Stereosternum and Brazilosaurus

Stereosternum tumidum (Cope 1885) Early Permian ~290 mya, 30 cm in length, was long considered a basal reptile without temporal openings. Not so. Crushing typically makes reconstruction difficult. Here both temporal openings are still open. Stereosternum was a diapsid derived from a sister to Claudiosaurus and phylogenetically preceded Mesosaurus and Wumengosaurus, which in turn preceded Hupehsuchus, Utatsusaurus and the thalattosaur Xinpusaurus.

Distinct from Claudiosaurus, the skull of Stereosternum was greatly elongated in the premaxilla and maxilla. The naris remained close to the orbit. The maxilla ascending process separated the expanded lacrimal from the naris. The jugal produced a quadratojugal process and the quadratojugal was shorter in larger (adult?) specimens. The upper temporal fenestra was reduced by expansion of the parietal. The squamosal shared a long border with the quadratojugal in small specimens, but not in large specimens. The teeth were larger and fewer in small specimens. The small specimens had a smaller lateral temporal fenestra.

The dorsal ribs were all thicker (pachyostotic). The caudal chevrons were deeper, creating a sculling tail. Pedal digit V was greatly elongated.

The BHM 999 specimen attributed to Stereosternum tumidum by Modesto 1999 (above) appears to be closer to Mesosaurus.

The complete reptile family tree is here.

Brazilosaurus sanpaukebsus (Shikama and Ozaki 1966; Piñeiro et al. 2021; Permian, Brazil) nests between Sterosternum and Mesosaurus in the LRT. Note the large diapsid skull architecture and slender postorbital region.

Andrews CW 1910. Descriptive Catalogue of the Marine Reptiles of the Oxford Clay, British Museum. (Natural History) Part 1:1-205.

Carroll RL 1988. Vertebrate paleontology and evolution. New York: W. H. Freeman & Co.

Cope ED 1886. A contribution to the vertebrate paleontology of Brazil. Stereosternum tumidum, gen. et sp. nov. Proceedings of the American Philosophical Society 23 (121):1-21.

Jiang D-Y, Rieppel O, Motani R, Hao W-C, Sun Y-I, Schmitz L and Sun Z-Y. 2008. A new middle Triassic eosauropterygian (Reptilia, Sauropterygia) from southwestern China. Journal of Vertebrate Paleontology 28:1055–1062.

Laurin M and Reisz R 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society, 113: 165–223.

Maisch MW 2010. Phylogeny, systematics, and origin of the Ichthyosauri - the state of the art. Palaeodiversity 3: 151–214.

Modesto SP 1999. Observations on the structure of the Early Permian reptile Stereosternum tumidum Cope. Palaeontol. Afr. 35, 7–19.

Piñeiro G, Ferigolo J, Mones A and Demarco PN 2021. Mesosaur taxonomy reappraisal: Are Stereosternum and Brazilosaurus valid taxa? Revista Brasileira de Paleontologia 24(3):205–235. A Journal of the Brazilian Society of Paleontology.

Shikama T and Ozaki H 1966. On a Reptilian Skeleton from the Palaeozoic Formation of San Paulo, Brazil. Transactions and Proceedings of the Palaeontological Society of Japan. New Series. 64: 351–358.

Vieira PC, Mezzalira S, Ferreira FJF 1991. Mesossaurídeo (Stereosternum Tumidum) e crustáceo (Liocaris Huenei) no Membro Assistência da Formação Irati (P) nos municípios
de Jataí e Montevidiu, estado de Goiás. Revista Brasileira de Geociências, 21:224-235.

Wu X-C, Cheng Y-N, Li C, Zhao L-J and Sato T 2011. New Information on Wumengosaurus delicatomandibularis Jiang et al., 2008, (Diapsida: Sauropterygia), with a Revision of the Osteology and Phylogeny of the Taxon. Journal of Vertebrate Paleontology 31(1):70–83.

wiki/Stereosternum
wiki/Brazilosaurus

Marine Younginiformes

Galesphyrus

Claudiosaurus

Enaliosauria

Anarosaurus

Parapachypleurosauria

Luopingosaurus

Mesosauria

PIMUZ-A-III-0192

SMF R 4710

Stereosternum