.; Kitching, J.W. (1977). "Triassic cynodont reptiles from Antarctica". American Museum Novitates (2611): 1–30. hdl:2246/2011.
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