Hundreds of fossils discovered in Scotland may show which creatures crawled out of the water to populate the land after a mass extinction 359 million years ago, according to research that also resets the timeline for animals with fingers and toes.
The fossils are from a mysterious 30-million-year blank in the record known as Romer’s Gap, named for Alfred Romer, the paleontologist who first recognized it, according to a study in the journal Proceedings of the National Academy of Sciences.
The period has confounded researchers' efforts to understand how creatures evolved because of a lack of evidence. The fossils found in Scotland from four-legged life forms demonstrate that having five fingers and toes arose 20 million years earlier than scientists thought.
“Everything is getting pulled back in the fossil record,” said Jennifer Clack, a paleontologist at the University of Cambridge in the U.K. and study author. “This gives us a clue to how quickly that the ability to walk on land with a conventional foot evolved -- much faster than previously thought.”
The discoveries suggest that the gap in the record was due only to collection failure. Before today, only one full skeleton and a few isolated bones dating from this period had been found.
The new information suggests that life recovered from a known mass extinction event at the end of the Devonian period, which was about 359 million to 416 million years ago, more quickly than researchers had once thought. Fish groups evolved into big freshwater forms including lungfishes, which can breathe air, and rhizodonts, which are now extinct. By 345.3 million years ago, animals that are usually considered to be land-dwellers had appeared.
“This is a great opportunity to see a block of time that was sort of blacked out for us because of volcanic activity,” said Bob Carroll, a paleontologist at Redpath Museum at McGill University in Montreal, in a telephone interview. “On the sea to the land, we have, in a way, a better window.”
It also challenges the idea that there was a low-oxygen period where only aquatic animals could survive, Clack said. Because the environment had a lot of volcanic activity, there may have been an unusually high level of carbon dioxide in the air, some researchers had thought.
Clues in Charcoal
Charcoal deposits found along with the fossils suggest that wasn’t the case, and future research may focus on the general environment these animals lived in, she said. Charcoal is plant material that’s been partly burned without being entirely incinerated, so scientists can identify the plants it was made from.
The Scottish fossils are from newly excavated locations that include the coastal section at Burnmouth north of Berwick on Tweed, the banks and bed of the Whiteadder River near Chirnside, the banks of the Tweed River near Coldstream and sedimentary strata near Tantallon Castle. All are to the east of Edinburgh. Two previously known locations, Dumbarton in western Scotland and Blue Beach near Horton Bluff in Nova Scotia, Canada, also contributed fossils.
Besides the sites where these fossils were found, there are some indications that another cache of animals from this period may be in a site near Pittsburgh. There are contemporary deposits near there, she said.