FOWLER, D.W. & HALL, L.E. (2009) Scratch-digging sauropods, revisited: similarity of sauropod pes unguals to tortoises, and inferred nest-excavating behaviour, Dinosaur Eggs & Babies, abstracts volume: 18

ABSTRACT

Sauropod dinosaurs were the heaviest animals ever to have walked on land. Their columnar limbs were highly adapted for weight support, often being compared to those of modern elephants. However, in ungual morphology they differ significantly. Early in their evolution, sauropod manual unguals underwent severe reduction and were lost on all digits except digit (D)-I. The pes unguals followed a markedly different trend, becoming hypertrophied and flattened with an unusual orientation. Despite the highly derived condition of sauropod pes unguals, little consideration has been given to their form and possible function. While in other dinosaurs, the unguals might have played a significant role in locomotion, sauropod unguals seem to have had no role in weight support. An early suggestion of a scratch digging function (aiding in nest building) was dismissed by recent workers without comparison to non-mammalian scratch digging taxa, proposing instead a substrate-gripping function during locomotion. In contrast, here we show that the morphology of sauropod pes unguals is very similar to that observed in some tortoises, especially the extant gopher tortoise Gopherus, a specialized burrower, and other extinct non-mammalian taxa inferred to have been specialist scratch diggers. Excavation of nests by tortoises uses different limb movements than observed in mammalian scratch-diggers, partially accounting for their different morphology. We suggest that the similarly shaped pes claws of sauropod dinosaurs were used in a comparable fashion, for movement of sediment during nest excavation. Despite being only an intermittent behaviour, successful nest-building is essential to the propagation of an individual’s genes and would be strongly selected for. Some species of non-burrowing tortoises which also excavate nests show increased average pes claw sizes in females. This presents the intriguing possibility that given a much larger dataset than is currently available, gender may be determinable in sauropods by analysis of claw size.