Was Mesosaurus a Fully Aquatic Reptile?

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.

Conflicting evidence for the use of caudal autotomy in mesosaurs

“…However, mesosaurs may have had the ability to occasionally come up onto land 31 , and the ability to escape potential terrestrial predators using caudal autotomy in order to return to the water would have been beneficial. Furthermore, it has been suggested that mesosaurs may have been cannibalistic, with adults preying on juveniles 32 .…”

Microanatomy and growth of the mesosaurs <i>Stereosternum tumidum</i> and <i>Brazilosaurus sanpauloensis</i> (Reptilia, Parareptilia)

“…The Permian family Mesosauridae is involved in several key events of the history of life and that of natural sciences. First of all, mesosaurs are exclusively found in early Permian black-shale and limestone deposits from both southeastern South America (the Irati Formation in southern Brazil and the Melo Formation in Uruguay) and southwestern Africa (the Whitehill Formation in Namibia and in western South Africa) (Oelofsen and Araújo, 1987). Because of this geographical distribution, which supports the theory that the two continents were once connected, mesosaurs were among the fossils cited by Alfred Wegener as a line of evidence to corroborate his theory of continental drift (Du Toit, 1937).…”

“…The majority of studies place them among Parareptilia (e.g. Modesto, 2006 Dougall et al, 2017Dougall et al, , 2018Tsuji and Müller, 2009), whereas others suggest that mesosaurs may instead represent the basalmost clade of Reptilia sensu Modesto and Anderson (2004) (Laurin and Reisz, 1995;Laurin and Piñeiro, 2017).…”

“…However, the degree to which they were aquatic is controversially discussed, and whether they might have been partially terrestrial is also still debated. Morphological evidence suggests limited in-land locomotive abilities (Modesto, 2006;Núñez Demarco et al, 2018). For some authors (Romer, 1956;Carroll, 1982;Chiappe and Chinsamy, 1996), the specialized dentition of mesosaurs was used for filter feeding, while Pretto et al (2012) see in the tooth microstructure of Stereosternum evidence for an active aquatic predatorial behaviour.…”

A Reassessment of the Taxonomic Position of Mesosaurs Based on Two Data Matrices

“…Mesosaurs, known from the Early Permian of southern Africa, Brazil and Uruguay, are the oldest known amniotes with a primarily, though probably not strictly, aquatic lifestyle (Nuñez Demarco et al, 2018). Despite having attracted attention of several prominent scientists, such as Wegener (1966), who used them to support his theory of continental drift, and the great anatomist and paleontologist von Huene (1941), who first suggested the presence of a lower temporal fenestra in Mesosaurus, several controversies still surround mesosaurs.…”