When fossil collectors Robert Sinibaldi and Joseph Branin went for a regular dive in the Steinhatchee River on Florida, back in June 2022, they expected a day like any other of the dives they’d completed there.

A difficult dive in a riverbed packed with tannins, where visibility is low and the chance of discovering anything of note is even lower.

Preparing to move on from the site, the pair took one last look at the riverbed, when Branin spotted something unusual.

The horse teeth that he noticed at the bottom of the river encouraged them to look in more detail at the same spot. When they did, they saw not only a hoof core, but a tapir skull too, before more and more fossils started to reveal themselves. And they were extremely well preserved too, as Sinibaldi explained in a statement:

“It wasn’t just quantity, it was quality. We knew we had an important site, but we didn’t know how important.”

And the surprises just kept coming. When the pair took the fossils to experts at the Florida Museum of Natural history, the palaeontologists were able to examine them, determining that they derived from the Pleistocene era, in a time known as the middle Irvingtonian.

To date, 552 of these valuable fossils have been unearthed from the site.

Why is this so important? Well the middle Irvingtonian is incredibly undocumented, with little in the way of archaeological evidence and fossils preserved from that time.

It is suspected that the reason that the fossils found here were so well preserved were as a result of a sinkhole that the unsuspecting creatures fell into. Preserved in the nutrient-rich earth until, eventually, the river reached out to the sinkhole and exposed the fossils, these horses and tapir, and other creatures have been held underground for over 700,000 years.

In fact, middle Irvingtonian fossils are so rare, that only one other site in Florida has so far been located.

And the fossils located by the Steinhatchee River offer a rare glimpse into the evolution of a number of species for whom changes in anatomy or physiology have long been unexplained.

For example, in the case of Holmesina, a now-extinct kind of armadillo, it has long been known that the creature evolved to become bigger and bigger – from a 150 pound animal, to 475 pounds, at which point it was reclassified as Holmesina septentrionalis. But until now, evidence of the progression of this species’ evolution has been lacking.

The findings of the Steinhatchee River site provide new, fossilised evidence that explains this progression, as Rachel Narducci, vertebrate paleontology collections manager at the Florida Museum, explained in the statement:

“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.

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.”

And it wasn’t just big armadillos. In fact, over 75% of the fossils belonged to an early species of horse, giving researchers vital information about how horses adapted to this particular environment.

Though research into its adaptations are limited by the lack of further fossils, the tapir skull also presented a lot of questions for the palaeontologists, as Richard Hulbert, retired vertebrate palaeontology collections manager at Florida Museum confirmed:

“We need more of the skeleton to firmly figure out what’s going on with this tapir. It might be a new species. Or it always could just be that you picked up the oddball individual of the population.”

Though no more information about the tapir can be confirmed yet, fossil collections at the site are ongoing, with a paper recently published in the journal Fossil Studies documenting the fascinating results revealed thus far.

But as sites such as this continue to uncover themselves, and daring fossil hunters like Sinibaldi and Branin continue their mission, we can only hope that more and more of our planet’s past continues to be revealed.

If you thought that was interesting, you might like to read about the mysterious “pyramids” discovered in Antarctica. What are they?