Today’s blog will highlight a number of putative clades that are in need of revision, are no longer valid or are redundant in light of the new reptile tree (which is larger than any prior attempt and encompasses all the major clades).
Reptilia – revision
Gauthier, Kluge & Rowe (1988) defined Reptilia as “the most recent common ancestor of extant turtles and saurians, and all of its descendants.” They separated mammals and their ancestors as outgroups to this clade. However, here mammals and their synapsid ancestors appear within the new Archosauromorpha and thus within the Reptilia as defined above. Diapsids, like Petrolacosaurus, were derived from basal synapsids, like Heleosaurus. So we’re all related.
Amniota – redundant
Gauthier, Kluge & Rowe (1988) defined Amniota as “the most recent common ancestor of extant mammals and reptiles, and all its descendants.” Here that definition is redundant with Reptilia, a term which goes back to Linneaus (1758).
Sauria – redundant
Gauthier, Kluge & Rowe (1988) defined Sauria as the most recent common ancestor of Lepidosauria and Archosauria and all of its descendants. Here that definition is redundant with Reptilia.
Sauropsida – paraphyletic
Gauthier (1994) defined Sauropsida as “Reptiles plus all other amniotes more closely related to them than they are to mammals,” based on traditional cladograms that indicated a basal split between the Synapsida and Sauropsida. Here this is a paraphyletic assemblage.
Lepidosauromorpha – retained and redefined
Gauthier, Estes and de Queiroz (1988) defined Lepidosauromorpha as Sphenodon, squamates and all saurians sharing a more recent common ancestor with them than they do with crocodiles and birds. That definition is retained here even though they erred when including “Younginiformes” in their taxon list. Here seven “Younginiformes” nest within the new Archosauromorpha. Lepidosauromorpha therefore becomes the first of the great clades dividing the Reptilia following Cephalerpeton, the basalmost known reptile. Lepidosauromorpha includes turtles, the tuatara, lizards and snakes among living examples. Diadectids, chroniosuchids, caseids, pterosaurs, among others, are extinct lepidosauromorphs.
Laurin 1991 redefined the Lepidosauromorpha as the clade originating with the most recent common ancestor of Palaeagama, Saurosternon, Paliguana, Kuehneosaurus and Lepidosauria. While that is a monophyletic clade, Laurin’s taxon list is also redundant with the list referred to Lepidosauriformes by Gauthier, Estes and de Queiroz (1988). A new node-based definition reflecting a greater presence for the new Lepidosauromorpha is proposed to include Paliguana, Thuringothyris, their most recent common ancestor and all of its descendants.
Lepidosauriformes – revision
Gauthier, Estes and de Queiroz (1988) defined the Lepidosauriformes as Sphenodon, squamates and all organisms sharing a more recent common ancestor with them than they do with younginiforms. Here that definition is equivalent to Lepidosauromorpha. Given a node-based definition, the new Lepidosauriformes is proposed to include Sphenodon, Paliguana, their last common ancestor and all its descendants. Such a clade encompasses the same taxon list that Gauthier, Estes and de Queiroz (1988) intended. Clark & Hernandez (1994) also recovered a monophyletic Lepidosauriformes originating with Saurosternon and Paliguana.
Archosauromorpha – retained
Gauthier (1986) defined Archosauromorpha as all Saurians sharing a more recent common ancestor with Archosauria than with Lepidosauria. That definition is retained. Gauthier’s taxon list included Prolacertiformes, Rhynchosauria, Trilophosaurus and Archosauriformes. Here Rhynchosauria and Trilophosaurus shift to the Rhynchocephalia (Sphenodontia) within the Lepidosauromorpha.
Benton (1990) redefined Archosauromorpha as the most recent common ancestor of Neornithes, Squamata, and all of the descendants of this common ancestor. In the present study, that definition is redundant with Reptilia.
Laurin (1991) proposed that Archosauromorpha include the most recent common ancestor of Prolacerta, Trilophosaurus, Hyperodapedon, archosaurs and all its descendants. Under the present tree topology this definition is also redundant with Reptilia.
Dilkes (1998) proposed a definition for Archosauromorpha that included Protorosaurus and all other saurians that are related more closely to Protorosaurus than to Lepidosauria. This is a suitable definition. Following Gauthier (1986), Dilkes (1998) and the results of this study, a node-based redefinition of the new Archosauromorpha is proposed to include Protorosaurus, Westlothiana, their last common ancestor and all of its descendants. That definition includes synapsids (including mammals), sauropterygians (including ichthyosaurs), mesosaurs, araeoscelidians and archosauriforms (including dinosaurs).
More coming in parts 2 through 4.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Benton MJ 1990. Origin and Interrelationships of dinosaurs, In Weishampel DB, Dodson P, and Osmólska H editors. The Dinosauria. 11–30. Berkeley: U Calif Press.
Clark JM and Hernandez RR 1994. A new burrowing diapsid from the Jurassic La Boca formation of Tamaulipas, Mexico, J Vert Paleo 14: 180–195.
Dilkes D 1998. The Early Triassic rhynchosaur Mesosuchus browni and the interrelationships of basal archosauromorph reptiles. Phil Trans R Soc B 353: 501–541.
Gauthier JA 1986. Saurischian monophyly and the origin of birds, In Padian K editor. The Origin of Birds and the Evolution of Flight, 1–55. Memoirs Calif Acad Sc 8.
Gauthier J, Kluge AG and Rowe T 1988. Amniote phylogeny and the importance of fossils. Cladistics 4: 105–209.
Gauthier J, Estes R and de Queiroz K 1988. A phylogenetic analysis of Lepidosauromorpha, In Estes R, Pregill G, editors. Phylogenetic relationships of the lizard families, 15–98. Stanford: Stanford U Press.
Gauthier JA 1994. The diversification of the amniotes. In: Prothero DR, Schoch RM editors. Major Features of Vertebrate Evolution: 129-159. Knoxville: Paleo Society.
Laurin M 1991.The osteology of a Lower Permian eosuchian from Texas and a review of diapsid phylogeny. Zoological Journal of the Linnean Society 101: 59–95.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.