Caseasauria

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[1] and the Permian, and include two superficially different families, the small insectivorous or carnivorous Eothyrididae, and the large, herbivorous, potentially aquatic[2] 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-Middle Permian, 306–272 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Fossil skeleton of Cotylorhynchus romeri
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	†Caseasauria Williston, 1912
Genera and families
†Caseidae †Eothyrididae †Phreatophasma?

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).[3]

Phylogeny

Most uncertainty in the phylogeny of synapsids lies among the earliest members of the group, including forms traditionally placed within Pelycosauria. As one of the earliest phylogenetic analyses, Brinkman & Eberth (1983) placed the family Varanopidae with Caseasauria as the most basal offshoot of the synapsid lineage. Reisz (1986) removed Varanopidae from Caseasauria, placing it in a more derived position on the stem. While most analyses find Caseasauria to be the most basal synapsid clade, the analysis of Benson (2012) placed a clade containing Ophiacodontidae and Varanopidae as the most basal synapsids, with Caseasauria occupying a more derived position. Benson attributed this revised phylogeny to the inclusion of postcranial characteristics, or features of the skeleton other than the skull, in his analysis. When only cranial or skull features were included, Caseasauria remained the most basal synapsid clade. Below is a cladogram modified from the analysis of Benson (2012):[4]

Tseajaia campi     Limnoscelis paludis   Amniota     Captorhinus spp.     Protorothyris archeri     Synapsida     Ophiacodontidae     Varanopidae       Eupelycosauria   Ianthodon schultzei       Edaphosauridae   Sphenacodontia   Haptodus garnettensis       Pantelosaurus saxonicus       Therapsida     Sphenacodontidae             Caseasauria Eothyrididae   Eothyris parkeyi     Oedaleops campi     Caseidae   Oromycter dolesorum       Casea broilii       Trichasaurus texensis       Euromycter rutenus (="Casea" rutena)       Ennatosaurus tecton       Angelosaurus romeri       Cotylorhynchus romeri       Cotylorhynchus bransoni     Cotylorhynchus hancocki

However, more recent examination of the phylogeny of basal synapsids, incorporating newly described basal caseids and eothyridids,[5] returned Caseasauria to its position as the sister to all other synapsids. Brocklehurst et al. (2016) [6] demonstrated that many of the postcranial characters used by Benson (2012) to unite Caseasauria with Sphenacodontidae and Edaphosauridae were absent in the newly discovered postcranial material of eothyridids, and were therefore acquired convergently.

See also

• Evolution of mammals
• Vertebrate paleontology
• Permian tetrapods

References

1. Robert R. Reisz and Jörg Fröbisch (2014). "The Oldest Caseid Synapsid from the Late Pennsylvanian of Kansas, and the Evolution of Herbivory in Terrestrial Vertebrates". PLoS ONE. 9 (4): e94518. doi:10.1371/journal.pone.0094518. PMC 3989228. PMID 24739998.
2. Lambertz, M.; Shelton, C.D.; Spindler, F.; Perry, S.F. (2016). "A caseian point for the evolution of a diaphragm homologue among the earliest synapsids". Annals of the New York Academy of Sciences. 1385 (1): 3–20. doi:10.1111/nyas.13264. PMID 27859325.
3. Maddin, H.C., Sidor, C.A. & Reisz, R.R. 2008. Cranial anatomy of Ennatosaurus tecton (Synapsida: Caseidae) from the Middle Permian of Russia and the evolutionary relationships of Caseidae. Journal of Vertebrate Paleontology (28): 160-180
4. Benson, R.J. (2012). "Interrelationships of basal synapsids: cranial and postcranial morphological partitions suggest different topologies". Journal of Systematic Palaeontology. 10 (4): 601–624. doi:10.1080/14772019.2011.631042.
5. Neil Brocklehurst, Robert Reisz, Vincent Fernandez, and Jörg Fröbisch (2016). "A Re-Description of 'Mycterosaurus' smithae, an Early Permian Eothyridid, and Its Impact on the Phylogeny of Pelycosaurian-Grade Synapsids". PLoS ONE. 11 (6): e0156810. doi:10.1371/journal.pone.0156810. PMC 4917111. PMID 27333277.CS1 maint: uses authors parameter (link)
6. Neil Brocklehurst, Robert Reisz, Vincent Fernandez, and Jörg Fröbisch (2016). "A Re-Description of 'Mycterosaurus' smithae, an Early Permian Eothyridid, and Its Impact on the Phylogeny of Pelycosaurian-Grade Synapsids". PLoS ONE. 11 (6): e0156810. doi:10.1371/journal.pone.0156810. PMC 4917111. PMID 27333277.CS1 maint: uses authors parameter (link)

• Reisz, R. R., 1986, Handbuch der Paläoherpetologie – Encyclopedia of Paleoherpetology, Part 17A Pelycosauria Verlag Dr. Friedrich Pfeil, ISBN 3-89937-032-5
• Laurin, M. and Reisz, R. R., 1997, Autapomorphies of the main clades of synapsids

External links

• Palaeos Vertebrates 390.100 Synapsida