The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology

Abstract

The cranial osteology of the aquatic reptile Mesosaurus tenuidens is redescribed on the basis of new and previously examined materials from the Lower Permian of both southern Africa and South America. Mesosaurus is distinguished from other mesosaurs in exhibiting an absolutely larger skull and possessing relatively longer marginal teeth. The teeth gradually angle outwards as one progresses anteriorly in the tooth row and become conspicuously procumbent at the tip of the snout. The suggestion that mesosaurs used their conspicuous dental apparatus as a straining device for filter feeding is based upon erroneous reconstruction of a high number of teeth in this mesosaur. Reinterpretation of the morphology and the organization of the marginal teeth of Mesosaurus suggests that they were used to capture individually small, nektonic prey. General morphological aspects of the skull support the idea that Mesosaurus was an aquatic predator and that the skull was well adapted for feeding in an aqueous environment. The anatomical review permits critical reappraisal of several cranial characters that have appeared in recent phylogenetic analyses of early amniotes. Emendation of problematic characters and reanalysis of amniote phylogeny using a slightly modified data matrix from the literature strengthens the hypothesis that mesosaurs form a clade with millerettids, procolophonoids and pareiasaurs within Reptilia.

Commentary: A Reassessment of the Taxonomic Position of Mesosaurs, and a Surprising Phylogeny of Early Amniotes

“…With Mesosauridae being considered the sister to all other parareptiles by most studies, whether or not lateral temporal fenestration is present in Mesosauridae would be important for determining if lower temporal fenestrae are primitive for Parareptilia. However, other specimens of Mesosaurus do not appear to exhibit temporal fenestration (Modesto, 2006), and specimens with supposed temporal fenestration, such as that presented in Piñeiro et al (2012), are extremely poorly preserved. Furthermore, Laurin and Piñeiro made no effort to reexamine the Mesosaurus specimens that had been previously described by Modesto (2006Modesto ( , 2010.…”

“…However, other specimens of Mesosaurus do not appear to exhibit temporal fenestration (Modesto, 2006), and specimens with supposed temporal fenestration, such as that presented in Piñeiro et al (2012), are extremely poorly preserved. Furthermore, Laurin and Piñeiro made no effort to reexamine the Mesosaurus specimens that had been previously described by Modesto (2006Modesto ( , 2010. This makes it difficult to say conclusively if lower temporal fenestration is present in Mesosaurus.…”

A case study of developmental palaeontology in <i>Stereosternum tumidum</i> (Mesosauridae, Parareptilia)

“…1; Modesto, 1999Modesto, , 2010. The fore-and hind limbs are elongate, characteristic for the aquatic lifestyle proposed for mesosaurs (Modesto, 2006;Piñeiro et al, 2012a, b;Villamil et al, 2016). Size and appearances of the fore-and hind limb stylopodia are similar, although the femora are slightly shorter.…”

“…The clade consists of the three monospecific genera Mesosaurus tenuidens, Stereosternum tumidum, and Brazilosaurus sanpauloensis from Lower Permian localities in Brazil and southern Africa Araújo, 1983, 1987). Their elongate skulls, slender teeth, paddle-like limbs, and thickened (pachyostotic) trunk ribs are indicative of a secondarily fully aquatic lifestyle and distinguish mesosaurs from all other Paleozoic amniotes (Modesto, 2006). Recent studies suggest that mesosaurs, or at least Mesosaurus, were slow swimmers living in shallow lagoon-like waters, possibly filter-feeding on pygocephalomorph crustaceans (Piñeiro et al, 2012b;Villamil et al, 2016).…”

“…Most recent studies suggest mesosaurs group basally within parareptiles; a small amniote clade that originated in the Late Carboniferous (Tsuji and Müller, 2009;Modesto et al, 2015; though see contradictory hypothesis of Laurin and Piñeiro, 2017). Evolutionary novelties in this clade include impedence-matching hearing in nycteroleterids (Müller and Tsuji, 2007), caudal regeneration in mesosaurs (Delfino and Sánchez-Villagra, 2010), derived feeding mechanisms and bipedality in bolosaurids (Reisz et al, 2007;Berman et al, 2000), and secondarily aquatic lifestyles as well as (ovo)viviparity in mesosaurids (Modesto, 2006;Piñeiro et al, 2012a), all of which occurred in parareptiles as early as the Early Permian. The first occurrence of congenital scoliosis is also reported from mem-bers of this clade (Szczygielski et al, 2017).…”