Mesosaurus tumidum (Cope, 1885) Stereosternum tumidum (meaning "rigid chest") (Stereos, Greek: “solid, firm”; Sternon, Greek: “chest, breastbone”) [2] is an extinct genus of mesosaur marine reptile from the Early Permian of Brazil and also the Great Karoo Basin of South Africa.

[7] Many of the ribs and the Haemal arches are thickened with bone or have a pachyostotic characteristic,[4] which would have helped the animal to be able to dive deep in the water as a counteracting force against buoyancy.

[4] With the pachyostosis of the postcrania elements and restricted movement at the appendicular joints, it has been suggested that mesosaurs could not have walked on land, but may have been able to push themselves across terrestrial substrates.

[9] When studying the histology of the ribs of Stereosternum and Mesosaurus, the kind of pachyostosis of these two species has, is very much characterized by hyperostosis of the periosteal cortex, increased bone density, which was due to the bone becoming more compact, the absence of free medullary cavity and the remnants of calcified cartilage matrix.

As counted by Osborn, there are about 60-64 caudal vertebrae, which is very predictable for an aquatic predator and these adaptations helped individuals to push and swim gracefully in the water.

[6] The presence of accessory articulations, like the zygantra and the zygosphenes, are there to supposedly to have dampened the twisting movements of the vertebral column.

[4] Many of the modifications of the bones within the skull seem to reduce drag and help propel the animal through an aqueous environment while swimming and feeding.

From looking at the teeth and how they are positioned, it seems that the replacement pulse occurred anteroposteriorly in Stereosternum, in a quasi-alternate polyphyodont pattern.

[5] Stereosternum tumidum was discovered by Edward Drinker Cope in 1886, while he was in Brazil and looking over some specimens housed in the Museu Nacional (Rio de Janeiro).

He also discussed the ribs of the animal and noting how they were long, cylindrical, curved and very robust, reminding him of both Ichthyosaurus and Mesosaurus.

The specimen that Cope had at the time contained only the posterior half of the animal and so he could not make a decision on whether to place it in Reptiles or in Batrachia.

[citation needed] There has also been evidence of small mesosaurid bones within the coprolites of mesosaurs as well, suggesting that cannibalism within this group did occur.

[15] There is ample evidence to suggest that mesosaurs may have been the oldest known amniotes that displayed extended embryo retention, which could have been either oviparous or viviparous within the same species.

Also, when looking at the anatomical structure of the mesosaur pelvic region, the evidence of the fusion of the ribs to the two sacral vertebrae, as well as a weak articulation between these ribs and the Iliac blade, suggests that mesosaurs had the capability to move on dry land and to deposit their eggs on land, plausibly close to water.

[18] Within this deeper part of the sea, the water column was very stratified, with a fresh and habitable upper layer that was on top of the anoxic, highly sulphurated, toxic bottom brines.

The lithology representing that bottom half of the water column are black shales and also carbonaceous oil is found in the same area.

[citation needed] Stereosternum tumidum and Brazilosaurus sanpauloensis have been recently considered as nomina dubia and synonymized to Mesosaurus by Graciela Piñeiro, Jorge Ferigolo, Alvaro Mones and Pablo Nuñez Demarco (2021), after a detailed anatomical and taphonomic study of more than 500 specimens from palaeontological collections of several countries, which demonstrated that most of the characters used to distinguish three mesosaur taxa are indeed derived of the preservation of specimens in different ontogenetic stages and others are influenced by taphonomy.

These results were also supported by morphometric analyses applied to specific region of the skeleton such as the vertebral column and limb bones by Pablo Nuñez Demarco, Jorge Ferigolo and Graciela Piñeiro (2022) and by an incredible similar article by Antoine Verrière and Jörg Fröbisch (2022), which was submitted posteriorly to the acceptation of the Nuñez Demarco et al. (2022) paper.

The isometry, and particularly, the high interrelation between metatarsals and phalanges permit us to suggest that the mesosaur hind limb is subject to notable modularity.

This evidence strongly argues that the differences previously described to support three mesosaur species in Western Gondwana, might instead reflect natural intraspecific variability, taphonomic features or even possible sexual dimorphism, as recently suggested by Piñeiro et al. (2021).