Pareiasaur phylogeny and the origin of turtles

Abstract The evolutionary relationship of all the valid species (and thus genera) of pareiasaurs are assessed through a phylogenetic analysis of these taxa together with turtles,Owenetta,Barasaurus,Sclerosaurus, procolophonids, lanthanosuchids, nyctiphruretids, and nycteroleterids. 128 osteological characters were used, and almost all relevant taxa were examined. The results confirm that among these taxa, pareiasaurs and turtles form a robust clade, to the exclusion of all other taxa including procolophonids. However, pareiasaurs might not be the monophyletic sister group of turtles, as previously suggested. Rather, there is some evidence that pareiasaurs are paraphyletic with respect to (i.e. ‘ancestral to’) turtles. Among pareiasaurs, the early, large, heavily ossified forms such asBradysaurusare most distantly related to turtles. These forms are characterized by rather smooth skulls, and dermal armour restricted to the dorsal midline. More closely related to turtles are forms such asScutosaurus,Pareiasuchus, andElginia. These taxa form a distinct clade of pareiasaurs, characterized by a very ‘mammal-like’ pelvis, elaborate cranial ornamentation and a loose covering of osteoderms over the entire dorsum. The late, dwarf pareiasaursNanoparia,Anthodon, andPumiliopareiaare the nearest relatives of turtles. These forms exhibit otherwise uniquely turtle features such as a rigid covering of dermal armour over the entire dorsal region, expanded flattened ribs, cylindrical scapula blade, great reduction of humeral torsion (to 25°), greatly developed trochanter major, offset femoral head, and reduced cnemial crest of the tibia. Thus, many features thought to be restricted to turtles (and thus to have evolved simultaneously with the turtle shell) actually arose earlier, at various points along the pareiasaurian stem lineage. The identification of the nature and sequence of anatomical changes leading to the origin of turtles, and the possibility that turtles are derived from dwarf pareiasaurs, should have important implications for speculations on the evolutionary biology of turtle origins.