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Caseasauria

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Caseasauria is one of the two main clades of early synapsids, the other being the Eupelycosauria. Caseasaurs are currently known only from the Late Carboniferous and the Permian, and include two superficially different families, the small insectivorous or carnivorous Eothyrididae, and the large, herbivorous Caseidae. These two groups share a number of specialised features associated with the morphology of the snout and external naris.

Caseasaurs
Temporal range: Late Carboniferous-Late Permian, 306–254 Ma
Fossil skeleton of Cotylorhynchus romeri
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: †Caseasauria
Williston, 1912
Families

†Caseidae
†Eothyrididae

The ancestor of caseasaurs can be traced back to an insect eating or an omnivorous reptile-like synapsid from the Pennsylvanian time of the Carboniferous, possibly resembling Archaeothyris, the earliest known synapsid. The caseasaurs were abundant during the later part of the Early Permian, but by the Middle Permian caseasaur diversity declined because the group was outcompeted by the more successful therapsids. The last caseasaurs became extinct at the end of the Guadelupian (Middle Permian).

Description

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Among the most conspicuous characteristics uniting caseasaurs are an enlarged nostril and a snout tip that overhangs the tooth row.[1]

Early caseasaurs, including all eothyridids, were relatively small animals. However, most caseids reached larger sizes, and some caseids, such as Cotylorhynchus and Alierasaurus, were among the largest terrestrial animals of the early Permian. These large herbivorous taxa reached a length of 4 to 6 meters (13 to 20 ft) and a mass of 330 to 500 kilograms (730 to 1,100 lb).[2]

Evolution

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Caseasaurs first appear in the fossil record in the late Carboniferous, alongside many other early amniote groups. The earliest known synapsid, Asaphestera from the Bashkirian age, may be an eothyridid caseasaur.[3] The earliest definitive caseasaur is Eocasea.[4]

Caseids thrived during the Kungurian age, and numerous large herbivorous caseids are known from the Kungurian of the United States.

Caseasaurs are one of the two pelycosaur clades known to have survived into the Guadalupian epoch, along with varanopids. Two caseasaur taxa are known from the Guadalupian of Russia, representing the geologically youngest known caseasaurs: the small, possibly omnivorous or insectivorous Phreatophasma, and the large, herbivorous Ennatosaurus.[5]

Classification

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Seymouriamorpha

Phylogenetic position of Caseasauria within Reptiliomorpha, showing the possible alternate positions of Diadectomorpha. Varanopidae may belong to Sauropsida.

Caseasauria is generally regarded as the most basal clade of synapsids, with all other synapsids being grouped in the clade Eupelycosauria. However, not all studies have supported this position. In 2012, Roger Benson argued that most of the characters supporting a basal position for caseasaurs pertained to the skull, and presented a phylogenetic analysis incorporating more postcranial data that resolved a clade comprising ophiacodontids and varanopids as the basalmost synapsid clade.[6] However, new postcranial data from eothyridids and basal caseids established that caseasaurs were more basal than ophiacodontids and varanopids after all, with the characters supporting a more derived position for caseasaurs being the result of convergent evolution between caseids and more derived synapsids.[4][7] The diadectomorphs, conventionally regarded as anamniote tetrapods, may prove to be synapsids even more basal than Caseasauria.[8][9]

Eocasea martini

Martensius bromackerensis

Casea broili

Oromycter dolesorum

Trichasaurus texensis

Casea nicholsi

Euromycter rutenus

Ennatosaurus tecton

Angelosaurus romeri

Alierasaurus ronchii

Cotylorhynchus romeri

Cotylorhynchus bransoni

Cotylorhynchus hancocki

Phylogeny of Caseidae, after Berman et al. 2020[10]

Most caseasaurs are divided into two families, Eothyrididae and Caseidae. The affinities of the earliest definitive caseasaur, Eocasea, are uncertain, with some phylogenetic analyses finding it to be a caseid and others finding it to be a basal caseasaur belonging to neither family.[11][12]

Three genera are typically regarded as belonging to the family Eothyrididae: Eothyris, Oedaleops, and Vaughnictis. However, some phylogenetic analyses have failed to resolve the eothyridids as a clade, instead finding them to be paraphyletic with respect to Caseidae.[13][11] Asaphestera has been provisionally regarded as an eothyridid as well, without being included in a phylogenetic analysis.[14]

The remaining caseasaurs belong to the family Caseidae.

List of species

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Species of Caseasauria
Genus Species Year Named Family Age Location Notes
Eocasea E. martini 2014 incertae sedis Gzhelian May be a caseid
Asaphestera A. platyris 1934 Eothyrididae Bashkirian Synapsida incertae sedis; may be an eothyridid
Eothyris E. parkeyi 1937
Oedaleops O. campi 1965
Vaughnictis V. smithae 1965 Asselian–Sakmarian Originally assigned to the genus Mycterosaurus
Callibrachion C. gaudryi 1893 Caseidae
Datheosaurus D. macrourus 1904
Oromycter O. dolesorum 2005
Phreatophasma P. aenigmatum 1954 Roadian
Martensius M. bromackerensis 2020
Ruthenosaurus R. russellorum 2011
Casea C. broilii 1910
C. halselli 1954
C. nicholsi 1954
Euromycter E. rutenus 1974 Originally described as Casea rutena
Ennatosaurus E. tecton 1956 The geologically youngest known caseasaur
Caseopsis C. agilis 1962
Caseoides C. sanangelensis 1953
Arisierpeton A. simplex 2019
Alierasaurus A. ronchii 2014
Cotylorhynchus C. romeri 1937
C. hancocki 1953
C. bransoni 1962
Angelosaurus A. dolani 1953
A. greeni 1962
A. romeri 1962
Trichasaurus T. texensis 1910

Paleoecology

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The paleoecology of caseids is debated. They are typically interpreted as terrestrial animals of dry, upland habitats. However, Caseids exhibit a similar bone microstructure to cetaceans and pinnipeds, which has led to the hypothesis that they led an aquatic lifestyle.[15][16] This hypothesis has been challenged on the grounds that their bone microstructure specifically resembles fully pelagic animals, and is unlike the bone microstructure of semiaquatic animals, but that the body plan of caseids is inconsistent with a pelagic lifestyle.[16] Moreover, caseid fossils are predominantly associated with arid upland deposits.

See also

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References

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  1. ^ Angielczyk & Kammerer 2018, p. 125.
  2. ^ Angielczyk & Kammerer 2018, p. 127.
  3. ^ Mann et al. 2020, p. 15.
  4. ^ a b Reisz & Fröbisch 2014.
  5. ^ Brocklehurst & Fröbisch 2017.
  6. ^ Benson 2012.
  7. ^ Brocklehurst et al. 2016.
  8. ^ Marjanović & Laurin 2019.
  9. ^ Klembara et al. 2021.
  10. ^ Berman et al. 2020.
  11. ^ a b Spindler et al. 2018.
  12. ^ Ford & Benson 2020.
  13. ^ Sumida, Pelletier & Berman 2014, pp. 20–21.
  14. ^ Mann et al. 2020.
  15. ^ Lambertz et al. 2016.
  16. ^ a b Angielczyk & Kammerer 2018, p. 128.

Bibliography

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