A short time ago, we covered a new fossil from the time when modern humans were emerging in Africa. This week, we have a find in Arctic Canada that dates from the emergence of terrestrial vertebrates out of the seas about 375 million years ago. It's been known for a while that the lobe-finned fish were critical intermediaries in this process, but there were multiple changes involved in the transition, and the sequence in which they fell into place remained unclear. The new fossil, of a species named Tiktaalik roseae (Tiktaalik is the word for large, shallow water fish in the Inuktikuk language of Nunavut, where it was found) bridges that gap:
Although the body scales, fin rays, lower jaw and palate
are comparable to those in more primitive sarcopterygians [the
precursor fish], the new species also has a shortened skull roof, a
modified ear region, a mobile neck, a functional wrist joint, and other
features that presage tetrapod conditions.
In other words, most of the body was very fish-like, but the top of the head and neck were very much like those of the first amphibians. Meanwhile, much of the eventual terrestrial limb, including the shoulder, elbow, and parts of the wrist, were in place at the forelimb level, but not in the hind limb. Even the forelimb still ended in a fin, rather than digits, though. Although this beast was a predator, with a vaguely alligator-like head, the researchers suggest that it took advantage of its ability to drag itself onto land to escape predators. This is both because it would have moved too slowly to hunt down prey out of water, and it's unclear whether any prey was out of the water at the time.
The Nature News story linked above will take subscribers to the actual articles describing it. The discovery of many transitional fossils—I think I've covered about five in my six months of writing here—is telling for one of the ongoing debates in evolutionary studies. The gradualist school of thought believes that most evolutionary transitions did occur through multiple intermediate forms, many of which we just don't have fossils for. Those favoring punctuated equilibrium think that many of the gaps in the fossil record truly represent leaps between species. Although I do not doubt that some transitions may have occurred via sharp transitions, the apparent regular discovery of species that fill gaps suggest that gradual transitions may be the rule.