Your Daily Fossil



Reconstruction by DiBgd

When: Carboniferous (~354–344 million years ago)

Where: Scotland

What: Pederpes is a primitive tetrapod. This is one of the first and only tetrapods known from Romer’s Gap; a roughly 15 million long period of time at the start of the Carboniferous that for decades yielded almost no fossil vertebrates.  Thus, there were no clear transitional forms to bridge the gap from the Devonian ‘age of fishes’ and the first predominately aquatic tetrapods, such as Acanthostega, to the later Carboniferous with a diverse fauna of fully terrestrial tetrapods, such as Hylonomus. Over time it has become evident that this gap of fossils is not just due to misfortune in selection of fossil localities, but has a biological basis in reality. The end of the Devonian was one of the major mass extinction events, and it is now evident that the biota took some time to rebound from the catastrophe. Study of the rocks from this 15 million year interval shows that there was less oxygen in the air and waters during this time than before or after.

However, despite the paucity of animals around to become fossilized, we have started to build a picture of evolution of tetrapods during this critical time period. Pederpes was the first tetrapod found in Romer’s Gap, though not the first recognized. It took over 30 years for paleontologists to realize it was truly a tetrapod, and not a fish. Jennifer Clack published and named Pederpes in 2002; but the fossil was unearthed in 1971, and classified as a lobe finned fish.  Pederpes is the oldest tetrapod that was capable of fully terrestrial locomotion; the neutral position of its limbs was to face forward and down, rather than the outward posture seen in more primitive forms. Not to say this 3 foot (~1 meter) long animal was fully terrestrial; far from it. The morphology of its ear region shows it would hear better in the water than on land, implying it hunted prey in the water rather than on land. In the tetrapod lineage Pederpes falls outside all living clades of tetrapods, it can be considered to be ancestral to all living tetrapods (amphibians, reptiles, birds, and mammals).