The dinosaurs in the study were close ancestors of birds, and their fossils were found on top of unusually large clutches, said David Varricchio, a Montana State Universitypaleontologist and lead author of the paper. It's possible, he said, that the males mated with several females who laid their eggs in one large clutch. When the females left, the males incubated and protected the eggs on their own.
"Scientists have long debated which care system, male-only or both parents, evolved first," Varricchio said. "The new research indicates that male-only care came first, evolving within the closest dinosaur ancestors of birds."
Scientists came to that conclusion after examining the clutch size and internal bone structures in Troodon, Oviraptor and Citipati dinosaurs, Varricchio said. Previous studies, largely done at MSU, had found that the dinosaurs shared several reproductive features with living birds. They both produced asymmetric eggs with nearly identical eggshell, for example. Adult dinosaurs were also found on top of clutches, possibly brooding the eggs.
The new work suggests that unusually large egg clutches are associated with male-only care, Varricchio said. The researchers compared the clutch sizes to adult sizes in the three types of dinosaurs, as well as in their closest living relatives -- birds and crocodiles. Large clutches occur in those animals with male-only care.
To further test the theory, the paleontologists examined the bones of adult dinosaurs found on top of clutches in Montana and Mongolia. None of the bones contained tissue normally associated with egg-laying females. Female birds store minerals for egg-laying as extra tissue in the inside of their hollow limb bones.
Male-only care is common among large flightless birds like emus and rheas and the South American tinamous, Varrichio said.
"This is important research in that it tells us about when and how an important aspect of the modern avian reproductive system came to be," added co-author Greg Erickson, a 1991 MSU graduate and now associate professor at Florida State University.
"Prior to the discovery that birds are in fact theropod dinosaurs, these animals seemed almost alien among reptiles and scientists were at a loss to explain how the modern avian condition evolved," Erickson said.
Theropods are a group of largely carnivorous dinosaurs. The group also includes birds and is believed to be the group from which birds evolved.
Varricchio said researchers had observed previously that dinosaur nests contained large numbers of eggs and the eggs were unusually large for the size of the adult dinosaurs.
"But no one had tried to understand how big or how unusual these clutches were until our study," he said.
The parental care study found that Troodon, Oviraptor and Citipati all had large eggs compared to their body size, but it leaves some unanswered questions for future research, Varricchio said. Scientists, for example, don't know exactly what the male dinosaurs were doing on top of the eggs. They could've been incubating them, protecting them or shading them from the sun. Another mystery is the reason behind the dinosaurs' sudden death while perching over eggs. One theory regarding the Mongolian dinosaurs is that they died from sandstorms or collapsing sand dunes.