A 400 million-year-old fossil of a coelacanth fin, the first finding of its kind, fills a shrinking evolutionary gap between fins and limbs. University of Chicago scientists describe the finding in a paper highlighted on the cover of the July/August 2007 issue of Evolution & Development.
The fossil shows that the ancestral pattern of lobed fins closely resembles the pattern in the fins of primitive living ray-finned fishes, according to the scientists.
This ends intense debate about the primitive pattern for lobed fins, which involves the ancestry of all limbs, including our own, said author Michael Coates, Ph.D., associate professor of organismal biology and anatomy at Chicago.
According to the researchers, the fossils pattern is similar to the branching arrangement still embedded in the fins of paddlefishes, sturgeons and sharks.
To understand the developmental evolution of the limbs of tetrapods [four-limbed vertebrates], we shouldnt be looking at the fins of our nearest living fish relativeslungfishes and coelacanthsbecause theyre far too specialized, Coates said.
Part of the reason why this is an interesting discovery is that people think of coelacanths animals as archetypal living fossils, said Matt Friedman, evolutionary biology graduate student at Chicago and lead author of the paper. But its a common misconception. If you look deep in the fossil record to the first members of that group, they are really different and very diverse.
Until now, many biologists have looked at lungfish as a primitive model of the evolution of tetrapods. Our fossil shows that what weve been using to define a primitive state is actually very specialized, Friedman said, which means it might give a deceptive view of what evolution was like for these fins skeletons.
If youre going to figure out how limbs evolved, you need to have a good idea about pre-conditions, Friedman said. You need to know what the
Contact: Catherine Gianaro
University of Chicago Medical Center