The postcranial anatomy of Procolophon (Parareptilia: Procolophonidae) and its implications for the origin of turtles (French and English text)

A detailed account of the postcranial anatomy of the Gondwana, Triassic genus Procolophon has served to augment significantly the current state of knowledge relating to Permo-Triassic parareptiles. Presently Procolophon represents the most completely studied member of Parareptilia and the wealth of morphological data gathered herein has allowed for a critical assessment of its phylogenetic position within Parareptilia.; Procolophon represents a basal member of the Triassic clade Procolophonidae. In addition, the enigmatic taxon Sclerosaurus is included within Procolophonidae.; A functional interpretation of the overall anatomy of Procolophon and a comparison with living taxa using a phylogenetic methodology is performed. Derived traits are mapped onto respective fossil and extant phylogenies leading to a plausible reconstruction of the preferred procolophonid habitus; the data supports a lifestyle of a burrowing or fossorial organism.; Basal parareptile relationships are presented with the reevaluation of the Permian, Laurasian taxon Acleistorhinus pteroticus. The restudy presents a heretofore unrecognized relationship between the Russian lanthanosuchids and this taxon. In addition, the analysis supports the monophyly of Parareptilia and suggests a possible origin for the clade in the New World.; Support for a monophyletic Parareptilia and the recognition of a well corroborated monophyletic Procolophonidae including the detailed anatomical study of Procolophon allows for an assessment of competing views pertaining to turtle origins. A global analysis is undertaken using 172 characters and 38 taxa including basal representatives of all amniote clades and more recent members of crown clades. After careful consideration of all the data, an alternate view of turtle relationships is presented suggesting that turtles are highly derived members of the Crown-group Sauria and nested within Lepidosauromorpha.; The present state of our knowledge precludes a final word on the origin of turtles, however. Notwithstanding some recently published molecular support for the turtle/saurian relationship, dissenting views have reiterated a turtle/parareptile kinship. Detailed anatomical studies of other pertinent taxa including nycteroleterids, pareiasaurs (Parareptilia), and placodonts (Sauria), as well as additional molecular and developmental studies are underway and must be completed before any other opinions regarding turtle genealogy can or should be considered.