A comprehensive analysis of amniotr interrrlationships is presented in a11 attempt t o trst turtle intcrrclationsliips. 'The results rrftitc earlier hypotheses that turtles are related to parareptilrs, i.e. to procolophonids or pareiasaurs. Iristrad, turtles are shown to be the sistergroup of Sauropterygia, the two clades being nested within Sauria as sister-group of Lrpidosauriforrnes. This sccriario is also supportrd by several devc~opnicrital arid soft tissuc charactcrs which arc shown to I)r congrurnt with the current phylogeny. 'l'hc analysis strongly supports a moriophyletic Pararrptilia, sister-group of a monophylctic Eurcptilia. The I)iapsiida, howrvcr, is paraphyletic unless it includes turtles and sauropterygians. Additionally, the position of turtles within Diapsida ha5 major irnplications for the evolutionary history and/or significancc of many characters, i.r. tempor;il fiwrstration. 0
A morphological study of the postcranial skeleton of Procolophon trigoniceps from the Lower Triassic of South Africa and Antarctica is undertaken. Procolophon shares a sister-group relationship with the procolophonid Tichvinskia from the Lower Triassic of Russia and is a basal member of Procolophonidae. This clade also includes the enigmatic taxon Sclerosaurus, believed most recently to be a pareiasaur relative. Owenettids form a separate lineage from Procolophonidae and are predominantly restricted to the Permian of both South Africa and Madagascar. A phylogenetically based assessment is considered, in which specialized modern taxa (sand lizards) are compared to their nonfossorial sister clade, allowing for "key innovations" to be identified. A similar comparison between owenettids and procolophonids reveals a number of apparent "key innovations" within procolophonids that are suggestive of a burrowing lifestyle for Procolophon.
Recent molecular studies of amniote relationships show turtles to be diapsid reptiles, related to the archosauromorph branch of saurian phylogeny. This conflicts with palaeontological data which shows turtles to be related either to anapsids, or to the lepidosauromorph branch within diapsids. Archosauromorph relationships of turtles have previously been proposed on the basis of neontological (mostly soft anatomy) characters. This paper reviews the neontological character evidence for turtle relationships and shows that most, but not all, of these characters are invalid in the reconstruction of turtle relationships within Amniota.
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