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 of southeastern Poland, paleontologist Per E. Ahlberg and colleagues report in the Jan. 7 Nature. That age substantially predates previous estimates for when animals left the sea to live on land.
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 earlier than body fossils of tetrapods. 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 fully land-adapted creatures.
The findings "force a radical reassessment of the timing, ecology and environmental setting of the fish-tetrapod transition" the team contends. Previous studies have suggested that the first tetrapods hauled up on lakeshores or in freshwater deltas, but these trackways hint 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." Also peppering the surface are small craters made by falling raindrops--a sign that the ancient sediments were exposed to the air at least part of the day, the researchers say.
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--suggests that 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.
Other footprints at the site, however, were made by a larger, similar animal and include signs of toes. …