Mesosaur taxonomy reappraisal: are Stereosternum and Brazilosaurus valid taxa?

Mesosaurs are basal amniotes that lived at the beginning of the Permian or close to the Permo–Carboniferous boundary. Despite the several hundred specimens that have been found, including complete skeletons of adult and juvenile individuals, mesosaur taxonomy has been subjected to a high controversy over time. Currently, three monotypic genera, Mesosaurus tenuidens Gervais, Stereosternum tumidum Cope, and Brazilosaurus sanpauloensis Shikama & Ozaki are recognized, but identification of new specimens using the available diagnostic characters are arbitrary and influenced by high subjectivity. We performed anatomical and morphometric analyses to look for statistical support to these previously suggested basic diagnostic characters through an exhaustive anatomical revision of these characters and also of some new attributes discovered during the course of our study. We found a notable influence of taphonomic features in most of the diagnostic characters used to differentiate the three monotypic genera, including strong bias derived from the preservation of individuals in different ontogenetic stages, whose size and degree of ossification could have been controlled by particular environmental conditions that resulted in subtle polymorphisms of these and other few characters. Other features may even represent sexual dimorphism. After the detailed revision of the type specimens of the three currently acepted mesosaur taxa, for which we include here good-quality photographs, and considering the lack of statistical support for the most applied putative diagnostic features such as the different ratio found when comparing skull and cervical region lengths and the low or higher intensity of pachyosteosclerosis observed in dorsal ribs, which can be controlled by taphonomic and ecological conditions, we recognize Mesosaurus as the only mesosaurid taxon in the Parana and Karoo basins, probably including dwarf individuals. Therefore, S. tumidum and B. sanpauloensis are suggested here as nomina dubia taking into account that the autapomorphies that supported these taxa cannot be confirmed to be absent in Mesosaurus. Keywords: Mesosaurus, morphometrics, taxonomy, ?Early Permian, Gondwana. RESUMO – Mesossauros sao amniotas basais que viveram no inicio do Permiano ou proximo ao limite Permo–Carbonifero. A despeito das centenas de especimes encontrados, incluindo esqueletos completos de individuos adultos e juvenis, a taxonomia dos mesossaurostem sido tema de muita controversia ao longo do tempo. Hoje, tres generos monotipicos, Mesosaurus tenuidens Gervais, Stereosternum tumidum Cope, e Brazilosaurus sanpauloensis Shikama & Ozaki sao reconhecidos, mas a identificacao de novos especimes utilizando oscaracteres diagnosticos disponiveis e arbitraria e influenciada por alta subjetividade. Nos realizamos analises anatomica e morfometrica, tendo em vista obter um suporte estatistico para estes caracteres sugeridos como diagnosticos basicos por meio de uma exaustiva revisao anatomica destes caracteres, e de alguns novos caracteres descobertos durante o desenvolvimento de nosso estudo. Nos notamos uma notavel influencia de fatores tafonomicos na maioria dos caracteres diagnosticos utilizados para os tres taxons monotipicos e incluindo uma forte preconcepcao derivada da preservacao dos individuos em diferentes estagios ontogeneticos, cujos tamanho e grau de ossificacao poderiam ser controlados por condicoes ambientais que resultaram em sutis polimorfismos destes poucos caracteres. Outros aspectos podem mesmo representar dimorfismo sexual. Considerando a falta de suporte estatistico para a maioria dos putativos caracteres diagnosticos, tais comoas diferentes relacoes entre comprimento do crânio e da coluna cervical e a pressao das condicoes tafonomicas e ecologicas, que podem ter controlado um menor ou maior grau de paquiosteoesclerose nas costelas dorsais, nos reconhecemos Mesosaurus como o unico taxonde mesosaurideo nas bacias do Parana e Karoo, provavelmente incluindo populacoes anas, e sugerimos que S. tumidum e B. sanpauloensis devem ser considerados como nomen dubium, levando em conta que as autapomorfias que suportam estes taxons nao podem ser confirmadas como nao estando presentes em Mesosaurus. Palavras-chave: Mesosaurus, morfometria, taxonomia, ?base do Permiano, Gondwana.