About half a million years ago, several horses, sloths and armadillos fell into a sinkhole in Florida's Big Bend region and died. The sinkhole filled in with sediment over time, preserving the animals where they lay until fossil collectors Robert Sinibaldi and Joseph Branin discovered them in 2022.
The pair had been diving for years near Sinibaldi's property on the Steinhatchee River. Combing the riverbed for fossils isn't easy; the water is full of tannins, which significantly reduces visibility. "It's like diving in coffee," Sinibaldi said.
They were on their usual fossil hunting trip in June of 2022 and weren't having much luck. They were preparing to move on when Branin looked down and happened to see horse teeth. As they continued looking, they uncovered a hoof core, and then a tapir skull.
Their good feeling swelled to disbelief as the finds continue to rack up — many in pristine condition. "It wasn't just quantity, it was quality," Sinibaldi said. "We knew we had an important site, but we didn't know how important."
The Steinhatchee River likely followed a different course when the fossils were preserved, but as it meandered over the following millennia, the river edged closer to the sinkhole until, very recently, it eroded into the former pit and rinsed the fossils, leaving them exposed along the bed of the river.
He and Branin shared their findings with the Florida Museum of Natural History, where paleontologists determined they were preserved during an obscure period of the Pleistocene ice ages called the middle Irvingtonian.
"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 coauthor of a new detailed study of the site.
Before the discovery, there had only been one other Florida site with fossils from this time period.
An evolutionary transition without a fossil record
While there are few fossils from the middle Irvingtonian, there are plenty from the periods just before and after. Paleontologists know from the record that some species from the early Pleistocene go extinct, while others appear for the first time in the late Pleistocene.
Then there are the species that, for unknown causes, go through changes in body size and shape during that gap.
One of these includes members of the now-extinct genus Holmesina, which resemble modern armadillos, only larger. When the species Holmesina floridanus first appeared in Florida two million years ago, individuals averaged 150 pounds. Over time, the creatures became larger and larger until being classified as a new species known as Holmesina septentrionalis, which grew up to around 475 pounds.
"It's essentially the same animal, but through time it got so much bigger and the bones changed enough that researchers published it as a different species," Narducci said.
The fossils recovered from the Steinhatchee River site offer a rare look into how this process of speciation took place, with ankle and foot bones that match the size of the larger H. septentrionalis species while retaining features of the older, smaller H. floridanus species.
"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. Only later, it seems, would the animals evolve skeletal features to help support the heavier bodyweight.
Three-quarters of the 552 fossils recovered so far from the Steinhatchee River site belong to an early species of the subgroup of living horses that includes the domestic horse and its wild relatives, known as the caballines. "That gives us a good sample size to measure or make comparisons, and it also tells us a little bit about the environment," said Richard Hulbert, lead author of the paper and retired Florida Museum vertebrate paleontology collections manager.
Horses are adapted for living in open habitats. Since they made up such a large share of the new Irvingtonian fossil site, scientists can conclude the area used to be fairly open, much different from the heavily wooded landscape in the region today. If it had been densely wooded back then, they would expect to find more forest-dwelling animals such as mastodons and deer.
"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," Hulbert said. These components are often only found separately. The teeth were also unusually well-preserved.
"That was one of the first things I noticed about the site," Hulbert said. Additionally, dental wear and tear from eating were still visible, offering a valuable chance to research the diet of these early caballine horses.
Branin also collected a puzzling tapir skull, with a mix of features that have not been seen together before. Hulbert cautioned against designating it a new species, though. "We need more of the skeleton to firmly figure out what's going on with this tapir," he said. "It might be a new species. Or it always could just be that you picked up the oddball individual of the population."
Hulbert stressed that the Steinhatchee River site, like many of Florida's great vertebrate fossil sites, was not found by professionals. Hobby fossil collectors like Sinibaldi and Branin work with experts like Hulbert and Narducci to expand our collective understanding of Florida's natural history.
Branin called Florida a lucky state for his hobby. "We have a permit system that allows people to collect fossils on state-owned lands, unlike a lot of places where there's more barrier to entry to doing that," he said.
Further collection at the site will be a slow, ongoing process, given the logistical challenges of excavating an ancient sinkhole underwater. Still, the authors are hopeful about the fossils that have yet to be revealed.
