Just as the African savannas aren't all lions and tigers, North America's late Cretaceous wasn't all tyrannosaurs and triceratops. Researchers have found remains of the continent's smallest dinosaur, according to a new study. The Hesperonychus elizabethae, which, according to the study published today in the Proceedings of the National Academy of Sciences, probably looked a lot more like a bird than a tiny Tyrannosaurus rex, is the first of its subfamily to be discovered outside of Asia and postdates the disappearance of its Asian cousins by about 45 million years.

"It was a bit of a shock," says study co-author Nick Longrich, a paleontology research associate at the University of Calgary in Canada about discovering that the fossils belonged to an entirely new genus. The tiny bones—originally assumed to come from a youngster—had languished in a collection at the University of Alberta in Edmonton for 25 years before Longrich and a fellow researcher decided to take another look at them. On closer examination, they noticed that the pelvis was fused, an indication that the 75-million-year-old dino that it came from had reached maturity and stopped growing. Estimates are that the dinosaur probably weighed only a little more than four pounds (1.9 kilograms).

The Hesperonychus, which hails from the same family (Dromaeosauridae) as the Velociraptor (of Jurassic Park fame), was most likely also warm-blooded and fully feathered. "They're very closely related to birds," Longrich says.

Why haven't more little dinosaurs been spotted? "For the same reason we don't find a lot of bird fossils," says Alan Turner, a paleontologist and assistant professor at Stony Brook University in Long Island, N.Y., who wasn't involved in the study. Like modern birds, the bones of these early feathered creatures are hollow with thin walls, which means they're less likely to survive the millennia intact than a massive triceratops skull. What's more: it's a lot easier to spot a giant T. rex femur or thigh bone than it is to find more puny bones. But Longrich says he's optimistic more will probably start cropping up now that researchers know small dinosaurs were in North America. "Sometimes you don't see something," he says, "until you're prepared to see it."

The discovery comes on the heels of another landmark find by Longrich and study co-author, Paul Sereno, a paleontologist at the University of Chicago and National Geographic Explorer-in-Residence: the fossilized bones of the Albertonykus borealis, a chicken-size animal that ate insects.

Across the globe, in Asia's Gobi Desert, paleontologists have been studying a rare formation that includes more than two dozen skeletons of H. elizabethae's better-known relative Sinornithomimus. The find, which has been the subject of previous studies, shows that a herd of juvenile dinos were apparently traveling together when they were trapped and died in viscous mud near an ancient lake, according to the research published today in the international paleontology journal Acta Palaeontologica Polonica

"It's pretty rare [when] you can say how the animals died," says study co-author David Varricchio, an assistant professor of paleontology at Montana State University in Bozeman. "That's pretty exciting." The dinosaurs appear to have been mired up to the hips, which preserved the lower halves of their skeletons better than the top halves. The torsos and heads were still subject to the same hazards as most dinosaur fossils (weather, scavengers, etcetera) that make it difficult to glean information about an animal's life—and death. The fossils were from dinosaurs that ranged from two to seven years in age, says Varricchio, who estimated the ages by counting growth rings in the bones.

Herd behavior is not uncommon in animals today, especially in birds, which in paleontologist-speak are technically living dinosaurs. But a herd exclusively made up of juveniles is not so common in today's animal kingdom. One theory, Varricchio says, is that whereas adults were busy nesting, breeding and perhaps rearing young hatchlings, juveniles that were too young to breed were free to "mosey about on their own." "We have hints that this behavior might extend to all dinosaurs," he says.

Turner cautions that "Any kind of inferences about behavior or social structure in any extinct group—including dinosaurs—is difficult to do." On the other hand, he says, these kinds of deposits, where there are many individuals, are especially important in understanding intricacies of dinosaur life.

For Varricchio, who has been on plenty of dinosaur digs, the discovery struck a unique chord. "It was the only [location] in which I felt empathy for the animals," Varricchio tells ScientificAmerican.com. "I could understand how they had perished."