In June 2022, two fossil hunters were on a diving trip in Florida’s Steinhatchee River when they discovered something remarkable. Half buried in the riverbed were the pristine remains of giant sloths, tapirs, armadillos and horses.
Recognising the significance of their find, Robert Sinibaldi and Joseph Branin shared the discovery with the Florida Museum of Natural History, where experts dated the fossils to a little-understood period of the Pleistocene ice ages known as the middle Irvingtonian.
The team believe these ancient creatures met their fate half a million years ago when they tumbled into a vast sinkhole. Over time, sediment filled the chasm, preserving their remains. During the millennia that followed, the Steinhatchee River altered its course many times, inching closer to the long-buried sinkhole, until eventually the river eroded into the ancient pit, washing away sediment and exposing the fossils hidden beneath.
Before the discovery, there had only been one other site in Florida with fossils from this time period.
A chance discovery
Sinibaldi and Branin's incredible discovery was made in the Steinhatchee River, a short watercourse that rises in the swamps of northern Florida before flowing south to the Gulf of Mexico. The pair knew the river well and have been searching for fossils in its murky waters for many years.
Looking for fossils on Steinhatchee's riverbed isn’t easy, says Sinibaldi, explaining that the water is full of tannins, which significantly reduces visibility. “It’s like diving in coffee.”
They were ready to move on after a fruitless search when Branin spotted something in the sediment – horse teeth. Then they saw a hoof core and a tapir skull. By the end of their search, the collectors had unearthed an entire treasure trove of fossils, all in pristine condition.
“It wasn’t just quantity, it was quality,” Sinibaldi says. “We knew we had an important site, but we didn’t know how important.”
And the scientists at the Florida Museum agreed. “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,” explains Rachel Narducci, vertebrate palaeontology collections manager at the Florida Museum and co-author of a new detailed study of the site.
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Tracing evolutionary changes
Despite the scarcity of fossils from the middle Irvingtonian, palaeontologists have well-documented records from the periods before and after. During this time, some species went extinct, while others evolved – changing in size and shape for reasons yet to be fully understood.
Among these were Holmesina, an extinct genus of armadillo-like creatures. When Holmesina floridanus first appeared in Florida two million years ago, individuals weighed around 70kg. Over time, the animals increased in size, evolving into Holmesina septentrionalis, an enormous species which grew to a massive 215kg – that's heavier than a red deer stag.
“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 explains.
Clues from ancient horses
Of the 552 fossils recovered so far, three-quarters belong to an early species of caballine horses – the subgroup that includes modern domestic horses and their wild relatives. Richard Hulbert, lead author of the paper, says that this dominance suggests that the region was once an open landscape, starkly different from today’s dense forests.
“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 says. The teeth’s exceptional preservation also allowed scientists to study their diet in unprecedented detail.
The site also yielded a tapir skull with an unusual combination of features never seen before. While it could be a new species, Hulbert remains cautious, noting that more skeletal evidence is needed to confirm its identity.
Citizen scientists make a difference
Like many of Florida’s major vertebrate fossil discoveries, this one was not made by professional scientists but by dedicated fossil enthusiasts. Sinibaldi and Branin’s collaboration with experts like Hulbert and Narducci highlights the crucial role that amateur fossil hunters play in advancing our understanding of prehistoric life.
Excavation at the site will be an ongoing, challenging process, given its underwater location. However, the researchers are optimistic about the secrets yet to be unearthed from this ancient sinkhole.
Main image: US riverbed (not the riverbed where the study fossils were found/Getty
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