About 300 million years ago, an 8-inch-long amphibian plodded across a muddy tidal flat that covered what is today Franklin County in eastern Kansas.
This summer, three Kansas scientists uncovered the tracks, so well-preserved that they show the claws on the animal's toes and the trace of its tail it dragged behind.
Geologists Howard R. Feldman of the Kansas Geological Survey, based at Kansas University; Allen Archer of Kansas State University; and William Lanier of Emporia State University discovered the tracks as part of their research on notable fossil-forming localities in Kansas and several Western states. Their work, funded by the National Science Foundation, is aimed at learning more about fossil formation and the ways rocks are deposited.
The Franklin County tracks are probably from a five-toed amphibian that resembled today's salamander. They were preserved in a layer of siltstone, a fine-grained rock that is part of a rock bed called the Douglas Group.
THE SILTSTONE was deposited late in the Pennsylvanian Period of geologic history about 300 million years ago, when eastern Kansas was covered, alternately, by shallow oceans and coal swamps.
Near the end of the period, Franklin County was covered by a shallow bay that was flooded daily with water as tides came in, then exposed as tides went out. The climate was subtropical, warm and wet.
According to the geologists, the animal left the tracks as it foraged on the silt-covered tidal flats. Its prints were preserved when the next tide came in and covered the tracks with more silt.
"With every tide, a new layer of fine-grained silt was dropped," Lanier said. "The water was saturated with sediment, which settled out. Sometimes as much as an inch or more of new sediment was deposited every day."
This soft sediment was perfect for preservation of trackways. The scientists also found tracks of insects and patterns where sediment was disturbed by the fins of fish swimming through the shallow water. Many layers of the rock are even delicately pitted by raindrops, evidence of rain showers 300 million years ago.
THE DOUGLAS Group is well-known for plant fossils, especially of ferns and trees. But this is the first report of animal tracks from the area.
"Other collectors in the area had described traces in the rock, and it sounded like they had found footprints," said Feldman, who uncovered the tracks in a 3-foot-long rock slab. "So I went there specifically looking for the tracks."
Though rare, fossil footprints have been found before in other parts of Kansas. In the 1860s, geologist Benjamin Franklin Mudge noticed a set in the flagstone of a Topeka sidewalk and traced them to a quarry in Osage County. More recently, researchers found tracks in a quarry near Garnett, in Anderson County.
Still, scientists know little about the animal that created the Franklin County prints, mostly because they have found no remains of the animal itself.
"We have trackways but no bones," said Archer. "So this environment really only preserved one fraction of the life that was around at the time. But what it preserved, it preserved remarkably well."