Correlated progression and the origin of turtles
Abstract
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 region1,2 Our understanding of chelonian origins has been restricted by a paucity of information on intermediate forms3,4, however, and it is often assumed that they must have evolved saltationally5. It has been suggested that pareiasaurs, a group of large herbivorous anapsid reptiles, are the sister-group of turtles6. Here I show that certain pareiasaurs-dwarf, heavily armoured forms such a Nanoparia-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.
- Publication:
-
Nature
- Pub Date:
- February 1996
- DOI:
- 10.1038/379812a0
- Bibcode:
- 1996Natur.379..812L