Around 500,000 years ago, some horses, sloths, and armadillos had a very bad day—they fell into a sinkhole in what is now Florida and remained there until two men discovered their bones in 2022.
Fossil hunters Robert Sinibaldi and Joseph Branin discovered 552 exceptionally preserved animal fossils while diving in the Steinhatchee River. The findings, detailed in a study published late last year in Fossil Studies, shed light on a little-known period of the Pleistocene ice ages called the middle Irvingtonian.
“It’s like diving in coffee,” Sinibaldi said of the murky Steinhatchee River in a statement from the Florida Museum of Natural History. Branin first spotted horse teeth, which then led to the discovery of many other specimens, including the skull of a tapir (large pig-shaped herbivorous mammals still alive today). “We knew we had an important site, but we didn’t know how important,” Sinibaldi added.
According to the study, the unfortunate animals met their demise in a sinkhole, where their remains were preserved under layers of accumulated sediments for hundreds of thousands of years. Back then, the Steinhatchee River likely wasn’t where it is today. As erosion reshaped its course, the river eventually collided with the sinkhole, washing the fossils into the riverbed.
Sinibaldi and Branin submitted their findings to the Florida Museum of Natural History, where paleontologists confirmed their exciting origins.
“The fossil record everywhere, not just in Florida, is lacking the interval that the site is from — the middle Irvingtonian North American land mammal age,” said Rachel Narducci, vertebrate paleontology collections manager at the Florida Museum and a coauthor of the study. In fact, it’s only the second known middle Irvingtonian site in Florida.
A lack of fossil evidence from this obscure time period has left many questions unanswered about the evolutions of certain animals. For example, Holmesina floridanus—now-extinct creatures that looked like large armadillos and weighed an average of 150 pounds (68 kilograms)—evolved into a much larger species named Holmesina septentrionalis, where individuals were around 475 pounds (216 kg). Holmesina‘s ankle and foot bones from the Steinhatchee River fossils now provide insight into this transition, as they showcase features of both H. floridanus and H. septentrionalis.
“This gave us more clues into the fact that the anatomy kind of trailed behind the size increase. So, they got bigger before the shape of their bones changed,” Narducci explained.
Sinibaldi and Branin’s discovery also offers clues about what modern-day Florida may have looked like half a million years ago. An overwhelming majority of the fossils are of caballines, an early group of both domesticated and wild horses. Since horses evolved to live in open habitats, and given their prevalence within the Steinhatchee River fossils, the researchers suggest that Florida was also once an open landscape with fewer trees than today.
“What was great about the horses from this site is, for the first time, we had individuals that were complete enough to show us upper teeth, lower teeth and the front incisors of the same individual,” said Richard Hulbert, the lead author on the paper and a retired Florida Museum vertebrate paleontology collections manager. The wear on well-preserved teeth could also provide clues on what kinds of plants these early horses were eating.
Additionally, an unusual tapir skull uncovered along the riverbed might represent a new species—or could just be “an oddball individual of the population,” Hulbert explained. “We need more of the skeleton to firmly figure out what’s going on with this tapir.”
The findings join a growing list of vertebrate fossil sites in Florida discovered by hobby fossil hunters, highlighting the potential for successful collaborations between passionate amateurs and professional researchers.