Four feet in the past

The discovery of fossil trackways made by four-legged land vertebrates (tetrapods) almost 400 million years ago will cause a significant reappraisal of our understanding of tetrapod origins. The finds, reported by Per Ahlberg and colleagues, come from Zachelmie Quarry in the Holy Cross Mountains of Poland. Some of the tracks are so well preserved as to permit detailed examination of the foot morphology, which resembles that of the early, primitive tetrapod Ichthyostega. But it is their age that makes these tracks so special: 18 million years older than the earliest known tetrapod body fossils, and 10 million years older than the oldest elpistostegids — Tiktaalik , Panderichthys and their relatives, seen as transitional forms between fishes and tetrapods. The finds suggests that the elpistostegids that we know were late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates.