Mid-Devonian Tetrapod Tracks.

Tetrapods are four-legged vertebrates that were first thought to have appeared in the late Devonian Period, and they were thought to be descendants of elpistostegid fishes like Tiktaalik and Panderichthys. But in 2010, in Poland, scientists found tetrapod tracks from the middle of the Devonian Period, meaning tetrapods appeared earlier than previously thought.

Illustration by Robert Back

Since these tetrapod tracks are older than the earliest
elpistostegid, this means that the elpistostegids, like the famous Tiktaalik, were not ancestors of tetrapods. Instead, the direct fish ancestors of tetrapods are not currently known.

Some of the trackways that have been found indicate that these early tetrapods grew up to at least two meters long. They probably looked a lot like other Devonian tetrapods, such as Ichthyostega.

Scientists used to think that tetrapods had their origins in freshwater environments, such as swamps and lakes. But the tetrapod tracks found in the Zachelmie Quarry were found in what had been a marine environment, like a lagoon. This means that tetrapods may have had their origins in saltwater environments.

The trackways indicate that these tetrapods walked in a sprawling gait, like lizards. The trackways also show that the tetrapods that made them were not walking on land. They were actually partially submerged in shallow water. If they were sprawling on land, their stomachs would have left a drag mark. But there are no stomach drag marks in the trackways in the Zachelmie Quarry, which means the tetrapods that made the tracks were floating, and their feet were pushing them along the bottom of the lagoon.

Many thanks to Robert for showing me these tetrapod tracks from the Zachelmie Quarry, and for letting me use his really cool illustration of what the tetrapod that left those tracks is believed to have looked like. 


References:

http://news.bbc.co.uk/2/hi/8443879.stm

http://scienceblogs.com/notrocketscience/2010/01/fossil_tracks_push_back_the_invasion_of_land_by_18_million_y.php

http://www.nature.com/nature/journal/v463/n7277/edsumm/e100107-01.html

http://www.earthmagazine.org/earth/article/2e6-7da-1-7

http://www.multi.fi/~rback/