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Turtle origins: Chinlechelys tenertesta and convergence in modern cladistic analysis
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1  New Mexico Museum of Natural History and Science
Academic Editor: Angelos G. Maravelis


The phylogenetic relationships of turtles (Testudines) challenge cladistics by demonstrating the inherent weaknesses of this non-Darwinian method of phylogeny reconstruction. Recent cladistic analyses identify sauropterygians as the closest relatives of turtles, and even identify at least one sauropterygian, Pappochelys, as a turtle. This is based largely on convergence of several characters associated with environmental adaptation, including relative lengths of phalanges, dense gastralia placement (assumed proto-plastron) and a well-defined intertrochanteric fossa. The lack of failure testing to identify such convergence is important in the analysis of unusual taxa because it can force a taxon into the in-group as a methodological artifact, as with cladistic placement of Eunotosaurus within Testudines. Eunotosaurus was not placed in a wider vertebrate phylogeny, which later identified it is a basal member of the Caseidae. Characteristics shared by caseids and Testudines contributed to this mistake, including relative head to body size, posterior jaw articulation and a reduced number of dorsal ribs and vertebrae. Late Triassic Chinlechelys provides a useful window into these varied cladistic problems due to anatomical convergence. It demonstrates an intermediate step between a carapace with neighboring ribs and a carapace fused with ribs. This earlier state lacked the organizing role of the ribs in the carapace and had multiple rows of costals (dorsal osteoderms) arranged at an angle to the ribs, a condition only known in some pareiasaurs. This fits with an evo-devo model of gradual change creating a new structure (carapace) followed by adaptive radiation filling the new niches opened by that structure.

Keywords: Chinlechelys tenertesta; Eunotosaurus; Pappochelys; Testudines