The search for the missing link between amphibians and land-dwelling reptiles has been greatly enhanced by fossil discoveries in Scotland. Tetrapods – vertebrates with four limbs – made the transition from water to land somewhere between 360 and 340 million years ago, following the mass extinction event at the end of the Devonian period. This time period is known as ‘Romer’s Gap’ because of the severely limited number of fossils found across the globe charting these water-land transitions. An ambitious and intriguing project called TWeed (Tetrapod World: early evolution and diversification) began in 2011 with the aim of searching for and defining early tetrapod fossils. With the theory that the fossils must be there in the rocks to be found, particularly in light of the discovery of ‘Lizzie’ – Westlothiana lizziae, a link between amphibians and reptiles – in West Lothian in 1984, the TWeed group have recently uncovered new ‘Romer’s Gap’ fossils in South East Scotland.
Found in early Carboniferous sedimentary rocks (c.358-346 million years ago), the information gathered from the new tetrapod fossils will allow scientists to verify the environmental and geological conditions present after the mass extinction event which ultimately led to the evolution of modern life. The Scottish fossils are of such high quality and hold such a wealth of information that the TWeed project has labelled the place where they were found as a site of international scientific significance.
Tim Kearsey, geologist at the British Geological Survey, says: I have been interested in the major events that have affected the evolution of life on land my entire career. In my PhD I studied what happened during the Permian –Triassic mass extinction and currently I am part of the TW:eed Project (Tetrapod World: early evolution and diversification). The team is investigating the earlier known terrestrial tetrapods discovered from 360 million year old rocks in the Scottish Borders. This is one of only two areas in the world where such fossils have been found.
How our tetrapod ancestors (4 limbed vertebrates) came out of the swamps and adapted to life on land some 360 million years ago is still poorly understood. We don’t know how lungs, ears and other adaptations for land evolved. These new Scottish fossils, along with the fossils such as Westlothiana lizziae from West Lothian, shows that Scottish rocks contain a remarkable record through what is arguably one of the most significant events in evolution and the story of how and why our ancestors made the transition from water to exploit the land fully.