Back in 2004, a fossil tetrapod from the Canadian Arctic achieved near-celebrity status, at least for mineralized skulls. The newly discovered specimen, named Tiktaalik roseae, made appearances on the Colbert Report and PBS NewsHour. (The scientists involved got to share a bit of the spotlight, as well.) The find was noteworthy because it supplied a long sought-after link between fish and their four-legged descendants. We had plenty of fossil fish that fill the bill as ancestor and a number of early tetrapods, but we were missing some of the details about how our ancestors lost their "fishiness."

The old joke about finding a new fossil is that it simply creates two new gaps in the record, one on either side. But a bit after Tiktaalik, there was an actual gap: Romer's Gap, named for the scarcity of fossils that covered the first appearance of four-limbed animals on land. Several new finds in Scotland are now filling in Romer's gap.

In the 1950s, paleontologist A.S. Romer pointed out a 30 million year long break in the tetrapod fossil record, a blank space that picked up his name. Later work whittled the gap down to 15 million years, but the transition was still pretty stark.

Prior to the gap, tetrapods were amphibious, using their four limbs to navigate shallow water and riverine environments. On the other side of the gap, many were already fully terrestrial. The early, amphibious tetrapods still had fish-like tail fin structures and appendages with more than five digits. Their terrestrial descendants had fewer digits and their skulls were smaller and less flattened.

Romer’s Gap begins with an extinction event at the end of the Devonian, about 359 million years ago. Geologists have found evidence that the extinction was caused by an abrupt glacial period accompanied by large swings in sea level. Tetrapods weren’t the only group to come out of the catastrophe looking a bit different than they did on the way in, but it’s not entirely clear how the glacial period drove so many terrestrial and marine animals to extinction.

In a new study published in the Proceedings of the National Academy of Sciences, researchers from Cambridge and the University of Southampton describe the discovery of a number of new fossils that will help fill in Romer’s Gap and allow us to test some ideas about that extinction event.

The fossils come from four sites in Scotland, just east of Edinburgh. All the fossils were found in the same layer of mudstone, which was deposited towards the end of the 15 million year gap. The mudstone yielded a number of tetrapods as well as a host of other organisms; the tetrapods represented include both terrestrial and aquatic species.

The finds suggest that Romer’s Gap is mainly a gap in collection. Rocks of the right age simply haven’t been sought out and worked thoroughly enough. Rather than a scenario where tetrapods changed dramatically, either right after the end-Devonian extinction or at the end of Romer’s Gap, the researchers see evidence of a gradual transition.

Other hypotheses about this time period will also be testable at these new sites. In 2006, a group of paleontologists proposed that atmospheric oxygen may have dropped sharply, stressing the early tetrapods’ newly-adapted respiratory systems and sending them back to the water. Such a change would also impact terrestrial arthropods, which were also uncovered at the new sites in Scotland. A close look at those arthropods may allow researchers to rule out the effect of atmospheric oxygen.

Gaps like Romer's are exciting for paleontologists, because where there’s an unknown, there’s something fascinating waiting to be discovered. These finds in Scotland promise to supply plenty of material for paleontologists to learn about what was going on during Romer’s Gap, which may soon be a misnomer.

This should also be a lesson for young scientists—don’t be too flattered if your name becomes attached to an unexplained phenomenon. It won’t last.

PNAS, 2012. DOI: 10.1073/pnas.1117332109 (About DOIs).