Correlated progression and the origin of turtles

Turtles exhibit some of the most extreme postcranial modifications found in vertebrates. The dorsal vertebrae and ribs have fused with dermal armour, forming a totally rigid box-like trunk region's Our understanding of chelonian origins has been restricted by a paucity of information on intermediate forms(3,4), however, and it is often assumed that they must have evolved saltationally(5). It has been suggested that pareiasaurs, a group of large herbivorous anapsid reptiles, are the sister-group of turtles(6). Here I show that certain pareiasaurs--dwarf, heavily armoured forms such a Nanoparin--approach the chelonian morphology even more closely than previously thought. Evolutionary trends within pareiasaurs, such as the elaboration of the dermal armour, shortening and stiffening of the presacral region, and increased reliance on limb-driven as opposed to axial-driven locomotion, suggest that the rigid armoured body of turtles evolved gradually, through 'correlated progression'(7).