Editing Mesosaur (section)

==Phylogeny==
The phylogenetic position of mesosaurs has 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]].<ref name=GKT88>{{cite book |last=Gauthier |first=J.A. |author2=Kluge, A.G. |author3= Rowe, T.  |year=1988 |chapter=The early evolution of the Amniota |title=The Phylogeny and Classification of the Tetrapods |volume=1 |editor=Benton, M.J. |publisher=Clarendon Press |location=Oxford |isbn=978-0198577058 |pages=103–155}}</ref> 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 – [[Anapsid|anapsids]] (specifically [[turtle]]s) 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 This view of placing turtles outside of diapsids is now outdated and the majority of modern paleontologists believe that the [[Turtle|Testudines]] (turtles and allies) are descended from diapsid reptiles that lost their temporal fenestrae. More recent morphological [[Phylogenetics|phylogenetic]] studies with this in mind placed turtles firmly within diapsids,<ref>{{Cite journal |last1=deBraga |first1=M. |last2=Rieppel |first2=O. |year=1997 |title=Reptile phylogeny and the interrelationships of turtles |url=https://academic.oup.com/zoolinnean/article-pdf/120/3/281/16872152/j.1096-3642.1997.tb01280.x.pdf |journal=Zoological Journal of the Linnean Society |volume=120 |issue=3 |pages=281–354 |doi=10.1111/j.1096-3642.1997.tb01280.x |doi-access=free}}</ref><ref>{{Cite journal |last1=RUTA |first1=MARCELLO |last2=CISNEROS |first2=JUAN C. |last3=LIEBRECHT |first3=TORSTEN |last4=TSUJI |first4=LINDA A. |last5=MÜLLER |first5=JOHANNES |date=2011-04-27 |title=Amniotes through major biological crises: faunal turnover among Parareptiles and the end-Permian mass extinction |journal=Palaeontology |volume=54 |issue=5 |pages=1117–1137 |doi=10.1111/j.1475-4983.2011.01051.x |s2cid=83693335 |issn=0031-0239|doi-access=free }}</ref><ref>{{Cite journal |last1=Borsuk−Białynicka |first1=Magdalena |last2=Evans |first2=Susan E. |year=2009 |title=A long−necked archosauromorph from the Early Triassic of Poland |journal=Paleontologica Polonica |volume=65 |pages=203–234}}</ref><ref>{{Cite journal |last=Evans |first=Susan E. |year=2009 |title=An early kuehneosaurid reptile (Reptilia: Diapsida) from the Early Triassic of Poland |journal=Paleontologica Polonica |volume=65 |pages=145–178}}</ref> and, more commonly, as a sister taxon to [[Archosaur|Archosauria]] (made up of [[Crocodilia|crocodiles]], [[Dinosaur|dinosaurs]] – including [[Bird|birds]] – and allies).<ref>{{Cite journal |last1=Field |first1=Daniel J. |last2=Gauthier |first2=Jacques A. |last3=King |first3=Benjamin L. |last4=Pisani |first4=Davide |last5=Lyson |first5=Tyler R. |last6=Peterson |first6=Kevin J. |date=2014-05-05 |title=Toward consilience in reptile phylogeny: miRNAs support an archosaur, not lepidosaur, affinity for turtles |url=http://dx.doi.org/10.1111/ede.12081 |journal=Evolution & Development |volume=16 |issue=4 |pages=189–196 |doi=10.1111/ede.12081 |issn=1520-541X |pmc=4215941 |pmid=24798503}}</ref> Furthermore, Anapsida is rarely considered a valid clade in recent phylogenetic analyses.<ref>{{Cite journal |last1=Modesto |first1=Sean P. |last2=Anderson |first2=Jason S. |date=2004-10-01 |editor-last=Lutzoni |editor-first=François |title=The Phylogenetic Definition of Reptilia |journal=Systematic Biology |language=en |volume=53 |issue=5 |pages=815–821 |doi=10.1080/10635150490503026 |pmid=15545258 |issn=1076-836X|doi-access=free }}</ref><ref>{{Cite journal |last1=Tsuji |first1=Linda A. |last2=Muller |first2=Johannes |year=2009 |title=Assembling the history of the Parareptilia: phylogeny, diversification, and a new definition of the clade |journal=Fossil Record |volume=12 |issue=1 |pages=71–81 |doi=10.1002/mmng.200800011 |doi-access=free}}</ref> 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.<ref name=MA04>{{cite journal |last=Modesto |first=S.P. |author2=Anderson, J.S. |year=2004 |title=The phylogenetic definition of Reptilia |journal=Systematic Biology |volume=53 |issue=5 |pages=815–821 |doi=10.1080/10635150490503026 |pmid=15545258 |doi-access=free }}</ref>
{{Cladogram|align=center|title='''Gauthier ''et al.'', 1988'''<ref name=GKT88/>
|clades={{clade| style=font-size:70%;line-height:80%
|label1=[[Amniota]]
|1={{clade
   |1=[[Synapsida]]
   |2={{clade
      |label1=<span style="background:#F2CECE">'''[[Parareptilia]]'''</span>
      |1={{clade
         |1='''MESOSAURIDAE'''
         |2={{clade
            |1=[[Procolophonia]]
            |2={{clade
               |1=[[Millerettidae]]
               |2=[[Pareiasauria]]}} }} }}
      |label2=<span style="background:#DAF2CE">'''[[Reptilia]]'''</span>
      |2={{clade
         |label1=[[Anapsida]]
         |1={{clade
            |1=[[Captorhinidae]]
            |2=[[Testudines]]}}
         |label2=[[Romeriida]]
         |2={{clade 
            |1=''[[Paleothyris]]''
            |label2=[[Diapsida]]
            |2={{clade
               |1=[[Araeoscelidia]]
               |2=[[Sauria]]
}} }} }} }} }} }}
}}

[[File:Mesosaur Fossil.jpg|thumb|Fossil of a South American Mesosaur]]

The study of Laurin and Reisz (1995) was the second major phylogenetic analysis of amniotes.<ref name=LR95>{{Cite journal|last=Laurin |first=M. |author2=Reisz, R.R. |year=1995 |title=A reevaluation of early amniote phylogeny |journal= Zoological Journal of the Linnean Society |volume=113 |issue=2 |pages=165–223 |doi=10.1111/j.1096-3642.1995.tb00932.x |url=http://www.erin.utoronto.ca/~w3reisz/pdf/laurin_reisz1995.pdf}}</ref> 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.<ref name=MA04/>

{{Cladogram|align=center|title='''Laurin and Reisz (1995)'''<ref name=LR95/>
|clades={{clade| style=font-size:80%;line-height:80%
|label1=[[Amniota]]
|1={{clade
   |1=[[Synapsida]]
   |label2=<span style="background:#CECEF2">'''[[Sauropsida]]'''</span>
   |2={{clade
      |1='''MESOSAURIDAE'''
      |label2=<span style="background:#DAF2CE">'''[[Reptilia]]'''</span>
      |2={{clade
         |label1=[[Anapsida]]
         |1={{clade
            |1=[[Millerettidae]]
            |label2=[[Procolophonia]]
            |2={{clade
               |1=[[Pareiasauria]]
               |label2=[[Testudinomorpha]]
               |2={{clade
                  |1=[[Procolophonidae]]
                  |2=[[Testudines]]}} }} }}
         |label2=[[Eureptilia]]
         |2={{clade
            |1=[[Captorhinidae]]
            |label2=[[Romeriida]]
            |2={{clade
               |1=''[[Paleothyris]]''
               |label2=[[Diapsida]]
               |2={{clade
                  |1=[[Araeoscelidia]]
                  |2=[[Sauria]]
}} }} }} }} }} }} }}
}}

More recent phylogenetic analyses, such as that of Modesto (1999), support that of Gauthier ''et al.'' (1988) by placing mesosaurs with parareptiles.<ref name=MSP99/> However, these phylogenies follow Laurin and Reisz (1995) in placing Parareptilia within crown-group Reptilia, meaning that mesosaurs are once again members of Reptilia. Using Laurin and Reisz's node-based definition of Sauropsida as "The last common ancestor of mesosaurs, testudines and diapsids, and all its descendants",<ref name=LR95/> Sauropsida and Reptilia are equivalent groupings; mesosaurs and testudines are more closely related to each other than either group is to diapsids,{{Ref_label|A|a|none}} meaning that the clade containing testudines and diapsids (which would be crown-group Reptilia) must also contain mesosaurs. Since Reptilia was named earlier than Sauropsida, it is used most often in modern phylogenetic analyses.<ref name=MA04/>

{{Cladogram|align=center|title='''Modesto, 1999'''<ref name=MSP99/>
|clades={{clade| style=font-size:80%;line-height:80%
|label1=[[Amniota]]
|1={{clade
   |1=[[Synapsida]]
   |label2=<span style="background:#DAF2CE">'''[[Reptilia]]'''</span>
   |2={{clade
      |label1=[[Anapsida]]
      |1={{clade
         |1='''MESOSAURIDAE'''
         |label2=<span style="background:#F2CECE">'''[[Parareptilia]]'''</span>
         |2={{clade
            |1=[[Millerettidae]]
            |label2=[[Procolophonia]]
            |2={{clade
               |1=[[Pareiasauria]]
               |label2=[[Testudinomorpha]]
               |2={{clade
                  |1=[[Procolophonidae]]
                  |2=[[Testudines]]}} }} }} }}
      |label2=[[Eureptilia]]
      |2={{clade
         |1=[[Captorhinidae]]
         |label2=[[Romeriida]]
         |2={{clade
            |1=''[[Paleothyris]]''
            |label2=[[Diapsida]]
            |2={{clade
               |1=[[Araeoscelidia]]
               |2=[[Sauria]]
}} }} }} }} }} }}
}}

A 2017 phylogenetic analysis by Laurin (who had previously published the 1995 study) and Piñeiro recovered mesosaurs as a basal member of Sauropsida/Reptilia and no longer present within Parareptilia, with Parareptilia being redefined as including former members of [[Procolophonomorpha]] (found to be paraphyletic), [[Millerosauria]], [[Pareiasaur]]ia, and [[Pantestudines]], with the latter two being found to be [[sister group]]s to one another. Parareptilia was also found to actually nest inside Diapsida as the sister group to [[Neodiapsida]].<ref name="Graciela H 2017">{{Cite journal|last1=Laurin|first1=Michel|last2=Piñeiro|first2=Graciela H.|date=2017|title=A Reassessment of the Taxonomic Position of Mesosaurs, and a Surprising Phylogeny of Early Amniotes|journal=Frontiers in Earth Science|language=English|volume=5|page=88|doi=10.3389/feart.2017.00088|bibcode=2017FrEaS...5...88L|issn=2296-6463|doi-access=free|hdl=20.500.12008/33548|hdl-access=free}}</ref>

{{Cladogram|align=center|title='''Laurin & Piñeiro''', 2017 <ref name="Graciela H 2017">{{Cite journal|last1=Laurin|first1=Michel|last2=Piñeiro|first2=Graciela H.|date=2017|title=A Reassessment of the Taxonomic Position of Mesosaurs, and a Surprising Phylogeny of Early Amniotes|journal=Frontiers in Earth Science|language=English|volume=5|page=88|doi=10.3389/feart.2017.00088|bibcode=2017FrEaS...5...88L|issn=2296-6463|doi-access=free|hdl=20.500.12008/33548|hdl-access=free}}</ref> 
|clades={{clade|align=center| style=font-size:80%;line-height:80%
|label1=[[Amniota]]
|1={{clade
   |1=[[Synapsida]]
    |label2=<span style="background:#CECEF2">'''[[Sauropsida]]'''</span>/<span style="background:#DAF2CE">'''[[Reptilia]]'''</span>
   |2={{clade
     |1='''MESOSAURIDAE'''
     |2={{clade
        |1=[[Captorhinidae]]
        |label2=[[Diapsida]]
        |2={{clade
            |1=[[Araeoscelida]]
            |2 = {{clade
               |1=''[[Paleothyris]]''
               |2={{clade
                    |1=[[Neodiapsida]]
|label2=<span style="background:#F2CECE">'''[[Parareptilia]]'''</span>
                    |2={{clade
                        |1=[[Acleistorhinidae]]
                        |2={{clade
                           |1=[[Millerosauria]]
                           |2={{clade
                               |1=[[Owenettidae]]
                               |2={{clade
                                   |1=[[Procolophonidae]]
                                   |2={{clade
                                        |1=[[Pareiasauria]]
                                        |2=[[Pantestudines]]
}} }} }} }} }} }} }} }} }} }} }} }}
}}

In 2012 it was revealed that ''Mesosaurus'' has holes at the back of the skull called lower [[temporal fenestrae]], a characteristic once thought to be present only in synapsids and diapsids.<ref name=PFRL12>{{cite journal |last=Piñeiro |first=G. |author2=Ferigolo, J. |author3=Ramos, A. |author4= Laurin, M.  |year=2012 |title=Cranial morphology of the Early Permian mesosaurid Mesosaurus tenuidens and the evolution of the lower temporal fenestration reassessed |journal=Comptes Rendus Palevol |volume=11 |issue=5 |pages=379–391 |doi=10.1016/j.crpv.2012.02.001 }}</ref>  This confirmed the previous results of German paleontologist Friedrich von Huene, already published in 1941 <ref>{{Cite journal|last=Huene|first=F. von|date=1941|title=Osteologie und systematische Stellung von Mesosaurus|url=https://www.schweizerbart.de/papers/pala/detail/A092/70428/Osteologie_und_Systematische_Stellung_von_iMesosaurus_i|journal=Palaeontographica Abteilung A|volume=92|pages=43–58}}</ref> 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 (reptile)|Candelaria]]'', ''[[Bolosaurus]]'', and [[lanthanosuchoid]]s possess lower temporal fenestrae.

[[File:Skull anapsida 1.svg|thumb|left|200px|The skull of a generalized anapsid.]]
[[File:Skull synapsida 1.svg|thumb|left|200px|The skull of a generalized synapsid.]]

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''.<ref name=PFRL12/>