"Missing Link" Found

One of three newly-discovered specimens of the 383 million-year-old
Tiktaalik roseae.
These specimens fill a gap in the fossil record between aquatic and terrestrial animals.
Image: Ted Daeschler.

Making that transition from aquatic life to living on land was very important for vertebrates. However, there has been a gap in the fossil record at precisely that transitional point -- up until now, that is. Today, a group of scientists report that they found a group of three fossils from "crocodile-like" animals that neatly fit into the evolutionary progression as animals moved from water to land, causing many people to refer to them as the "missing link."

"We are capturing a very significant transition at a key moment of time. What is significant about the animal is that it is a fossil that blurs the distinction between two forms of life - between an animal that lives in water and an animal that lives on land," said palaeontologist, Neil Shubin, from the University of Chicago, who co-discovered the fossil with Edward Daeschler, from the Academy of Natural Sciences in Philadelphia.

Based on evolutionary theory and an analysis of the fossil record, scientists predicted that a creature once existed that possessed a unique blend of piscine and crocodilian characteristics. But the fossil record had a gap at precisely this point between Panderichthys, a fish that shows the beginnings of several land-friendly features, and Acanthostega, the earliest known tetrapod (four-limbed land-dwelling animals). Panderichthys lived approximately 385 million years ago while the earliest Acanthostega fossils are about 365 million years old.

The newly described Tiktaalik roseae fits between these two forms (see figure, right). This new species has scales on its back and webbed fins like a fish, but it also has eyes located on the top of its flat crocodile-like head, suggesting that it lived in shallow water, and it has a short neck -- a characteristic that all fishes lack (see artist's representation below, left).

"When we look inside the fin, we see a shoulder, we see an elbow, and we see an early version of a wrist, which is very similar to that of all animals that also walk on land," noted Shubin.

In 2004, the researchers found three nearly complete, well-preserved fossil skeletons of this new species in the Nunavut Territory, a region of the Canadian Arctic. The largest fossil is almost 3 meters (9 feet) long.

"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," commented Andrew Milner, a palaeontologist from the Natural History Museum, UK.

These findings will appear in this week's issue of the top-tier research journal, Nature. A cast of one of the fossils will be on display at the Science Museum in London beginning Thursday.

Thanks, Caren!