A reevaluation of early amniote phylogeny

Abstract

A new phylogenetic analysis of early amniotes based on 124 characters and 13 taxa (including three outgroups) indicates that synapsids are the sister-group of all other known amniotes. The sister-group of Synapsida is Sauropsida, including Mesosauridae and Reptilia as its two main subdivisions. Reptilia is divided into Parareptilia and Eureptilia. Parareptilia includes Testudines and its fossil relatives (Procolophonidae, Pareiasauria and Millerettidae), while Eureptilia includes Diapsida and its fossil relatives (Paleothyris and Captorhinidae). Parts of the phylogeny are robust, such as the sister-group relationship between procolophonids and testudines, and between pareiasaurs and the testudinomorphs (the clade including procolophonids and testudines). Other parts of the new tree are not so firmly established, such as the position of mesosaurs as the sister-group of reptiles. The new phylogeny indicates that three major clades of amniotes extend from the present to the Palaeozoic. These three clades are the Synapsida (including Mammalia), Parareptilia (including Testudines), and Eureptilia (including Sauria). In addition, the Procolophonidae, a group of Triassic parareptiles, are the sister-group of Testudines.

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

“…With the advent of phylogenetic systematics, numerous studies incorporated mesosaur taxa into their analyses, the first being the pioneering study by Gauthier et al (1988), which recovered mesosaurs as the sister to all other parareptiles. Later studies recovered mesosaurs in varying places within Amniota, with one study recovering mesosaurs as the basal-most lineage of a clade to which the name Sauropsida was attached (Laurin and Reisz, 1995). Numerous others recover it as the sister to all other parareptile clades (Tsuji et al, 2012;Modesto et al, 2015;MacDougall et al, 2016MacDougall et al, , 2017 replicating the results of Gauthier et al (1988), and one study recovers the clade being nested within Parareptilia as the sister taxon of bolosaurids (Bever et al, 2015).…”

“…The 2017 paper "A Reassessment of the Taxonomic Position of Mesosaurs, and A Surprising Phylogeny of Early Amniotes" by Laurin and Piñeiro is the latest publication to tackle the issue of the phylogenetic position of Mesosauridae. The results of their phylogenetic analysis reaffirm the Laurin and Reisz (1995) placement of Mesosauridae at the very base of Sauropsida (sensu Laurin and Reisz, 1995, in which Sauropsida = Mesosauridae + Reptilia). Furthermore, their phylogenetic analysis also produced the unique tree topology of having the remaining Parareptilia nested within Eureptilia as the sister taxon of younginiforms.…”

“…A modified version of the data matrix from MacDougall et al (2017), which was itself based in part upon the Laurin and Reisz (1995) matrix, was used for the phylogenetic analyses presented in this study. The matrix was modified by changing the codings for Mesosaurus so that they reproduce the codings used for Mesosauridae in Laurin and Piñeiro (2017).…”

“…The authors argued that this is because they are more familiar with the Laurin and Reisz (1995) matrix, and that this removes the issue of different systematists coding characters of an unfamiliar matrix differently. While we acknowledge that different researchers can code the same characters in different ways, this argument only holds up for matrices that include characters that are vague or not explained thoroughly.…”

“…The data matrix used in the phylogenetic analysis of Laurin and Piñeiro (2017) was based upon the Laurin and Reisz (1995) matrix. They updated it with the addition of two parareptile taxa, the lanthanosuchoid Acleistorhinus pteroticus and the procolophonoid Owenetta kitchingorum, as well as the stemturtle Odontochelys semitestacea.…”