Bad to the bone: Cannibal dinosaurs turned to eating each other in tough times
Could theropod dinosaurs have eaten dinosaurs of their own kind?
It looks like they did when food was scarce in ancient Colorado around 150 million years ago during the Late Jurassic period.
Theropods were a carnivorous group of dinosaurs whose members walked on two legs and ranged from small to very large in size.
Their bite marks have been found primarily on the bones of herbivorous dinosaurs (that ate plants) and also on the bones of other theropods, according to a study published Wednesday in the journal PLOS ONE.
Examining fossils in the Mygatt-Moore Quarry of Colorado, researchers found that theropods likely scavenged from the carcasses of their fellow dinosaurs.
Bite marks provide insight on several behaviors of extinct animals among their own species, including food chain interactions, feeding strategies, prey selection and competition, the study said.
But matching bite marks with specific perpetrators is challenging when marks may represent a small number of total bone markings, when marks between actors have similar characteristics and when similar animals inhabit the same environment, the study said.
“[With] trace fossils in general, it can be challenging to figure out who left them,” said first author Stephanie Drumheller-Horton, an adjunct assistant professor and part-time lecturer of paleontology at the University of Tennessee, Knoxville’s department of earth and planetary sciences.
That’s because other animals could have existed at the same excavation locations but haven’t been found yet. And sometimes the reasons why animals were fossilized at a certain site can be murky.
So when researchers are trying to narrow down the details, Drumheller-Horton said, they have to ask themselves, “Can I really match this up?”
In previous research, it was difficult to find such an association in a high diversity of theropods preserved within the Upper Jurassic Morrison Formation, which contains Mygatt-Moore Quarry.
This time, an unusually large volume of bite-marked bones recently discovered in the quarry allowed the researchers to test methods to narrow down the sources of marks and characterize the dinosaurs’ body sizes using measurements of the bite marks.
Out of the 2,368 vertebrate fossils surveyed from the quarry, the researchers found 684 (28.8%) specimens had at least one theropod bite mark.
The amount was surprising as it was significantly higher than in other dinosaur-predominant assemblages, or accumulations of fossils that include mostly dinosaur bones.
“Large fossil assemblages like this can provide such wonderful, detailed windows into the complexities of entire ecosystems in ways that single partial skeletons rarely do,” said Jason Schein, a paleontologist and executive director of the Bighorn Basin Paleontological Institute, who wasn’t part of the study.
Theropod bite marks are especially rare, representing only 4% or less of dinosaur-dominated assemblages, according to previous research.
That’s a much lower rate than the 13.1% to 37.5% frequencies of bones marked by mammals.
Mammals have higher rates because they’re messy eaters, Drumheller-Horton said, and they’re known to leave lots of marks. The idea behind dinosaurs’ marks is that maybe they were accidental, in that they were going after the meat and just happened to scrape the bones.
“We looked at a couple thousand fossils and about 30% of them had bite marks on them, which is up there in that sort of mammal range,” Drumheller-Horton said.
The findings left the authors with two possible explanations, she added.
Maybe the Mygatt-Moore Quarry “was weird and something was going on here where the theropods were having to just eat every available resource they could find, which does fit with some of what we know of the ecology,” she said.
“But the other side of that is that we paleontologists might have led ourselves astray a little bit because of the way we collect fossils in the field,” she said.
Former collection protocols meant only the best-preserved fossils were collected for study and display, which might have left an unusually large amount of marked remains in the quarry.
To provide more context for their findings, researchers would need to expand their collection of data on bite-marked bones to other sites.
A stressed ecosystem
The Mygatt-Moore Quarry is a dinosaur-dominated assemblage in the middle Brushy Basin Member within the Upper Jurassic Morrison Formation, located within the McInnis Canyons National Conservation Area near the Utah-Colorado border.
It was discovered in 1981 by couples Pete and Marilyn Mygatt and J.D. and Vanetta Moore as they hiked, the study said. For the last several years, the site has been managed by the Museums of Western Colorado and the United States Bureau of Land Management.
The quarry’s ecosystem has a lot of vegetation and both wet and dry seasons, Drumheller-Horton said, but no perennial standing water. Remains of turtles, fish, crocodiles and other aquatic invertebrates were rare, likely due to the lack of continuous standing water.
More than 30 years of excavations have unearthed thousands of vertebrate fossils such as herbivorous dinosaurs and bony fish. Some of that material was dug up by public citizens and volunteers, Drumheller-Horton said, which is what prompted her team’s project.
About six years ago, Julia McHugh, coauthor of the study, noticed significantly more bite marks on the fossils than she was expecting while working as curator of paleontology at the Museums of Western Colorado, Drumheller-Horton said.
That’s when they switched gears from the traditional protocol of only collecting attractive, identifiable fossils to bringing everything back to the lab at the Museums of Western Colorado to observe bone markings.
Scattered bite marks
Using angled light and a microscope, the authors inspected the bone markings.
Most of the bite marks were found on sauropod (herbivorous) remains, while theropod dinosaurs had the second highest amount (17%). Bite marks included punctures, scores, furrows, pits and striations, marks in which the individual denticles of a serrated tooth leave subscores on bone surfaces.
Denticles, or serrations, are small bumps on a tooth that give the tooth a serrated edge. In studies of dinosaurs, denticles are used to describe and classify their teeth.
Bitten areas were categorized according to nutritional value, based on designations from previous research. Low economy elements were less nutritious and closer to cartilage and ligaments, while high economy elements were more nutritious and near large muscles or internal organs.
Low economy areas had 52.8% of the bites, while high economy regions had 47%. Vertebrae and ribs had the majority of bite marks, which are, respectively, low and high economy.
The bite marks were up to an inch long by a third-inch wide in length.
To determine the identity and maximum size of theropods that could have produced the marks, the authors compared striation widths to recorded widths of other groups found at the quarry, since denticle widths increase with a theropod’s size.
The striated marks were consistent with denticles of the two largest theropod predators known from the site: Allosaurus, the most common, and Ceratosaurus. One of the bite marks may be from an unusually large Allosaurus or a separate, large-bodied taxon such as Saurophaganax or Torvosaurus tanneri, which have been found in other parts of the Morrison Formation.
These findings tie in with trace fossils sometimes communicating “cryptic diversity,” Drumheller-Horton said. “We know we don’t have everything that was living at the site, but sometimes the trace fossils can give us a little window into who else was there even if we don’t necessarily have their skeletons.”
What drove scavenging behavior
The distribution of the bite marks on all parts of the remains, particularly those found on other theropods, suggested the marks were evidence of scavenging rather than active predation, the study said.
“That tells us a little bit about how completely the remains were being used by the predators,” Drumheller-Horton said. “We have bite marks on everything from really meaty bits like ribs and long bones in the legs, all the way down to things like little, tiny toe bones that if that’s what you’re eating on, you’re probably late to the party. All the better parts are gone, and that’s all you have left.”
Bite marks on high economy bones could mean the dinosaurs were preyed upon, but scavenging is what’s almost certain, Drumheller-Horton said. Maybe the meatier parts were eaten by dinosaurs that just happened to show up to the remains earlier than others.
The carcasses were likely buried slowly since quarry sediment accumulated gradually. This would have left remains exposed for longer periods of time, and to more carnivores than expected in dry seasons when easier sources of food might not have been readily available.
“This suggests that the Mygatt-Moore Quarry might preserve a stressed paleoecosystem, in which any available remains would be more fully processed to ensure utilization of every available nutrient source,” the study said.
Though the findings highlighted food chain dynamics and the quarry’s ecology, they could be influenced by former collection protocols that prioritized appealing fossils and left damaged bones at the site.
“I get really excited by beat-up, marked-up, broken bones because that tells you sort of the history of the fossil,” Drumheller-Horton said. “All different kinds of things can mark up bones and this could be something that happened [during any part of the animal’s life, death, burial or fossilization]. Each one of those steps tells you something.
“If we’re only collecting the nice stuff and we’re leaving sort of the ugly, beat-up stuff out in the field, that’s going to skew your idea of what the history of the fossils at that site actually was.”
“I think this [study] will cause several researchers to go back and take a look at what is preserved on bones collected in large death assemblages to see what variety should be considered the norm,” said Jason Poole, head preparator and paleo illustrator at the Bighorn Basin Paleontological Institute, who wasn’t involved in the study.
“Collection bias is a problem historically, and it persists today. Workers who are diverse in their collection of fossils from any given site will be more apt to have a better understanding of the ecosystems that the fossils and geology represent.”
The researchers are working toward follow-up studies to establish a good basis for comparison for examining bite-marked bones within other sites.
