Oenosaurus muehlheimensis (Rauhut et al. 2012; Late Jurassic; BSPG 2009 I 23) was originally considered a rhynchocephalian with an unusual fusion of all teeth to make crushing plates. Here Oenosaurus nests at the base of the Mesosuchus + Rhynchosaurus clade, derived from trilophosaurs and all nesting within the clade Rhynchocephalia. The tiny size of Oenosaurus is just one more example of phylogenetic miniaturization at the genesis of a new clade. This taxon is a late survivor of an Early Triassic radiation.

Mesosuchus browni (Watson 1912) Early Triassic ~240 mya, ~40cm in length, was found in a sandstone block along with Euparkeria, hence the name, Mesosuchus, which indicates an affinity with crocodilians. Further excavation revealed the two skeletons (Broom 1913a,b) and the relationship of Mesosuchus to rhynchosaurs. Romer (1956) considered rhynchosaurs and sphenodontians to be related, but Benton (1985) and Carroll (1988) mistakenly split them apart by placing too much emphasis on the lack of fusion in the teeth and tarsus rather than considering the suite of characters that more parimoniously unites them. Mesosuchus was derived from a sister to the sphenodontian, Brachyrhinodon and the trilophosaur, Trilophosaurus. Mesosuchus phylogenetically preceded Priosphenodon, Hyperodapedon and the rest of the Rhynchosauria.

Distinct from Brachyrhinodon, the skull of Mesosuchus had distinct teeth not fused to the skull bones. The nares were joined without a central premaxilla bone dividing them. The temporal bars were more gracile with larger temporal openings. The jugal was more gracile. The quadrant leaned anteriorly. The premaxilla extended down further.

The ectopterygoids extended laterally, largely contacting the jugals. This is an autapomorphy seemingly shared only with Howesia, suggesting that these two shared a clade and that more derived rhynchocephalians were derived closer to Brachyrhinodon which had the anteriorly oriented ectopterygoids, not in contact with the jugal.

The cervicals were longer and taller. The vertebral spines were taller.

The scapulocoracoid was larger. The manus was smaller.

The pelvis had deeper ventral elements and a broad pubic apron. The tarsal elements were not fused together.

Click to enlarge. The palates of several derived rhynchocephalians leading to toward the rhynchosaur, Hyperodapedon. Here is a best guess, given the data, as to how the unique palate of rhynchosaurs evolved. There are no closer sisters known. If you have them, please send them.

Benton MJ 1983. The Triassic reptile Hyperodapedon from Elgin, functional morphology and relationships. Philosophical Transactions of the Royal Society of London, Series B, 302, 605-717.

Benton MJ 1985. Classification and phylogeny of diapsid reptiles. Zoological Journal of the Linnean Society 84: 97-164.

Butler R, Ezcurra M, Montefeltro F, Samathi A, Sobral G 2015. A new species of basal rhynchosaur (Diapsida: Archosauromorpha) from the early Middle Triassic of South Africa, and the early evolution of Rhynchosauria. Zoological Journal of the Linnean Society 10.1111/zoj.12246.

Broom, R. 1913a Note on Mesosuchus browni, Watson, and on a new South African Triassic pseudosuchian (Euparkeria capensis). Record of the Albany Museum 2:394-396.

Broom R 1913b. On the South African pseudosuchian Euparkeria and allied genera. Proceedings of the Zoological Society London 1913, 619-633.

Carroll, RL 1988. Vertebrate Paleontology and Evolution. WH Freeman and Company.

Dilkes DW 1998. The Early Triassic rhynchosaur Mesosuchus browni and the interrelationships of basal archosauromorph reptiles. Philosophical Transactions of the Royal Society, Lond. B 353: 501-541.

Rauhut OWM, Heyng AM, López-Arbarello A, Hecker A 2012. A New Rhynchocephalian from the Late Jurassic of Germany with a Dentition That Is Unique amongst Tetrapods. PLoS ONE 7(10): e46839. doi:10.1371/journal.pone.0046839

Romer AS 1956. Osteology of the Reptiles. University of Chicago Press, Chicago.

Watson, DMS 1912. Mesosuchus browni, gen. et spec. nov. Rec. Albany Museum 2, 296-297.

wiki/Mesosuchus
wiki/Eohyosaurus
wiki/Oenosaurus