Meeting Abstract

S3-1.3  Saturday, Jan. 5  Historical evolution of early tetrapod movement PIERCE, SP*; HUTCHINSON, JR; CLACK, JA; The Royal Veterinary College, UK; The Royal Veterinary College, UK; University Museum of Zoology, Cambridge, UK spierce@rvc.ac.uk

Conceptualizations of the evolution of tetrapod locomotion have changed drastically in the past 50 years. When early tetrapod fossils were first discovered, the animals were reconstructed as salamander-like in their mode of locomotion, walking around on four sturdy legs. In fact, the "prototetrapod" was envisaged as a terrestrially capable creature with a fish-like body and modified pectoral/pelvic fins equipped with weight supporting joints and the beginnings of digits, but no sacrum. 'Conquest of land' was seen as the driving force in the evolution of limbs. However, intensive re-examination of fossil material and the discovery of key specimens has gradually redefined our perception of the tetrapod bauplan. The prevailing theory is that early tetrapods were primarily aquatic in habit and that limbs evolved before the ability to ‘walk’ on land. New fossil footprints have challenged this idea by inferring early tetrapods were walking - perhaps partially supported by water - 20 million years before any known tetrapod body fossils. Another recent study has posited that sarcopterygian fishes evolved hindlimb powered locomotion, which was later exapted for usage in tetrapods. However, our recent work on the late Devonian tetrapod Ichthyostega has demonstrated that its limb joints did not permit a walking gait like that of a living salamander, and that land locomotion was forelimb-driven. Considering that other closely related stem tetrapods seem to have had a similar limb joint structure, this may have been an ancestral state, although the anatomy of earlier Devonian tetrapods remains unknown. The historical transformation of locomotion potential, and the drivers of land dwelling in the earliest limbed vertebrates, has thus changed drastically, with several different hypotheses having been put forward over the past few years. New information and methodological techniques are helping to refine and shape our understanding of this pivotal evolutionary event.