Early tetrapods - the first four-legged vertebrates - diversified rapidly after the late Devonian mass extinction 358 million years ago to quickly split into two groups: the ancestors of amphibians, and the ancestors of reptiles, birds and mammals. New research, published online in Nature Ecology & Evolution this week, describes fossils from Scotland that help to fill a 25 million year hole in the tetrapod fossil record - a key time in their evolution about which little is known.
Tetrapods from the late Devonian period, before the mass extinction, were relatively fish-like with many digits. The fossils from after the period known as Romer’s Gap have fewer digits and more closely resemble today’s land vertebrates. The reason for the apparent gap in the fossil record is not known, but it has been suggested that low oxygen levels limited the number and diversity of tetrapods.
In their study, Jennifer Clack and colleagues describe five new and diverse species from the Tournaisian period, which occurred just a few million years after the mass extinction, as well as fragments from another seven taxonomic groups. They identify two of the species as early amphibians, and three as more distantly related early tetrapods, which suggests an early split between amphibians and amniotes (reptiles, birds and mammals). The authors also examine oxygen levels by looking at fossilized charcoal, and refute the idea that there was a reduction in atmospheric oxygen.
The authors note that filling the gap in the fossil record gives insight into the critical initial stages of tetrapod evolution, showing that the number and diversity of species increased from early on without being limited by a tough low-oxygen environment.