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What do horses, armadillos, sloths, and tapirs all have in common? Theyre all animals, sure. Theyre all mammals, too. But theres something more striking, more special, about them: Around 500,000 years ago, they all fell into the same sinkhole along Floridas Steinhatchee River. Sediments filled their sinkhole thereafter, trapping them there until they were found, fossilized, in 2022. Today, these animals provide insights into Floridas Middle Irvingtonian Period, a forgotten age from which very few fossils have been found. The fossil record everywhere, not just in Florida, is lacking the interval that the site is from, said Rachel Narducci, a collections manager from the Florida Museum and an author of a new study about the site in Fossil Studies, in a press release. Researchers say that the abundance of horses at the site hints at what the landscape looked like around 500,000 years ago, while the assemblage as a whole with armadillos, sloths, and tapirs as well as horses reveals the trajectories of evolution throughout the period.A Land of Horses Of the over 550 fossils found in the sinkhole thus far, around 75 percent are fossils from early horses from the genus Equus, whose evolutionary lineage led, eventually, to todays living horses. Because these early horses were anatomically adapted to live their lives out in the open, their presence at the site provides a picture of the landscape around that period. [That] tells us a little bit about the environment, said Richard Hulbert, a retired collections manager from the Florida Museum and another author of the new study, in the press release. Indeed, the early horses indicate that the forests that cover the area today were once much more open.In addition to revealing the landscape of the site, the horse fossils also allow researchers to make comparisons to todays horses and to analyze Middle Irvingtonian diets. Scrapes and scratches are still visible on the horses teeth, the researchers say, making them a valuable resource for recreating the food sources that were available in the region.What was great about the horses from this site is, for the first time, we had individuals that were complete enough to show us [the] upper teeth, [the] lower teeth, and the front incisors of the same individual, Hulbert added in the release. That was one of the first things I noticed about the site.Massive Armadillos, Sloths, and Tapir MixesAn abundance of fossils from before and after the Middle Irvingtonian have helped researchers reconstruct this periods lost evolutionary lines, illuminating which lineages died and emerged and which lineages transformed in terms of their size and shape.For instance, armadillo-like animals from the Holmesina genus were much smaller before the Middle Irvingtonian (before 700,000 years ago) than they were after ( 500,000 years ago). The Steinhatchee River sinkhole, with its handful of Holmesina fossils, helps reveal this mysterious shift in size. It gave us more clues into the fact that the anatomy kind of trailed behind the size increase, Narducci said in the release. They got bigger before the shape of their bones changed.Fossils from sloths and tapirs were also found at the Steinhatchee site, with a skull showing a strange mixture of tapir traits. We need more of the skeleton to firmly figure out whats going on with this tapir, Hulbert said in the release. It might be a new species. Or it always could just be that you picked up the oddball individual of the population.A Sunken SinkholeThough the Steinhatchee sinkhole likely sat alongside the river in the Middle Irvingtonian, it sits submerged in the river today. The sinkholes discoverers fossil collectors Robert Sinibaldi and Joseph Branin thus found the fossils while diving in June 2022, after years of dives in the area. A fossil of horse teeth was found first, followed by a hoof bone and a tapir skull. It wasnt just quantity, it was quality, Sinibaldi said in the release. We knew we had an important site, but we didnt know how important.Researchers suspect that there are other fossils in the sinkhole, one of only two Middle Irvingtonian sites in all of Florida. Submerged in the Steinhatchee, these fossils will provide further insights into the world around 500,000 years ago if found, exposing even more of a Florida that was long ago lost to time.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
