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

Abstract

We reassess the phylogenetic position of mesosaurs by using a data matrix that is updated and slightly expanded from a matrix that the first author published in 1995 with his former thesis advisor. The revised matrix, which incorporates anatomical information published in the last 20 years and observations on several mesosaur specimens (mostly from Uruguay) includes 17 terminal taxa and 129 characters (four more taxa and five more characters than the original matrix from 1995). The new matrix also differs by incorporating more ordered characters (all morphoclines were ordered). Parsimony analyses in PAUP 4 using the branch and bound algorithm show that the new matrix supports a position of mesosaurs at the very base of Sauropsida, as suggested by the first author in 1995. The exclusion of mesosaurs from a less inclusive clade of sauropsids is supported by a Bremer (Decay) index of 4 and a bootstrap frequency of 66%, both of which suggest that this result is moderately robust. The most parsimonious trees include some unexpected results, such as placing the anapsid reptile Paleothyris near the base of diapsids, and all of parareptiles as the sister-group of younginiforms (the most crownward diapsids included in the analyses). Turtles are placed among parareptiles, as the sistergroup of pareiasaurs (and in diapsids, given that parareptiles are nested within diapsids). This unexpected result offers a potential solution to the long-lasting controversy about the position of turtles because previous studies viewed a position among diapsids and among parareptiles as mutually exclusive alternatives.

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

“…The matrix was modified by changing the codings for Mesosaurus so that they reproduce the codings used for Mesosauridae in Laurin and Piñeiro (2017). Furthermore, numerous synapsid and eureptilian taxa were added to increase the sample of non-parareptilian taxa.…”

“…The character codings that were changed for Mesosaurus are as follows: 7 (1→0), 8 (-→0), 9 (-→1), 10 (-→1 Laurin and Piñeiro (2017).…”

“…Furthermore, temporal fenestration has been shown by several studies to be highly variable within Reptilia, raising concern about its utility as a phylogenetic character in analyses of Reptilia. Here we show how these two factors contribute to the results that were obtained by Laurin and Piñeiro (2017), and how future studies can alleviate these problems.…”

“…While the results of Laurin and Piñeiro (2017) are interesting, there are issues with the methodology and interpretations of the study that render them problematic. Firstly, it is perplexing that the authors utilize an outdated data matrix that largely ignores the extensive body of parareptile literature that has been produced in the last two decades.…”

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

“…Hypothesized viviparity in Mesosaurus is another potential indicator of a fully aquatic lifestyle (Piñeiro et al, 2012a). 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.…”