March 19, 2009 -- New findings on mass dinosaur graves, where several juveniles died together, suggest that young dinosaurs banded together to improve their chances for survival, according to two new studies.
Together, two new studies present three gory ways in which the young dinosaur groups probably met their end: mud traps, droughts and predators.
Paul Sereno, a University of Chicago paleontologist, and his team studied the remains of a herd of more than 25 young, bird-like dinosaurs of the species Sinornithomimus dongi that died together 90 million years ago at what is now the Gobi Desert.
"These animals died a slow death in a mud trap, their flailing only serving to attract a nearby scavenger or predator," Sereno said, explaining that plunging marks in the mud surrounding the skeletons recorded the dinosaurs' failed attempts to escape while predators picked away at their fleshy hips.
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"I was saddened because I knew how the animals had perished," added Sereno, whose findings were published in Acta Palaeontologica Polonica. "It was a strange sensation and the only time I had felt that way at a dig."
Researchers Timothy Myers and Anthony Fiorillo of the Huffington Department of Earth Sciences at Southern Methodist University focused their attention on two other juvenile dinosaur fossil sites, which are described in a paper that will be published in next month's Palaeogeography, Palaeoclimatology, Palaeoecology.
The first, at Mother's Day Quarry in Montana, contains the remains of several young sauropods that died en masse during the Upper Jurassic. Skin impressions indicate soft tissue was still present when the animals were buried at the site.
"During droughts, modern animals tend to cluster around water sources," Myers told Discovery News. "The herd of sauropods preserved at the Mother's Day Quarry may have done the same."
He and Fiorillo also studied the remains of three juvenile Alamosaurus sanjuanensis at the Upper Cretaceous site Big Bend in Texas. The minimally weathered bones suggest the young sauropods died together in a single event.
"Given their proximity to a lake shore, it's possible that they succumbed to drought as well," Myers said.
The three sites add to a growing body of evidence that dinosaurs were gregarious animals that sometimes segregated themselves based on age. This appears particularly true for sauropods, "since the difference in body size between immature and fully-grown sauropods was vast," said Myers.
"Since individuals that are the same sex, age and body size will have similar routines, herds composed of similar individuals will be more cohesive," he added.
While no modern animal is a perfect analog for sauropod behavior, he said, they might have behaved somewhat similar to hoofed mammals, like elks, that form distinct herds to move "together as a unit."
Modern adult elks, however, exhibit fairly extensive parental care. For sauropods, it's possible that youngsters were left to fend for themselves not long after hatching.
Glenn Storrs, vice president for collections research and curator of vertebrate paleontology at the Cincinnati Museum Center, has worked at the Mother's Day Quarry for a decade. He told Discovery News that this site "contains the best existing evidence that age segregation of juvenile sauropods has occurred."
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