Mesosaurs ("middle lizards") are members of the extinct reptilian order Mesosauria and family Mesosauridae that lived during the Early Permian period. Mesosaurs were the first known aquatic reptiles, having apparently returned to an aquatic lifestyle from more terrestrial ancestors. Fossils have been found in southern Africa (Namibia, South Africa[1]) and South America (Brazil, Uruguay, Paraguay[1]), around the shorelines of the former Irati–Whitehill sea, an epicontinental sea that covered parts of southern Pangaea during the Early Permian. Most authors consider mesosaurs to have been aquatic,[2][3] although adult animals may have been amphibious, rather than completely aquatic, as indicated by their moderate skeletal adaptations to a semiaquatic lifestyle.[4][5] Similarly, their affinities are uncertain; they may have been among the most basal sauropsids[6][7] or among the most basal parareptiles (in the case of which parareptiles were basal sauropsids).[8][9]
Mesosaurs have been described as small-medium sized reptiles, with hatchlings around 10–12 centimetres (3.9–4.7in) long, and young adult specimens around 80–90 centimetres (31–35in) in length, with most collected specimens being around 70 centimetres (28in). The largest specimens reach a length of 2 metres (79in), though such specimens are rare as fossils, perhaps because large individuals had different environmental preferences than younger individuals.[10] The bodies of mesosaurs are slender with an elongate tail,[1] which was proportionally narrow from side to side.[11] The bones exhibit pachyosteosclerosis (the property of being both thick and relatively dense).[1] The limbs are paddle-like,[11] with preserved soft tissue from some individuals indicating that the feet were webbed.[12] The rostrum (the front part of the skull) is relatively elongate[1] and mouth exhibits numerous, slender conical teeth, which underwent replacement, as well as palatal teeth at the roof of the mouth.[11][1] The three genera of mesosaurs have generally been distinguished by the shape of their teeth, recent studies have suggested that the teeth actually changed shape of the course of growth, and that only Mesosaurus is actually a valid genus, with the other two genera representing juvenile or fragmentary specimens of Mesosaurus.[1]
Paleobiology and paleoenvironment
Recently described embryos show that pachyostosis of the ribs (which were thicker and denser than in terrestrial tetrapods) developed even before hatching, which suggests that mesosaurs were able to swim at birth, or shortly thereafter. They were apparently not very fast swimmers, with an optimal swimming speed estimated to have been between 0.15 and 0.86m/s.[13] They have long been thought to have been coastal forms that probably inhabited relatively shallow water,[14] but recent research suggests that at least those from Uruguay inhabited a hypersaline environment, rather than a coastal marine environment.[15] It has been suggested that at least adult mesosaurs were not fully aquatic and had some ability to move terrestrially on land.[5][12]
Mesosaurs are thought to have been aquatic predators that used their elongate conical teeth as a trap to restrain prey, likely crustaceans belonging to the extinct order Pygocephalomorpha.[1] Mesosaurs may also have engaged in cannibalism.[16] Their reproductive mode is somewhat uncertain because association between adults and possible embryos in utero suggests viviparity, as in many aquatic reptiles, but a potentially isolated egg has also been found.[17]
Fossils of mesosaurs have been found in southern Africa (Namibia, South Africa) and South America (Brazil, Uruguay).[1] Mesosaurs lived on the shore surrounding the Irati–Whitehill sea, an epicontinental sea that existed in the southern part of GondwananPangaea covering parts of south and eastern South America and southern Africa during the Early Permian for a period of up to 4 million years during the late Kungurian age of the Early Permian, around 275 million years ago, before drying up, resulting the extinction of the mesosaurs.[18]
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.[19] 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,[20][21][22][23] and, more commonly, as a sister taxon to Archosauria (made up of crocodiles, dinosaurs – including birds – and allies).[24] Furthermore, Anapsida is rarely considered a valid clade in recent phylogenetic analyses.[25][26] 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.[27]
The study of Laurin and Reisz (1995) was the second major phylogenetic analysis of amniotes.[28] 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.[27]
More recent phylogenetic analyses, such as that of Modesto (1999), support that of Gauthier et al. (1988) by placing mesosaurs with parareptiles.[8] 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",[28] Sauropsida and Reptilia are equivalent groupings; mesosaurs and testudines are more closely related to each other than either group is to diapsids,[a] 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.[27]
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.[29]
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.[30] This confirmed the previous results of German paleontologist Friedrich von Huene, already published in 1941 [31] 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 skull of a generalized anapsid.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.[30]
Notes
^ Studies using molecular phylogenetics, which examine the genes and proteins of living organisms, suggest that testudines (turtles) are diapsids. These studies show that mesosaurs do not form a clade with turtles that excludes diapsids, but fossil evidence still suggests that mesosaurs form a group with parareptiles. In most recent studies, Reptilia is not used as a crown group and still contains mesosaurs and Parareptilia.
↑Canoville, A. and M. Laurin. 2010. Evolution of humeral microanatomy and lifestyle in amniotes, and some comments on paleobiological inferences. Biological Journal of the Linnean Society 100:384–406.
↑Modesto, S. 2006. The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology. Zoological Journal of the Linnean Society 146:345-368.
↑Piñeiro, G. 2008. Los mesosaurios y otros fosiles de fines del Paleozoico; pp. 179–205 in D. Perera (ed.), Fósiles de Uruguay. DIRAC, Montevideo.
12Pablo Nuñez Demarco et al. Was Mesosaurus a Fully Aquatic Reptile? Front. Ecol. Evol, published online July 27, 2018; doi: 10.3389/fevo.2018.00109
↑Laurin, M. and R. R. Reisz. 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society 113:165-223.
123Modesto, S.P. (1999). "Observations of the structure of the Early Permian reptile Stereosternum tumidum Cope". Palaeontologia Africana. 35: 7–19.
↑MacDougall, M.J.; Modesto, S.P.; Brocklehurst,N.; Verriere, A.; Reisz, R.R.; Fröbisch, J. (2018). "Response: A Reassessment of the Taxonomic Position of Mesosaurs, and a Surprising Phylogeny of Early Amniotes". Front. Earth Sci. 6:99 doi: 10.3389/feart.2018.00099
↑Villamil, J. N.; Demarco, P. N.; Meneghel, M.; Blanco, R. E.; Jones, W.; Rinderknecht, A. S.; Laurin, M.; Pineiro, G. (2015). "Optimal swimming speed estimates in the Early Permian mesosaurid Mesosaurus tenuidens (Gervais 1867) from Uruguay". Historical Biology. 28 (7): 963–971. doi:10.1080/08912963.2015.1075018. S2CID85846750.
↑Oelofsen, B. and D. C. Araújo. 1983. Palaeoecological implications of the distribution of mesosaurid reptiles in the Permian Irati sea (Paraná basin), South America. Revista Brasileira de Geociências 13:1–6.
↑Rivaldo R. Da Silva; Jorge Ferigolo; Piotr Bajdek; Graciela H. Piñeiro (2017). "The feeding habits of Mesosauridae". Frontiers in Earth Science. 5: Article 23. doi:10.3389/feart.2017.00023.
12Gauthier, J.A.; Kluge, A.G.; Rowe, T. (1988). "The early evolution of the Amniota". In Benton, M.J. (ed.). The Phylogeny and Classification of the Tetrapods. Vol.1. Oxford: Clarendon Press. pp.103–155. ISBN978-0198577058.
↑Borsuk−Białynicka, Magdalena; Evans, Susan E. (2009). "A long−necked archosauromorph from the Early Triassic of Poland". Paleontologica Polonica. 65: 203–234.
↑Evans, Susan E. (2009). "An early kuehneosaurid reptile (Reptilia: Diapsida) from the Early Triassic of Poland". Paleontologica Polonica. 65: 145–178.
12Piñeiro, G.; Ferigolo, J.; Ramos, A.; Laurin, M. (2012). "Cranial morphology of the Early Permian mesosaurid Mesosaurus tenuidens and the evolution of the lower temporal fenestration reassessed". Comptes Rendus Palevol. 11 (5): 379–391. Bibcode:2012CRPal..11..379P. doi:10.1016/j.crpv.2012.02.001.
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