Problems of the ancestry of turtles

Abstract

The unquestioned unity of the Chelonia provides a necessary basis for establishing their interrelationships and determining the evolutionary history within the group. On the other hand, the host of uniquely derived features of the oldest known turtles make it extremely difficult to establish their ancestry among more primitive amniotes. This is illustrated by the great diversity of taxa that continue to be proposed as putative sister-taxa of turtles without general acceptance of any. Nearly every major clade of early amniotes from the late Paleozoic and early Mesozoic has been proposed as a possible sister-taxon of turtles, from synapsids to anapsids and diapsids, including pelycosaurs, captorhinomorphs, procolophonids, pareiasaurs, aquatic placodonts and crocodiles, but none possess derived characters that could be synapomorphic with the unique skeletal structure and patterns of development of the chelonian skull, carapace or plastron, which had reached an essentially modern configuration by the Late Triassic. Numerous molecular biologists have attempted to establish the closest sister-group of turtles through analyses of a host of living species, but there is no way for them to preclude turtles from having evolved from one or another of the Paleozoic or early Mesozoic clades that have become extinct without leaving any other living descendants. On the other hand, recent studies of the genetic and molecular aspects of the development of the carapace and plastron imply unique patterns of evolutionary change that cannot be recognized in any of the other amniote lineages, living or dead. This, together with the retention of a skull without temporal fenestration implies a very early divergence from a lineage that probably retained an anapsid skull configuration. This problem may be resolved by more detailed study of the enigmatic genus Eunotosaurus, from the Late Permian of South Africa.