By Michael Hawkins
Researchers have recently discovered fossilized tracks which push the evolution of tetrapods back by 18 million years.
The discovery from southeast Poland shows imprints with the clear outline of toes. The distance between the tracks as well as their particular pattern also offer a glimpse into the size of the animal, which is estimated to be between 6 and 7 feet long.
Tetrapods are four-footed or four-limbed vertebrates. All birds, reptiles, amphibians, dinosaurs, and mammals fall into that category.
Until recently, tetrapod fossils had been dated between 375-380 million years in age. Tiktaalik roseae, discovered and described by Neil Shubin et al in 2004 on Ellesmere Island in Canada, was one of these fossils. Shubin had actually specifically predicted he would discover Tiktaalik where he did based upon evolutionary science at the time: there were no tetrapods 390 million years ago but there were tetrapods 360 million years ago. Thus, he should find a species showing features of both fish and tetrapods somewhere between those dates. He was successful, but now his discovery needs some minor explanation in light of tetrapod evolution actually being pushed back to 397 million years ago (and probably more).
The fact is that this new discovery represents a quantitative change, not a qualitative one. That is, the relations between fish and tetrapods and the evolutionary time line laid out for them has not been altered. What has been changed is the quantity of time now evident for tetrapod evolution. It is actually not entirely surprising; Tiktaalik still represents an awesome transition between marine and land vertebrates, but it wasn’t the only creature able to drag itself out of the oceans. Shubin, in fact, had described Tiktaalik as a representative of the sort – key word, sort – of ancestor tetrapods have in his original paper. We should expect to find other tetrapods walking around in ancient times; we just didn’t know exactly how ancient those times were.