Ancient altercations between musk turtles and alligator gar recorded in Florida's fossil record

Ancient altercations between musk turtles and alligator gar recorded in Florida's fossil record Lisa Lock Scientific Editor Robert Egan Associate Editor Sometime between 5.5 and 5.6 million years ago, two shell crushers squared off in the languid currents of an ancient Florida river. The fossils they left behind, discovered by paleontologists at the Florida Museum of Natural History, reveal the identity of the combatants and the outcome of their encounter. In one corner, weighing in up to 370 pounds and growing up to 10 feet long, was one of North America's largest freshwater fish, the alligator gar. The common name partially comes from their long snouts, which superficially resemble those of alligators but most obviously differ by having several additional rows of teeth. Their skulls are wrapped in thick cantilevered muscles that can pull the jaw down from full gape in close to a tenth of a second. This rapid movement, usually made in anticipation of a meal, creates a substantial amount of negative pressure that pulls the surrounding water—and anything suspended in it—toward the gar's gullet and down into the lightless lower reaches of its alimentary canal. On the opposing end was a humble musk turtle. These animals are small, even by turtle standards, and furtive, preferring to spend most of their time trawling muddy river bottoms and only rarely leaving the water to bask on logs and tree limbs, which they abandon at the slightest sign of danger. But "when [their] slow anger is aroused," musk turtles can become a formidable adversary. Many of them have heads that are proportionally larger than other turtles and a correspondingly strong bite force that they use to crush snail and clam shells. If threatened, they may resort to vigorously biting anything within their reach, which—given that some have necks they can twist at a 180-degree angle and extend all the way to their hind feet—is considerably large. If that doesn't work, musk turtles will often emit small drops of volatile acid from glands beneath their arms and legs. The resulting smell has been variously described as reminiscent of burning tires, cheesy garbage, bitumen and body odor. Fossils found clumped together in unique 'turtle death layer' Jason Bourque, fossil preparator and resident turtle expert at the Florida Museum, found evidence of the encounter after studying hundreds of turtle fossils over the course of 10 years. All of them came from Montbrook, a late Miocene fossil site in North Florida full of buried treasure. Since its discovery in 2015, paleontologists have found an elephant graveyard, the oldest skull of a smilodontine saber-toothed cat, the oldest North American deer, a new species of heron and the bones of a giant otter known only from Florida, Mexico and California. But most of the fossils delicately removed from the quartz sand grains and the occasional seam of mineral-rich clay are less noteworthy to everyone except the new volunteer recruits. Today, Montbrook is located in a dry field, but the area was once bisected by a meandering river in the late Miocene. Consequently, the largest proportion of vertebrate fossils at the site come from fish. The second most common vertebrate group at Montbrook are turtles, which suits Bourque just fine. Their preponderance at the site has given him a practically endless supply of specimens to work on. He was especially pleased when the museum field crew of staff, students and volunteers found what came to be known as the turtle death layer. "There were layers at the dig site where we could barely find dirt as we probed the ground with our screwdrivers, just a pavement of turtle, fish and alligator bones scrambled together," Bourque said. He guessed that this section of the excavation had once been a depression where the last remnants of river water pooled during a drought. "When the river came back, it did so as a rapidly flowing channel that jumbled all of these bones into a pile." Most of the turtles in the death layer belonged to sliders, snapping and softshell turtles, but occasionally Bourque would come across a few well-preserved musk turtles. The latter elicited excitement and frustration in varying degrees—excitement because the fossil record of musk turtles up to that point was nearly non-existent. He knew this fact firsthand from having studied the few fossils that did exist during the process of naming two new extinct musk turtle species in early 2015, one from the Gray fossil site in Tennessee and the other from the Bone Valley formation in Florida. The study he co-authored describing the new species was published in Historical Biology just months before the death layer at Montbrook was discovered, which accounts for the frustration. Musk turtles are a uniquely North American group of animals, and the two new extinct species were the oldest known representatives of the group. But fossils of the species found at Bone Valley were limited to a few shattered shell fragments. This was enough for Bourque to know he was dealing with something never seen before but less than ideal for the purposes of describing a new species. He had to decide whether to name a new species based on a handful of broken bones or wait for additional fossils that might never see the light of day. "So we named it as a composite description, my co-author and I saying to each other, we're not ever going to find anything more complete than this, so we might as well just name it. And then, not even a year later, Montbrook comes along." At first glance, the death layer at Montbrook seemed to be littered with the remains of the same species he'd found in Bone Valley. If he'd waited just a few more months, he chided himself, he'd have had a much better idea what the turtle once looked like when he described it as a new species. Turtle escapes jaws of death, takes teeth as trophy Excitement ultimately outweighed frustration when Bourque noticed a few peculiarities while measuring the fossils. Most noticeably, the shells from Montbrook were smaller than the ones from Bone Valley, and the shells' armpits—called axillary notches—were larger. Based on these and other characteristics, it soon became clear he was dealing with yet another new species of musk turtle. This time there were more than enough fossils to base a species description on. This brought Bourque's total up to three musk turtles that he'd helped co-discover and name: one from Tennessee and two from Florida. The Montbrook shells were remarkably complete but far from pristine. Some looked like they'd been stepped on by an elephant, a very real possibility with all the gomphotheres that were stomping around Florida at the time. Others had fractures, cracks, scratch marks and broken buttresses. Just like human bones, turtle shells are capable of mending themselves after an injury. By looking for the telltale scars this left behind, Bourque could see that most of this damage had occurred when the turtles were alive rather than during the process of fossilization. Many of the shells were also covered in what looked like the turtle equivalent of acne scars. These show up frequently on both modern and fossil shells and are often attributed to parasites. "It's common to find these infections on mud and musk turtles, that they often get from microbials and, reportedly, leeches," Bourque said. While leeches might be safe on the shell of an ordinary turtle with a limited range of cervical motion, musk turtles have been documented extending their long, flexible necks back over their shells to remove and eat the offending parasites. Possibly as a consequence, Bourque said "it's incredibly rare to find what made those marks." So Bourque initially paid little attention to the indentations he found on several shells. But people in the business of reconstructing skeletons out of fossilized bones—most of them minute and misshapen if not missing entirely—are meticulous. After placing the shells he'd glued back together under a microscope for one last round of careful inspection and cleaning, he abruptly came up short. Embedded in one of the pits was a small but unmistakable tooth fragment that had broken off just below the tip. "At first, I thought it was an accident. We find so many disarticulated teeth, and I thought maybe it had magically washed into the pit on its own. Then I found another one stuck into a hole right next to the first." The broken teeth were exceptionally small and thus easy to miss with the naked or spectacled eye. Still, they were orders of magnitude larger than the pixel-sized teeth of leeches and thus had to have come from something else. Bourque took time away from cleaning and gluing bones to do some detective work. Identifying an animal with only a broken tooth smaller than a grain of rice to go on would ordinarily be a difficult task, but he had a solid lead from having looked at fossils coming out of Montbrook for years. "I immediately thought of gar. Their teeth fall out pretty regularly, and we find hundreds and hundreds of their scales and teeth at Montbrook. But that was almost too easy." Bourque covered his bases by comparing the teeth of several animals that lived at the time and might have felt plucky enough to bite down on a geodesic bunker made of reinforced bone. He briefly considered actual alligators, which are also common at Montbrook and have a surfeit of pluck when it comes to chomping on anything that bears a passing resemblance to food. But this didn't pan out. "When I looked at the teeth in cross section, they looked more like fish. Alligators have more laminated teeth that look irregular if they break," he said. Knowing the teeth belonged to fish narrowed things down significantly. Florida has a stunning diversity of freshwater fish in its rivers, lakes and canals, but few of them have diets that include turtle meat. Alligator gar, which are known to eat crabs and small turtles, are one of the only exceptions. The proverbial smoking gun came in the form of a mummified alligator gar skull that Bourque borrowed from the museum's South Florida archaeology collection. The tips of several teeth had been chipped off at some point when the animal was alive. The mouth of an alligator gar looks like the entrance to a porous cave haphazardly encrusted with dozens of sharp, jagged speleothems. Their lower jaw contains a row of large, recurved teeth called dermopalatine fangs that puncture prey and make it difficult for anything to slip away once their jaws have closed. Smaller rows of needle-like teeth surround the fangs and even sprout from the roof of their mouths. The upper jaw is similar but has an additional row of fangs for good measure. With so much dental redundancy packed into a small space, Bourque said, alligator gar can afford to lose or break a few teeth here or there. Turtle death layer may have been a favorite lunch spot rather than a drought-stricken pool Bourque named the new species Sternotherus pugnatus, the epithet being a Latin variation of the word pugnacious. It's not the first time this adjective has been used to describe the behavior of musk turtles once their slow anger has been sufficiently roused, but it seemed especially fitting for a scrappy species that went up against a predator several times bigger than itself and came out with only a few minor bite marks. It may have managed to escape by deploying the stink bomb musk turtles are famous for. Even the most avaricious predator would be disinclined to choke down a meal marinated in acid. Several years had elapsed between the time the Montbrook dig team discovered the turtle death layer and Bourque identified the teeth. But he still put off publishing his results for another few years. Despite finding hundreds of Sternotherus pugnatus shells at Montbrook, none of them had any corresponding skulls or limbs. After years of waiting for them to make an appearance, Bourque realized the turtles might not have been congregating during a drought after all. Though he primarily works with fossil specimens, Bourque will occasionally collect modern species for comparison. When that happens, he doesn't go snorkeling or attempt to catch turtles basking along a riverbank. Instead, he looks for birds. "They're so commonly eaten by birds, and when we find them, they're typically missing all of their extremities. But on the inside, we'll find the cervical vertebrae and the pectoral girdle and pelvic girdle that are up in the shell where the birds couldn't reach. A few of the ones at Montbrook still have those inside the shell." Many carnivorous birds have a favorite tree they consistently return to whenever they catch a meal. Other animals, like snakes, that are aware of this behavioral pattern will sometimes wait below to eat the scraps. It's possible the fossilized musk turtles were once dropped by birds beneath just such a tree. At present, there's not enough evidence to say whether Sternotherus pugnatus escaped the jaws of death only to be done in by a beak or laid low by a drought. But regardless of the ending, this footnote in Florida's rich natural history shows what life was once like in the Sunshine State—and possibly still is, if alligator gar haven't yet learned to avoid musk turtles. Publication details Jason R. Bourque, A small flat-shelled musk turtle from the Late Miocene of Florida and new Pleistocene records of Sternotherus (Kinosternidae), Historical Biology (2026). DOI: 10.1080/08912963.2025.2602686 Journal information: Historical Biology Provided by University of Florida