First footprints, revisited

GeoLog has previously reported on the first footprints made on land, tracks in 530-million year old sandstone made by some sort of invertebrate animal, probably an arthropod—a millipede, perhaps. Now comes word of the oldest known footprints of a four-legged vertebrate, or tetrapod, in rocks 395 million years old.

This find is significant because the tracks predate the oldest known tetrapod body fossils by 18 million years, and if these tracks are indeed made by tetrapods, the find would push back the timing of the vertebrate transition from water to land.

In the absence of accompanying body fossils, it is difficult to definitively match trackways with track-makers, and one alternative explanation is that these trackways could have been made by lobe-finned fishes, precursors to the tetrapods.

[One more note: The trackways represent several different animals ranging from an estimated 50 centimeters long to 2 and a half meters long—quite a large animal for so early in tetrapod evolution.]

Sources: Grzegorz Niedźwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz & Per E. Ahlberg ^{Nature 463, 43-48 (7 January 2010) Tetrapod trackways from the early Middle Devonian period of Poland}

Perkins, S., 2010, Fossil footprints could push back origin of first four-limbed animals. Science News January 30, 2010.