The Homologies and Early Evolution of the Shoulder Girdle in Turtles

The homologies of the highly distinctive shoulder girdle of turtles are reinterpreted in the light of recent phylogenetic studies. The acromion process is an extension of the scapula blade: it is not a modified anterior coracoid, contrary to recent suggestions. In modern turtles, the acromion process articulates with the plastron (the ventral portion of the turtle shell). However, recent phylogenetic analyses indicate that the acromion arose in the common ancestor of pareiasaurs and turtles, long before the plastron evolved. In pareiasaurs and primitive turtles, the acromion projects anteriorly and forms a strong, mobile articulation between the shoulder girdle and the clavicle. In modern turtles, the acromion has changed its orientation and morphology, projecting ventrally and contacting the plastron. Nevertheless, even in modern turtles the acromion still retains its original function: it supports the shoulder girdle, and allows the shoulder girdle to pivot during locomotion, thus increasing stride length.