Locomotion in the earliest tetrapods: testing models of terrestriality

The water - to - land transition made by vertebrates during the Devonian period, between 370 and 360 million years ago, was one of the most profound in evolutionary history, and ultimately allowed the appearance of all land vertebrates including humans. The changes involved fundamental modifications to skeletons - from supports for swimming musculature in water to robust weight-bearing struts and cantilevers for walking and running on land, as well as changes to physiological and sensory systems. These latter changes also had influences on the skeleton that in turn affected the locomotor patterns. Over the last few years we have a gained a much improved fossil record of the animals that represent this transition. It has vastly improved since the days when only two taxa were available (the fish Eusthenopteron and the tetrapod Ichthyostega): we now have the elpistostegids Panderichthys and Tiktaalik, and the tetrapod Acanthostega. These newly discovered skeletons are well enough known to permit hypotheses about relationships and sequences of character acquisition. However, this is only part of the story. It is now evident that the two Devonian tetrapods whose skeletons are adequately known (Acanthostega and Ichthyostega) differ radically not just from modern tetrapods, but from each other. They represent divergent answers to some of the problems of emerging from the water. Ichthyostega and Acanthostega are evolutionary icons of importance comparable to that of other critical 'transitional forms' such as Archaeopteryx (the first bird), Australopithecus (e.g., 'Lucy'), and Ambulocetus (an early 'walking whale'). Acanthostega though possessing limbs with digits was still basically aquatic. On the other hand, the popular image of Ichthyostega as a sturdy land-going quadruped has been shown to be quite erroneous: in fact it was highly aberrant skeletally, having morphological traits that suggest both aquatic and terrestrial specialisations. Its mode of locomotion appears to have been more like that of a seal than a salamander, and its skeleton, according to work by the PI and colleagues, seems to have allowed dorsoventral flexion using a regionalised vertebral column and differentiated musculature as in mammals. We want to test both these diverse models of early tetrapod morphology for their accuracy quantitatively and in three dimensions, which our earlier attempts were unable to do. This we will do by subjecting the specimens to recently developed high-tech methods including microCT, CT and laser scanning, 3D reconstruction and manipulation software that allow us to do this for the first time. Next, we will examine the range of movements of the skeletons to assess what they were capable of, and quantify key indices of biomechanical performance such as regional stiffness of the vertebral column as well as internal bone architecture. For this, we will compare particular parts of the skeleton - centra, neural arches, ribs, limbs and girdles - with modern tetrapods to try to understand, using modern quantitative techniques, how the Devonian tetrapods moved, to what conditions they were most suited, and thus to what environments and life styles they were best adapted. This will help to illuminate the earliest conditions of the transition from water to land and ultimately lead to greater understanding of how modern tetrapod skeletons evolved. This will not only enhance our understanding of the transition from water to land, but lay the foundations and establish the starting conditions for the eventual appearance of the truly terrestrial members of the tetrapod crown group, and thus our own remote ancestors.