Paleontologists have found the earliest known evidence that dinosaurs lived in herds — unlike reptiles, and more like penguins and other birds do today — and socialized with each other by age groups.
The scientists, working a rich deposit of fossils at a site in Argentina’s province of Santa Cruz, at the southern tip of South America, found more than 100 eggs and the skeletons of 80 individuals ranging in age from embryos to adults.
All of the fossils, including the embryos inside the eggs, are of the species Mussaurus patagonicus. These dinosaurs were about 10 feet high and 26 feet in length when fully grown, with a long tail balanced by an equally long neck that ends in a head that seems too small for the enormous animal it is attached to. This is the only place Mussaurus remains have ever been found.
Little is known about the behavior of dinosaurs, but this large number of fossils, and their distribution at the site, has given scientists new information about their social lives. The study appeared in Scientific Reports on Thursday.
The bones and eggs are spread over about 250 acres — a small area for finding so many fossils of the same species. Most of the eggs were found in clutches of eight to 30 in nests close together, which suggests that the animals used a common breeding ground. Within the nests, the eggs are arranged in trenches that the animals apparently excavated for the purpose.
The scientists found eggs, neonates, juveniles and adults clustered close to each other, which indicates that the animals lived in socially cohesive groups, rather than gathering only temporarily to breed and lay eggs. Age groupings like this, the authors write, suggest that the animals maintained social connections with each other across their life spans.
Among the specimens are 11 one-year-olds, and an analysis of the bones suggests that they were probably members of a single brood, buried together. The researchers also found many adults close to each other, in natural resting poses, suggesting that the animals lived and died together.
Often fossils are found in large numbers at one location not because the animals died together, but because a stream or river transported bones of different ages and species, piling them up and burying them under the silt. But these Mussaurus bones were found in deposits made from windblown dust, and the authors conclude that they probably died simultaneously in periodic droughts. There were at least three episodes of mass death at the site.
“The sediments also have evidence that the animals still had soft tissue when they were buried,” said the lead author, Diego Pol, a researcher at the Museo Paleontologico Egidio Feruglio in Trelew, Argentina. “This indicates a simultaneous death.”
The Latin name for the species is clearly a misnomer. The site had been examined by other researchers in the late 1970s, but they found only a few juveniles, small enough to hold in the palm of one hand. They named them Mussaurus, which is Latin for “mouse lizard.” These new excavations, which began in 2012, have revealed a much more extensive assemblage of bones, and an animal considerably larger than a mouse.
It has been known for some time that dinosaurs sometimes lived in herds, but the behavior was found only in dinosaurs that lived, at the earliest, about 150 million years ago. But the long-necked, plant-eating Mussaurus flourished 193 million years ago, meaning that dinosaurs likely lived in herds earlier than previously thought.
X-ray analysis by the researchers of the growth patterns in the bones indicates that the animals did not reach adult size until they were at least 15 years old.
“During all this time young individuals were vulnerable and subject to predation,” Dr. Pol said. “This adds to the interpretation that herd behavior was beneficial for the species, to protect the young during their growth.”
Body shape — the long neck and tail that come to mind immediately when picturing a dinosaur — may also be a factor in Mussaurus’s evolutionary success.
“Once it appeared, that body became the dominant form for millions of years,” Dr. Pol said. “So we’re very interested in the evolution of that form. Behavior was, maybe, another element in that successful evolutionary recipe.”