Fossil find turns back evolutionary clock

The point in evolution when fish took their first tentative steps on to land has been put back by several million years because of the discovery of a fossil species that is half fish, half amphibian.

Two fragments of the lower jawbone of a primitive land animal about 4ft long were found in Latvia, fossilised in rock estimated to be about 375 million years old. This is about five million years earlier than the time fish were previously believed to have made the transition from a marine to a terrestrial environment.

Scientists at the Natural History Museum in London, working with colleagues in Latvia and Estonia, have not yet formally named the new species, but they are convinced it represents a "missing link" in the chain of events leading to one of the most important moments of life on Earth.

Per Ahlberg, a palaeontologist at the museum, said the animal's jawbone contains rows of sharp teeth, showing that it must have been acrocodile-like predator inits swampy habitat.

Four-legged animals with backbones made only one transition to land. All 25,000 living species of vertebrates, including humans, are descended from the same stock of fish-like tetrapods, which became the first amphibians.

Dr Ahlberg announced the discovery at the Nature's Treasurehouses conference, which is an attempt by the museum to bring together natural history scientists with specialists from other academic spheres.

Dr Ahlberg said: "This [discovery] is about our own ancestry. We came from this route. It tells us where we come from, and also perhaps who we are. The fossil is a perfect intermediate condition. It's forced the reinterpretation of another fossil, an animal previously thought to be a fish."

The fossilised jaw, excavated from a site in Latvia, has an arrangement of bones more advanced than that of an animal called Panderichthys, the most tetrapod-like fish. The find has made scientists re-examine another animal, called Elpistostege, which was thought also to be a fish. Elpistostege will now be reclassified as a true tetrapod. "Thus we suddenly have two potential transitional forms where previously we had none," Dr Ahlberg said.

Details of the find will be published in the journal Palaeontology in a paper written by Dr Ahlberg, Ervins Luksevics of the Latvian University and Elga Mark-Kurik of Tallinn Technical University in Estonia.