The discovery of the Tiktaalik fossil back in 2006 was a cause of great celebration because it was a 385 million year old fossil that showed the early stages of forelimbs that indicated it was a transitional one from sea to amphibian. Neil Shubin, the leader of the team, wrote a highly engaging book Your Inner Fish that showed how much our human bides were shaped by our fishy ancestors.
Now more discoveries have been made by the same team in the same region that show the development of rear legs too.
Tiktaalik is what is termed a tetrapodomorph – a type of transitional vertebrate.
About 2.5m in length, it looked somewhat like a fish in that it had scales and fins with webbing; but it also had a flat head, and shoulder, forearm and wrist bones that echoed later, fully land-living, four-limbed animals.
The early analysis of Tiktaalik was based on fossils gathered in Canada’s High Arctic, on Ellesmere Island in 2004. This assessment concentrated just on the creature’s front end.
It is only now, after cleaning and preparing other specimens gathered at the same time as the original finds, that the team has been able to say something definitive about Tiktaalik’s hind region.
In the new collection is a thick, powerful rear fin. But the big surprise is the pelvic girdle that would have supported its hind fins.
There are only limited impressions of these appendages in the fossil rock, but it is clear from the size of the pelvis that Tiktaalik’s back fins, too, would have been big.
You can see the full paper here.
Ed Yong discusses the transition of fins to legs, especially the acquisition of the ability to control legs in ways that are not required for fins.
Anti-evolutionists are always demanding to see transitional fossils and some scientists were expecting them to be confounded by the discovery of Tiktaalik. I was sure it would not do anything to the majority of them since in their minds, the discovery of any intermediate stage merely creates two new intermediate states that they will demand be also filled. And so it came to pass.
This new finding too will not persuade hard-core creationists. But over time the weight of evidence will chip away at the foundations of disbelief.