Fossilized footprints found in an abandoned quarry in Poland hint that four-limbed creatures called tetrapods evolved much earlier and in a radically different environment than previously thought.
The footprints — many individual impressions, as well as some arranged in sets called trackways — are preserved in 395-million-year-old rocks in the Holy Cross Mountains, in the southeastern part of the country, paleontologist Per E. Ahlberg and colleagues report in the Jan. 7 Nature. That age substantially predates the time frame that paleontologists have pinned as the sea-to-land transition.
Evidence suggests that the carbonate rocks were laid down as sediments in the intertidal areas of a tropical shoreline, possibly in a lagoon, says Ahlberg, of Uppsala University in Sweden.
The presence of footprints in rocks of this age is surprising: The tracks date to 18 million years before body fossils of tetrapods show up in the geological record. And the footprints are about 10 million years older than body fossils of creatures such as Tiktaalik and Panderichthys (SN: 6/17/06, p. 379), believed to represent the transition from lobe-finned fish to creatures fully adapted to life on land.
Ahlberg and his colleagues contend that the findings “force a radical reassessment of the timing, ecology and environmental setting of the fish-tetrapod transition.” While previous studies have suggested that the first tetrapods hauled up on lakeshores or in freshwater deltas, these trackways hint that the water-to-land transition could have happened in a shallow marine setting.
In some areas of the quarry, the fossilized footprints are so common that they fall on top of and partially obliterate one another, forming what Ahlberg and his colleagues call a “densely trampled surface.” Small craters peppering the surface were made by falling raindrops — a sign that the ancient sediments were exposed to the air at least part of the day, the researchers contend.
The arrangement of footprints in one trackway — including stride length and relative spacing, as well as the absence of an impression from a sagging tail — suggest a tetrapod between 40 and 50 centimeters long left the tracks. Individual footprints in this set didn’t have sharply defined edges, possibly because the creature traipsed through squishy sediment in shallow water, the researchers note.
Other footprints at the site, however, were made by a larger, similar creature and include signs of toes. Most of those prints are about 15 centimeters wide and were probably made by a tetrapod about 2.5 meters long, the team estimates.
Because the rocks don’t contain any body fossils, it’s difficult to interpret what type of organism made the tracks, says Jennifer A. Clack, a paleontologist at the University of Cambridge in England. “If the isolated footprints are what they seem to be, the creature was relatively enormous,” she notes. And, she adds, “if fossils of the creature that made these tracks are ever found, they could well upset the apple cart of what people thought about tetrapod evolution.”
Constantly changing water levels make intertidal areas a dynamic environment, Clack says. In such a setting — where substantial quantities of food might have been stranded in shallow water when tides rolled out — evolution would have favored adaptable creatures, she contends.
The newly described footprints and trackways “are a very exciting find,” says Anthony J. Martin, a paleontologist at Emory University in Atlanta. “The combination of trampled surfaces, multiple trackways with tetrapodal gait patterns [and] distinct individual tracks with distinct digit impressions … makes a very persuasive argument that these structures are tetrapod trace fossils.”
Other scientists, however, aren’t convinced that tetrapods left the fossilized footprints.
“These are interesting trace fossils,” says Ted Daeschler, a paleontologist at the Academy of Natural Sciences in Philadelphia. However, he notes, trace fossils are notoriously difficult to interpret with confidence. Also, he adds, the arrangement of individual footprints in these trackways reflects a style of walking not seen again in the fossil record until 50 million years later. (No trackways that can be attributed to Tiktaalik or Panderichthys have been discovered.) “All together, the new evidence isn’t strong enough to make me discard the current, well-supported ideas about the timing of tetrapod evolution,” he says.
Many of the newly described footprints are indistinct and possibly not even made by tetrapods, but finding anything from that far back in time is still exciting, comments Robert L. Carroll, a paleontologist at the McGill University’s Redpath Museum in Montreal. The indistinct tracks could be impressions made in soft sediments by lobe-finned fish, he notes. And the footprints may have been made by members of a yet-to-be-discovered group of four-limbed creatures that went extinct and didn’t lead to today’s tetrapods.