A tough river turtle, Boremys, not only survived the meteorite impact that likely wiped out the dinosaurs, but it also seemed completely unfazed by the catastrophic event, according to a new Society of Vertebrate Paleontology paper.
The discovery shows that the Cretaceous-Tertiary extinction 65-million-years ago was non-random, meaning that some groups perished completely (non-avian dinosaurs and many marine species), some groups suffered heavy losses, while other groups, like turtles, did very well.
"We believe that aquatic turtles were particularly resilient to the meteorite impact because they naturally possess a wide behavioral repertoire that allows them to survive bad times," co-author Walter Joyce of the University of Tubingen's Institute for Earth Sciences told Discovery News.
"When it gets too cold, aquatic turtles naturally will hibernate," he added. "When it gets too hot or dry, aquatic turtles will estivate (dig themselves into mud holes and wait out the problem). These are tools that come in handy during regular times, but apparently also during meteorite impacts."
Lead author Tyler Lyson of Yale University, Walter Joyce, Georgia Knauss and Dean Pearson recovered the remains of Boremys from the Hell Creek and Fort Union rock formations in southwestern North Dakota and eastern Montana. Boremys belonged to a group of extinct river turtles known as the baenids.
Based on the finds, this particular species preferred the swampy areas that surrounded large, tropical rivers. It flourished from approximately 80 to 42 million years ago.
The jaws of this turtle are relatively non-specializing, according to Joyce, so it probably ate a varied diet including soft plants, small mollusks, insects and fish. The smallest Boremys had a shell length of 9.8 inches, while the shell of the largest was 31.5 inches long.
Although the hearty turtle is not closely related to any modern turtles, the researchers believe it behaved similar to extant North American pond turtles, such as what are known as the "painted" turtles or river cooters.
Debate still swirls as to what exactly happened 65 million years ago. A growing consensus among paleontologists is that a single large meteorite strike at the Yucatan Peninsula did in the dinos and numerous other animals.
Joyce thinks "most animals did not die the day of the meteorite impact, but rather during the week to month-long aftermath. "While large land animals must have dropped dead by the hundreds, it appears that many small to medium-sized aquatic amphibians, such as frogs and salamanders, as well as reptiles (turtles, the crocodile-resembling champsosaurs, actual crocodilians, and lizards) generally did well, likely because all of these groups naturally have techniques that help them to survive bad times," Joyce said.
In terms of what did finally kill off tough Boremys, this turtle and other baenids could not withdraw their heads under their shells as living turtles can.
He said, "It therefore appears that baenids may have survived the great meteorite impact with few problems, but could not defend themselves against small predatory mammals."
Several experts informed Discovery News that they agree with the new findings. Nick Fraser, Keeper of Natural Sciences at National Museums Scotland, said the study "highlights that one or two groups of animals were somehow apparently unaffected by what can only be described as extremely harsh environmental changes as a result of the end Cretaceous impact."
Don Brinkman, Director of Preservation & Research at the Royal Tyrrell Museum, was surprised by the Boremys finds, given the prior fossil record. He agrees that the discovery strengthens “evidence that turtles in general were little affected by the extinction event."
Jason Head of the University of Nebraska-Lincoln said, "The extinction in ocean environments was catastrophic, but the situation on land was much more complex with only some groups of animals, including dinosaurs, going extinct and others, like turtles, appearing to sail through the event."
Today's turtles, however, aren’t faring as well.
As James Parham, a Field Museum of Natural History researcher, told Discovery News, "The sad irony is that these hardy animals that have existed for 220 million years are now going extinct because of human activities. They survived the asteroid, but they can't survive our species."