CORRELATED PROGRESSION AND THE ORIGIN OF TURTLES

Turtles exhibit some of the most extreme postcranial modifications fou nd in vertebrates. The dorsal vertebrae and ribs have fused with derma l armour, forming a totally rigid box-like trunk region's Our understa nding of chelonian origins has been restricted by a paucity of informa tion on intermediate forms(3,4), however, and it is often assumed that they must have evolved saltationally(5). It has been suggested that p areiasaurs, a group of large herbivorous anapsid reptiles, are the sis ter-group of turtles(6). Here I show that certain pareiasaurs--dwarf, heavily armoured forms such a Nanoparin--approach the chelonian morpho logy even more closely than previously thought. Evolutionary trends wi thin pareiasaurs, such as the elaboration of the dermal armour, shorte ning and stiffening of the presacral region, and increased reliance on limb-driven as opposed to axial-driven locomotion, suggest that the r igid armoured body of turtles evolved gradually, through 'correlated p rogression'(7).