The phylogenetic position of mesosaurs has historically had an important bearing on the definition of
Reptilia. In one of the first major phylogenetic studies of amniotes (vertebrates laying eggs on land) Gauthier
et al. (1988) placed Mesosauridae in a group called
Parareptilia. Parareptilia means "at the side of reptiles" and was placed outside the
clade Reptilia, which was considered a
crown group. As a crown group, Reptilia included the
most recent common ancestor of the then believed to be the two main lineages of living reptiles –
anapsids (specifically
turtles) and diapsids (all other living reptiles) – and all descendants of that common ancestor. This view of placing turtles outside of diapsids is now outdated and the majority of modern paleontologists believe that the
Testudines (turtles and allies) are descended from diapsid reptiles that lost their temporal fenestrae. More recent morphological
phylogenetic studies with this in mind placed turtles firmly within diapsids, and, more commonly, as a sister taxon to
Archosauria (made up of
crocodiles,
dinosaurs – including
birds – and allies). Furthermore, Anapsida is rarely considered a valid clade in recent phylogenetic analyses. In this sense, Reptilia was a
node-based taxon because the first reptilian common ancestor would have been a "node" on the phylogenetic tree. Under this phylogeny, many extinct forms traditionally regarded as reptiles including mesosaurs were excluded from the group because they were outside the node. Cladogram after Gauthier et al. 1988: Like Gauthier
et al., Laurin and Reisz used Reptilia as a crown group and placed mesosaurs outside the group. Their phylogeny differed in that the parareptiles of Gauthier
et al. were now regarded as close relatives of turtles, within crown group Reptilia. Laurin and Reisz adopted the name
Sauropsida as a node-based taxon including the last common ancestor of mesosaurs and Reptilia. Traditionally, amniotes are divided into two groups: a mammal lineage called
Synapsida and a reptile lineage called either Reptilia or Sauropsida. In fact, the study of Gauthier (1994) defined Sauropsida as all amniotes more closely related to reptiles than to mammals, which meant that Sauropsida was a
stem-based taxon encompassing the entire reptilian lineage or reptilian "stem" of Amniota (Synapsida was the mammalian stem). Under this phylogeny, the only group that prevents Sauropsida from being equivalent to Reptilia is mesosaurs. }} A 2017 phylogenetic analysis by Laurin (who had previously published the 1995 study) and Piñeiro recovered mesosaurs as a basal member of Sauropsida and no longer present within Parareptilia, with Parareptilia being redefined as including former members of
Procolophonomorpha (found to be paraphyletic),
Millerosauria,
Pareiasauria, and
Pantestudines, with the latter two being found to be
sister groups to one another. Parareptilia was also found to actually nest inside Diapsida as the sister group to
Neodiapsida. Cladogram after Laurin & Piñeiro, 2017: The condition in the skull of
Mesosaurus is most similar to that in synapsid skulls because both lack the upper temporal fenestrae of diapsids. Lower temporal fenestrae are so far known only in
Mesosaurus, but may be present in all mesosaurs. The presence or absence of temporal fenestrae is an important consideration in the phylogeny of mesosaurs and other amniotes because the three major groups of amniotes -Synapsida, Diapsida, and Anapsida- have been named after the number of holes in their skull; Diapsida means "two arches" in reference to the two bars that close off the upper and lower fenestra, Synapsida means "fused arch" in reference to a single bar at the bottom of the skull closing a single fenestra, and Anapsida means "no arch" in reference to skulls that lack any bars or fenestrae. Mesosaurs were traditionally classified as anapsids because they were thought to have lacked temporal fenestrae. However, the occurrence of fenestrae in amniotes has been recognized a highly variable feature within the group for many years prior to their discovery in
Mesosaurus; many anapsids such as
Candelaria,
Bolosaurus, and
lanthanosuchoids possess lower temporal fenestrae. The phylogenetic position of mesosaurs influences the current understanding of how amniotes evolved temporal fenestrae. If the phylogeny produced by Laurin and Reisz (1995) is correct in that mesosaurs are basal sauropsids, the lower temporal fenestra may be a primitive feature in amniotes, present in amniote's most recent common ancestor. Synapsids would have retained their fenestrae, and so too would sauropsids except for turtles and most parareptiles. Another possibility under Laurin and Reisz's phylogeny is that lower temporal fenestrae evolved independently in mesosaurs, synapsids, diapsids, and some parareptiles, and that the lack of fenestrae is a primitive feature in amniotes. If instead mesosaurs are members of Parareptilia, the presence of temporal fenestrae is probably not a primitive feature in amniotes, and the lower temporal fenestrae in mesosaurs may be characteristic of a lineage of basal parareptiles that also includes fenestra-bearing lanthanosuchoids and
Bolosaurus. |3= }} }} }} }} }} }} }} }} }}|style=|label1=
Sauropsida}} == References ==