Tetrapods pushed back 18 million years

The oldest tracks of four-legged animals have been discovered in Poland.

Rocks from a disused quarry record the “footprints” of unknown creatures that lived about 397 million years ago.

Scientists tell the journal Nature that the fossil trackways even retain the impressions left by the “toes” on the animals’ feet.

The team says the find means that land vertebrates appeared millions of years earlier than previously supposed.

This is especially interesting because Tiktaalik was discovered by Neil Shubin based upon a lack of land animals 390 million years ago but a prevalence 360 million years ago. He specifically looked for a place likely to have fossils that was 375 million years old in order to discover his transitional fossil. This new information doesn’t mean that he just got lucky – one would still expect to find transitional forms prior to true land animals – but a little luck was involved. (It was actually involved no matter what he wanted to find and when he wanted to find it because fossilization is so rare anyway.)

One important fact to note about Tiktaalik is that it likely lived in freshwater. This is key because a marine environment is less conducive to a full move onto land than a freshwater lake or river, and Tiktaalik shows evidence that it is closely related to later fully land animals. Think about it for a moment and it becomes obvious: you need to be able to drink freshwater, not salt water, in order to fully utilize the land. If your ancestors lived in freshwater, then the first transition has been made for you. That means the owners of these newly discovered footprints represent a transition of sorts, but they were still very much tied to a marine life, unlike Shubin’s discovery.