Into the Gap: Fossil find stands on its own four legs

A fossil originally misidentified as an ancient fish turns out to be the nearly intact remains of a four-limbed creature, perhaps one of the first to walk on land. The find is particularly important because it dates from an era noted for its dearth of terrestrial fossils that illuminate the evolution of the ancestors of today’s amphibians, reptiles, and mammals.

There’s been plenty of fossil material for studying the evolution of some fish into early four-legged animals, or tetrapods. However, aquatic vertebrates with rudimentary legs disappear from the fossil record about 360 million years ago, says Jennifer A. Clack, a paleontologist at the University Museum of Zoology in Cambridge, England. Well-preserved tetrapod specimens don’t turn up again until about 20 million years later, when many animals with modern-style limbs were strolling the planet.

That lengthy gap in the fossil record–nicknamed Romer’s gap, after a scientist who searched in vain for fossils of terrestrial creatures from that time–coincides with the period when animals evolved efficient land walking.

“We know the animals were there,” says Robert L. Carroll, a paleontologist at McGill University in Montreal. “We just don’t know what they were like.”

Enter Pederpes. Remains of the ancient creature were extracted from a lump of limestone discovered in Scotland in 1971. Impressions of fish scales and other fossils also embedded in the nodule had initially led scientists to consider the 65-centimeter assemblage of bones as an ancient fish that lived during Romer’s gap. However, a painstaking exhumation by Clack recently revealed the nearly complete skeleton of a tetrapod.

Enough of the animal’s feet was preserved to indicate that its toes pointed forward like those of modern tetrapods rather than sideways like those of its aquatic ancestors. Also, the creature’s narrow, deep skull suggests that Pederpes got oxygen into its lungs by deep breathing rather than by pumping its throat to inhale air.

Although the animal could walk on land, its ear structure and other features of its skull suggest that Pederpes spent much of its time immersed in the brackish waters of coastal wetlands. Clack describes the 1-meter-long Pederpes in the July 4 Nature.

Because there are relatively few fossil deposits that preserve terrestrial animals from Romer’s gap, there’s been precious little material for probing how early tetrapods evolved into land vertebrates, says Carroll. The diversity that shows up in the fossil record just after this crucial period suggests that at least 10 distinct groups of tetrapod species must have evolved during Romer’s gap.

The group to which Pederpes belonged was widespread. Scientists have found isolated bones of its close relatives–all of which appear to have died out before the end of Romer’s gap–in Australia, North America, and Ireland.