Raptorial Dinosaurs Did Not Hunt in Coordinated Packs, Paleontologists Say
An analysis of the fossilized teeth of Deinonychus antirrhopus, a species of wolf-sized dromaeosaurid dinosaur that lived between 115 and 108 million years ago (Cretaceous period) in what is now the United States, adds to the growing evidence that this and other raptors were not complex social hunters by modern mammalian standards.
The image of the highly intelligent, pack-hunting raptor has become engrained in scientific literature and popular works alike.
First proposed to explain the relatively common co-occurrence of large herbivorous dinosaurs and much smaller Deinonychus antirrhopus from the Cretaceous of North America, a wolf-like social hunting structure has become the standard depiction of raptors in popular works over the last three decades.
“Raptorial dinosaurs often are shown as hunting in packs similar to wolves,” said Dr. Joseph Frederickson, a vertebrate paleontologist and director of the Weis Earth Science Museum at the University of Wisconsin Oshkosh Fox Cities Campus.
“The evidence for this behavior, however, is not altogether convincing. Since we can’t watch these dinosaurs hunt in person, we must use indirect methods to determine their behavior in life.”
“Though widely accepted, evidence for the pack-hunting dinosaur proposed by Yale University paleontologist John Ostrom is relatively weak,” he added.
“The problem with this idea is that living dinosaurs (birds) and their relatives (crocodilians) do not usually hunt in groups and rarely ever hunt prey larger than themselves.
“Further, behavior like pack hunting does not fossilize so we can’t directly test whether the animals actually worked together to hunt prey.”
Paleontologists recently proposed a different model for behavior in raptors that is thought to be more like Komodo dragons or crocodiles, in which individuals may attack the same animal but cooperation is limited.
“We proposed in this study that there is a correlation between pack hunting and the diet of animals as they grow,” Dr. Frederickson said.
In Komodo dragons, babies are at risk of being eaten by adults, so they take refuge in trees, where they find a wealth of food unavailable to their larger ground-dwelling parents. Animals that hunt in packs do not generally show this dietary diversity.
“If we can look at the diet of young raptors versus old raptors, we can come up with a hypothesis for whether they hunted in groups,” Dr. Frederickson explained.
To do this, Dr. Frederickson and his colleagues from the University of Oklahoma and the Sam Noble Museum analyzed the chemistry of teeth from Deinonychus antirrhopus.
“Stable isotopes of carbon and oxygen were used to get an idea of diet and water sources for these animals,” he said.
“We also looked at a crocodilian and an herbivorous dinosaur from the same geologic formation.”
The paleontologists found that Cretaceous crocodilians, like modern species, show a difference in diet between the smallest and largest teeth, indicating a distinct transition in diet as they grew.
“This is what we would expect for an animal where the parents do not provide food for their young,” Dr. Frederickson said.
“We also see the same pattern in the raptors, where the smallest teeth and the large teeth do not have the same average carbon isotope values, indicating they were eating different foods. This means the young were not being fed by the adults, which is why we believe Jurassic Park was wrong about raptor behavior.”
The research is published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.
J.A. Frederickson et al. Ontogenetic dietary shifts in Deinonychus antirrhopus (Theropoda; Dromaeosauridae): Insights into the ecology and social behavior of raptorial dinosaurs through stable isotope analysis. Palaeogeography, Palaeoclimatology, Palaeoecology, published online May 3, 2020; doi: 10.1016/j.palaeo.2020.109780