Dinosaurs may have lived in 'social' herds 193 million years ago

Prehistoric creatures lived in social herds 193 million years ago, about 40 million years earlier than previously thought, an analysis published Thursday by Scientific Reports found.

The results, based on dating of sediments found among a discovery of fossils, suggest the dinosaur herd dates back to the early Jurassic period, the earliest evidence of social herding among dinosaurs, the researchers said.

Living in herds may have given this particular species of dinosaur, the Mussaurus patagonicus, an evolutionary advantage, according to the researchers.

Early dinosaurs originated in the late Triassic period, shortly before an extinction event wiped out many other animals, but the Mussaurus patagonicus survived and eventually dominated the terrestrial ecosystem in the early Jurassic period.

"We've now observed and documented this earliest social behavior in dinosaurs," co-author Jahandar Ramezani said in a press release.

"This raises the question now of whether living in a herd may have had a major role in dinosaurs' early evolutionary success and gives us some clues to how dinosaurs evolved," said Ramezani, a research scientist at the Massachusetts Institute of Technology in Cambridge.

The findings are based on an analysis of 100 exceptionally well-preserved early dinosaur eggs and partial skeletons of 80 juvenile and adult dinosaurs from a rich fossil bed in southern Patagonia, the researchers said.

Using X-ray imaging, they were able to examine the eggs' contents without breaking them apart.

Using the preserved embryos within the eggs, the researchers were able to confirm that the fossils were all members of Mussaurus patagonicus, a plant-eating dinosaur that lived in the early Jurassic period.

The Mussaurus patagonicus is classified as a sauropodomorph, a predecessor of the massive, long-necked sauropods that later roamed the Earth.

The researchers observed that the fossils were grouped by age, with dinosaur eggs and hatchlings found in one area and skeletons of juveniles in a nearby location, they said.

Meanwhile, remains of adult dinosaurs were found alone or in pairs throughout the field site.

This "age segregation" is a sign of a complex, herd-like social structure, suggesting the dinosaurs worked as a community, laying their eggs in a common nesting ground, while juveniles congregated in "schools," and adults roamed and foraged for the herd, the researchers said.

"This may mean that the young were not following their parents in a small family structure," Ramezani said.

"There's a larger community structure, where adults shared and took part in raising the whole community," he said.

The fossils identified so far were found in three sedimentary layers spaced close together, suggesting that the region may have been a common breeding ground where the dinosaurs returned regularly, perhaps to take advantage of favorable seasonal conditions.

Among the fossils they uncovered, the team discovered a group of 11 articulated juvenile skeletons, intertwined and overlapping each other, as if they had been thrown together.

However, given the remarkably preserved nature of the entire collection, it appears this particular herd of Mussaurus died at roughly the same time, perhaps in a flash flood, according to the researchers.

The sediments among the fossils included volcanic ash, which may contain zircon as well as uranium and lead, they said.

Based on uranium's half-life, or the time it takes for half of the element to decay into lead, the researchers were able to calculate the age of the zircon and the ash in which it was found.

They successfully identified zircons in two ash samples, all of which were dated to around 193 million years ago.

Taken together, the team's results show that Mussaurus and possibly other dinosaurs evolved to live in complex social herds as early as 193 million years ago, around the dawn of the Jurassic period.

Scientists suspect that two other types of early dinosaurs, Massospondylus from South Africa and Lufengosaurus from China, also lived in herds around the same time, although the dating for these dinosaurs has been less precise, according to Ramezani and his colleagues.

If multiple separate lines of dinosaurs lived in herds, the social behavior may have evolved earlier, perhaps as far back as their common ancestor, in the late Triassic period, the researchers said.

"Now we know herding was going on 193 million years ago," Ramezani said.

"This is the earliest confirmed evidence of gregarious behavior in dinosaurs. But paleontological understanding says, if you find social behavior in this type of dinosaur at this time, it must have originated earlier," he said.

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