Our record of tetrapod taxa from the Late Devonian has expanded greatly in recent years. Two of the best known are Ichthyostega and Acanthostega from Upper Devonian (Famennian) rocks of Greenland. The relative completeness of their skeletal remains has allowed us to gain new insights into how they could move, or rather, not move. Both were to a large extent aquatic animals: Acanthostega was primarily aquatic with little capability of terrestrial locomotion, whereas Ichthyostega was capable of some excursions on land. We used microCT and synchrotron scanning methods to explore the range of motion of the limb joints in Ichthyostega. We showed that it did not ‘walk’ in the traditional sense, but seems to have used a ‘crutching’ motion in which the limbs were used in a way comparable to that of a mudskipper. 

At the end of the Devonian, a mass extinction event resulted in the loss of many aquatic vertebrate taxa such as placoderms, many early lobe-finned fishes, and many species of acanthodians. Following this, chondrichthyans and ray-finned fishes radiated, but coelacanths and lungfishes were reduced in diversity. Tetrapods survived, but because of an apparent hiatus in the fossil records, we knew very little about them. By the time they reappear in numbers in the fossil record, in the mid-Carboniferous Visean stage, they are fully terrestrial and diverse. How they evolved from their aquatic predecessors into terrestrial forms was unknown. Recently, new material, of tetrapods, fish, arthropods and plants, from the earliest Carboniferous Tournaisian stage is helping to fill that gap. For example, the earliest known pentadactyl foot has been found, from this time, alongside new tetrapod taxa showing a range of previously unknown morphologies. This ‘work in progress’ heralds fresh understanding of the advent of tetrapods onto land and the establishment of modern-style ecosystems.