April 5, 2006

scientists comment on early tetrapod fossil find published in Nature

Fossils of a 375 million year old new species of ancient fish, named Tiktaalik roseae, found north of the Arctic Circle, fill an evolutionary gap in the transition between water and land animals.

Daeschler EB, Shubin NH & Jenkins Jr FA (2006) A Devonian tetrapod-like fish and the evolution of the tetrapod body plan, Nature 440: p757-763

Dr Marcello Ruta, Vertebrate Palaeontologist, University of Bristol, said:

“This is an amazing discovery that has joined many of the dots of a complex picture. When the early limbed vertebrates (tetrapods) evolved from their fish-like ancestors, they didn’t just jump out of water and start crawling around. Instead they gradually gained characteristics that foreshadowed the anatomical and functional features that we find in subsequent land vertebrates. Some of these features evolved further, thus enabling vertebrates to conquer the land, and cope successfully with the requirements of the new environments (breath in the open air, reproduce and be able to move around on land. If you could get in a time capsule and travel back in time to see the new animal described by Shubin and co-authors, you would say it looked like a fish. It has fins, scales and gills like a fish; but it also have a vaguely crocodile-like skull not united with the shoulder girdle, and enlarged ribs, like an early tetrapod, and its eyes are high up on the skull like those of a crocodile or hippopotamus. These eyes presumably enabled the animal, from time to time, to peer out of the water. It is the skeleton of the fins that is really interesting. The animal has a single bone, the humerus, like you or I have, representing the only bone connecting the anterior appendage to the shoulder girdle and there is a radius and ulna too but no wrists and no digits, and a full complement of fin rays as in a fish! It still doesn’t have a wrist or elbow joint that would allow it to prop itself up on land. But it could probably flex its fin and be capable of some degree of movement of its anterior appendage relative to the shoulder girdle. This would enable this extraordinary animal to rest on the floor of streams and support itself in part. This animal fits neatly between four legged animals and fish. The new fossil is known from several exceptionally well preserved and articulated specimens.”

Dr Andrew Milner, Natural History Museum, said:

“Previous fossils representing this evolutionary event have really been fish with a few land characteristics, or land vertebrates with a few residual fish characteristics. These fossils show an animal that sits bang in the middle between the fish and land animals. This material is amazing because it includes a nearly complete skeleton – which is always handy because instead of assembling the fossil from bits we can see the whole skeleton and be sure that this is how the animal was put together.

“It is unusual for a fossil like this to be found in such good condition. In the past scientists have looked in rocks in Greenland, but this fossil was found in slightly older rocks in the Canadian Arctic. The great thing about the Arctic is that is where you get the right sort of rocks; there is very little vegetation and no topsoil, and the good fossils are easier to locate.”