A single fossil site containing the remains of dozens of individuals from a range of age groups is the earliest evidence that long-necked, four-legged dinosaurs lived in herds.
“This is a stunning new fossil site,” Steven Brusatte, a paleontologist at the University of Edinburgh who wasn’t involved in the new study, wrote to me in an email. “This is convincing evidence that these plant-eating dinosaurs were social, and formed groups, and probably took at least some care of their eggs and young.”
A team led by Diego Pol from the Museum of Paleontology Egidio Feruglio discovered the fossils in the Laguna Colorada Formation in Patagonia, Argentina. They belong to Mussaurus patagonicus—a long-necked sauropod from the Early Jurassic who stood on four legs. Over the past 15 years, the team has been conducting research and excavations at the fossil site, resulting in the discovery of more than 100 eggs and nearly 80 skeletons of Mussaurus.
The fossils, dispersed across an apparent breeding ground, spanned the entire dinosaur life cycle—from embryos still tucked away inside eggs through to fully grown adults. Incredibly, the fossils were clustered into age-specific groupings—a sign that these gigantic herbivores lived in herds. At an estimated age of 192-million-years-old, these Early Jurassic fossils predate prior evidence of this complex social behavior among dinosaurs by around 40 million years. Details of this finding were published today in Scientific Reports.
“The specimens we have found showed that herd behavior was present in long-necked dinosaurs since their early history,” explained Pol in an email. “These were social animals and we think this may be an important factor to explain their success.”
The shared Mussaurus breeding ground was located on the margins of a dry lake. The climate was warm, but evidence of drought points to a possible cause of death and a reason for why some of the dinosaurs were buried by wind-blown dust.
Most of the eggs were grouped into clusters containing anywhere from eight to 30 eggs and placed along a series of trenches suggestive of a common breeding ground. X-ray imaging was used to identify the embryos as belonging to Mussaurus.
Analysis of the fossilized skeletons revealed the surprising presence of age-specific groups, including a cluster of 11 juveniles (all younger than one-year-old), a group of nine adolescents, and two adults. The discovery of age-specific groupings is potential evidence that Mussaurus individuals lived in herds, that they did so across their entire lives, and that they preferred to hang out with members of a similar age. I asked Pol to explain the presence of age-specific groupings.
“Mussaurus was tiny when it was born—the entire skeleton fits in the palm of your hand—but adults were 1.5 tons, which is roughly the weight of a hippo,” he responded. “The daily motion patterns, speed, and daily foraging was probably very different in newborns, youngsters, and adults.” He said it’s common for animals of the same size group to hang out together and coordinate their activities. This is especially the case, said Pol, “for youngsters that are small, inexperienced, and therefore more vulnerable to attacks from predators.”
This complex social behavior may have emerged as a consequence of increasing body sizes, which started among the sauropods between 227 and 208 million years ago. These dinosaurs, in order to meet their tremendous energy requirements, had to forage over long distances, requiring a new set of adaptive social skills, according to the study.
Ryan Felice, an anatomist from University College London who wasn’t involved in the research, described it as a “really exciting discovery.” As he explained, paleontologists already knew that non-avian dinosaurs were good parents, as evidenced by clusters of nests belonging to the Cretaceous dinosaur Maiasaura—a name that literally means “good mother lizard.”
“From those types of discoveries, we could infer that dinosaurs had a reproductive strategy similar to crocodiles today—the mother protects the babies when they are very small, but once they can fend for themselves the family breaks apart and everyone goes their separate ways,” Felice said. “What makes this discovery so exciting is that there are [hatchlings], juveniles, and fully grown adults of Mussaurus all in the same place. This means that multifamily groups got together not just for breeding and nesting but that they potentially formed life-long herds, more like today’s elephants or wildebeests.”
What makes the new discovery especially important is that Mussaurus is a fairly ancient dinosaur species, “so the authors have hypothesised that maybe social groups and parental care were things that evolved early in dinosaur history,” said Felice.
Brusatte offered a similar take.
“Because these are Early Jurassic dinosaurs, from the early stages of dinosaur history, it is the oldest record, from that first stage of dinosaur history, it is the oldest record of dinosaur social lives,” he explained. “It seems dinosaurs were highly social animals from the very beginning, which may have factored into their stupendous evolutionary success.”
Looking ahead, Pol and his colleagues will continue to inspect the site in hopes of acquiring a better understanding of the nests and how they were structured, along with searching for evidence of predators and the plants consumed by Mussaurus.