STEREOSTERNUM

SMF R-4710 (Rossman 2002) Here the Rossman “Stereosternum” nests at the base of the Mesosauria, close to Stereosternum. It is a mesosaurid with very short teeth. This is the most primitive mesosaur tested here. Derived from a sister to Anarosaurus, this specimen was basal to Stereosternum, Wumengosaurus and note the resemblance to Hupehsuchus. Distinct from Anarosaurus, the skull of the SMF specimen had a much longer rostrum, a shorter neck, taller dorsal spines and a shallower ventral pelvis.

At right is a subset of the large reptile tree highlighting relationships among mesosaurs, thalattosaurs and ichthyosaurs. A tree of the Reptilia is here.

Above is a selection of palates from Mesosaurus and sister taxa showing the evolution and shifting of the various elments. As a side note, In Carroll 1988 (figure 12-31, taken from Andrews 1910) the palate of Ichthyosaurus was illustrated, labeling the bones lateral to the pterygoids as palatines and the central bones, vomers, all in accord with the palate of archosaurs, etc. Here we can see that the ectopterygoid is actually the bone lateral ot the pterygoid and the palatine has become the medial element in mesosaurs, thalattosaurs and ichthyosaurs. 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.