Eighteen million years ago, a sinkhole opened up in what is now Gilchrist County, Florida. Thousands of unlucky animals fell in—frogs and snakes, ground doves and turkeys, even now-extinct camels and rhinoceroses that once roamed North America. Some likely survived for a time before starving or falling prey to predators that also took the plunge.
Today, that sinkhole provides paleontologists with a rich understanding of Early Miocene life. The Thomas Farm site, now managed by the Florida Museum of Natural History, was named for original owner Raeford Thomas. When he dug a well in 1931, he didn’t find water but he did uncover fossils. Since then, researchers have dug down about 10 m and they estimate that the sinkhole goes down another 10 m based on core samples of the earth, says vertebrate paleontology collections manager Richard Hulbert.
Thomas Farm is valuable due to the number of species—more than 100—and wide range of animal sizes represented, Hulbert says. Nearly 50,000 quality specimens have been identified, and there are tens of thousands still awaiting curators’ attention. Because the sinkhole was only open for a couple thousand years, it offers a snapshot of the animals that wandered ancient Florida. Limestone lining the sinkhole has helped preserve the fossils by preventing acidic groundwater from seeping in. The findings include thousands of horse fossils from at least three different species; Hulbert is currently working to prove that one of those groups is really two distinct species.
Other finds have included remnants of novel species, such as two skulls of a kind of mustelid, a family that includes wolverines and weasels. Proof of Zodiolestes freundi, christened in honor of the fossil's discoverer, volunteer John Freund of Gainesville, Florida, extended the animal's known range (1).
The layered clay and sand yield easily to scientists’ screwdrivers and picks; it takes just minutes to uncover a piece of tortoise shell or a small horse hoof. Excavators bag all of the soil, “literally tons of sediment,” says Hulbert. Then they run that silt through a sieve to catch every last tiny fossil, such as rodent teeth or snake vertebrae. For example, researchers identified an extinct species of bat, Primonatalus prattae, from these siftings (2).
The bats probably lived in caves in the sinkhole walls.
Researchers, students, and volunteers uncover so many fossils that they toss the lower-quality specimens on a scrap heap for visiting schoolchildren to “excavate.” However, there are plenty of good ones left. Hulbert still gets a rush when he or a volunteer unearths a fossil he knows is new.