Three-dimensional limb joint mobility in the early tetrapod Ichthyostega

Three-dimensional reconstruction and modelling of limb joint mobility in the early tetrapod Ichthyostega is used to provide insights into an important step in vertebrate evolution—the transition from swimming to walking. The locomotion of the early tetrapod Ichthyostega is a puzzle because it had an unusual skeleton — a robust ribcage, a spine adapted for up-and-down rather than lateral movement, and paddle-like limbs that seem unsuited to walking. This paper presents a three-dimensional reconstruction and computer modelling of limb-joint mobility based on Ichthyostega fossils. The models suggest that Ichthyostega was unable move its limbs in a walking pattern. Instead it may have shuffled on land using movements similar to those seen in seals. This implies that the transition from swimming to walking was a gradual process. The origin of tetrapods and the transition from swimming to walking was a pivotal step in the evolution and diversification of terrestrial vertebrates. During this time, modifications of the limbs—particularly the specialization of joints and the structures that guide their motions—fundamentally changed the ways in which early tetrapods could move1,2,3,4. Nonetheless, little is known about the functional consequences of limb anatomy in early tetrapods and how that anatomy influenced locomotion capabilities at this very critical stage in vertebrate evolution. Here we present a three-dimensional reconstruction of the iconic Devonian tetrapod Ichthyostega and a quantitative and comparative analysis of limb mobility in this early tetrapod. We show that Ichthyostega could not have employed typical tetrapod locomotory behaviours, such as lateral sequence walking. In particular, it lacked the necessary rotary motions in its limbs to push the body off the ground and move the limbs in an alternating sequence. Given that long-axis rotation was present in the fins of tetrapodomorph fishes5,6,7, it seems that either early tetrapods evolved through an initial stage of restricted shoulder8,9 and hip joint mobility or that Ichthyostega was unique in this respect. We conclude that early tetrapods with the skeletal morphology and limb mobility of Ichthyostega were unlikely to have made some of the recently described Middle Devonian trackways10.