Evolution: Oldest-known caecilian found

The oldest-known fossil of a worm-like amphibian known as a caecilian is reported in this week’s Nature. The finding sheds light on the origins of caecilians and their position in the amphibian group.

Modern-day caecilians are limbless amphibians that look like worms or snakes. Their origins are poorly understood, due to a paucity of relevant fossils. In this study, Ben Kligman and colleagues analysed the fragmented, fossilized remains of at least 76 individual caecilians, found in the Upper Triassic Chinle Formation of Petrified Forest National Park, Arizona, USA. Dated at 220 million years old, the findings extend the caecilian fossil record by 35 million years.

The creature combines features found in the common ancestor of living caecilians with those of an extinct group of four-legged amphibians called the dissorophoid temnospondyls. It is also missing certain features that are present in modern caecilians, such as the tenticular organ. As such, it bridges the gap between modern caecilians and extinct tetrapods and confirms the position of caecilians within Lissamphibia: the group that also contains frogs and salamanders.