"Teen" Dinosaurs Roamed in Herds, Mass Grave Suggests
National Geographic News
Like teenagers at the mall, young dinosaurs may have wandered in herds—fending for themselves while adults were busy nesting, according to a new report on one of the world's best preserved fossil sites.
About 90 million years ago a herd of more than 25 birdlike dinosaurs got stuck in the mud at the edge of a drying lake and perished together in modern-day China, said study co-leader Paul Sereno, a University of Chicago paleontologist.
Nearly complete skeletons of the plant-eaters were found at the Gobi desert site—some stacked on top of each other.
The dig site is etched with an ancient tragedy, said Sereno, who is also a National Geographic explorer-in-residence. (The National Geographic Society owns National Geographic News.)
Plunge and scratch marks are preserved in the long-hardened mud, showing the young dinosaurs' futile attempts at escape.
The dinosaurs' flailing likely attracted predators that feasted on the meatiest parts of the young—the hips, Sereno said. Only hip bones are missing from the fossilized bodies.
"Best Documented Case"
"This is the best documented case we have for preservation of an actual dinosaur population," Sereno said.
Though other dinosaur herds have been found fossilized, the newly announced Gobi site is the first to be found with whole dinosaur skeletons.
The remains are all from a single dinosaur species, Sinornithomimus dongi—"Chinese bird mimic"—first discovered by a Chinese-Japanese team in the 1990s.
In the rib cages of some specimens, Sereno's team found "stomach stones"—rocks apparently swallowed on purpose to aid digestion—and the carbonized remains of the last plants the dinosaurs had consumed.
Judging from the animals present at the site and their ages, as determined by radiometric dating of the surrounding volcanic rocks, the herd was probably made up of one- to seven-year-olds, said David Varricchio, a Montana State University paleontologist. While the parents were devoting time to reproduction, the young took off on their own, said Varricchio, a co-author on the study published in December 2008 in the journal Acta Palaeontologica Polonica.
Sereno said via email: "We hope to learn more about the pace of growth and the number of nesting seasons per year from the skeletons."
The Gobi discovery "fits nicely with the emerging picture that dinosaurs were in many ways more like birds," said Hans-Dieter Sues, a paleontologist at the National Museum of Natural History in Washington, D.C.
"About everything we know about dinosaurs shows they're more like their descendants, birds," Sues added.
For instance, the group of young dinosaurs found at the Gobi site was not a coincidence, Sues said—such age groupings are seen in modern-day birds and was likely a behavior that evolved from dinosaurs.
Dinosaurs also demonstrated other complex, birdlike social behavior, such as brooding their eggs.
"There's a great element of serendipity" in the discovery of the Gobi mass grave, he said. "[Much] of what we know about social structures is based on such mass deaths."