No abstract

Mesosaurs have been considered strictly aquatic animals. Their adaptations to the aquatic environment are well known and include putative viviparity, along with the presence of several skeletal characters such as a long, laterally compressed tail, long limbs, the foot larger than the manus, and presence of pachyosteosclerotic bones. They were also described as possessing non-coossified girdle bones and incompletely ossified epiphyses, although there could be an early fusion of the front girdle bones to form the scapulocoracoid in some specimens. Some of these features, however, are shared by most basal tetrapods that are considered semiaquatic and even some terrestrial ones. The study of vertebral columns and limbs provides essential clues about the locomotor system and the lifestyle of early amniotes. In this study, we have found that the variation of the vertebral centrum length along the axial skeleton of Mesosaurus tenuidens fits better with a semi-aquatic morphometric pattern, as shown by comparisons with other extinct and extant taxa. The present study allows us to suggest that whereas well-preserved mesosaur skeletons are mostly represented by juveniles and young adults that inhabited aquatic environments, more mature individuals might hypothetically have spent time on land. This is also supported, to an extent, by taphonomic factors such as the scarce representation and poor preservation of remains of mature individuals in the fossiliferous levels, and also by anatomy of the appendicular bones, and particularly the strongly ossified epiphyses and tarsus.

Mesosaurid biology has been subject of continuous debate since the first description of Mesosaurus tenuidens by Paul Gervais in 1867. Controversy surrounds their environmental and feeding preferences. Most studies suggested that mesosaurids were marine reptiles and perhaps piscivorous predators. Nonetheless, recent work suggests that they inhabited a salty, eventually hypersaline shallow epicontinental sea and that pygocephalomorph crustaceans were their preferred food item. Here, we present results of the first biomechanical study about optimal swimming capabilities in Mesosaurus tenuidens, which along with the comparative analysis of the limb morphology support the hypothesis that these animals were slow swimmers living in shallow waters. The study is based on the revision of several almost complete mesosaurid specimens and isolated, well-preserved bones housed in palaeontological collections in Uruguay, Brazil, France and Germany. We studied the relative size and proportions of the bones, as well as their morphology and anatomical position to produce a three-dimensional reconstruction of the original appearance of an undamaged, complete skeleton. Our results suggest a fairly low optimal swimming speed for mesosaurids, which is consistent with capture of fairly slow prey like pygocephalomorphs, possibly by filter-feeding, rather than by active pursuit of fast prey.

The Permo-Triassic archosauromorph record is crucial to understand the impact of the Permo-Triassic mass extinction on the early evolution of the group and its subsequent dominance in Mesozoic terrestrial ecosystems. However, the Permo-Triassic archosauromorph record is still very poor in most continents and hampers the identification of global macroevolutionary patterns. Here we describe cranial and postcranial bones from the Permo-Triassic Buena Vista Formation of northeastern Uruguay that contribute to increase the meagre early archosauromorph record from South America. A basioccipital fused to both partial exoccipitals and three cervical vertebrae are assigned to Archosauromorpha based on apomorphies or a unique combination of characters. The archosauromorph remains of the Buena Vista Formation probably represent a multi-taxonomic assemblage composed of non-archosauriform archosauromorphs and a ‘proterosuchid-grade’ animal. This assemblage does not contribute in the discussion of a Late Permian or Early Triassic age for the Buena Vista Formation, but reinforces the broad palaeobiogeographic distribution of ‘proterosuchid grade’ diapsids in Permo-Triassic beds worldwide.

The native species of amphibians and reptiles of Uruguay were categorized according to the IUCN Red List criteria. Out of 47 amphibian species, seven are listed as Critically Endangered (CR), five as Endangered (EN), one as Vulnerable (VU), three as Near Threatened (NT), and two as Data Deficient (DD); the remaining species are considered to be Least Concern (LC). Among the 64 species of reptiles evaluated, one is listed as Critically Endangered (CR), seven as Endangered (EN), two as Vulnerable (VU), one as Near Threatened (NT) and seven as Data Deficient (DD); the rest are considered to be Least Concern (LC). The use of these results as an additional criterion in the definition of protected areas in Uruguay will contribute towards the conservation of the aforementioned threatened species and their associated ecosystems.