Society sans frills.

Past studies suggest that horned dinosaurs such as Triceratops and their relatives, a group known as ceratopsians, lived in herds and used the frills on their skulls and other ornamentations to identify members of their own species, as did many other dinosaurs (SN: 8/13/05, p. 103). Now, the discovery of the fossils of several young dinosaurs in one small space suggests that an ancestor of ceratopsians exhibited social behavior millions of years before their group gained large distinctive decorations.

Researchers found the remains of six juvenile psittacosaurs in 128-million-year-old rocks in northern China. Those rocks appear to have been laid down quickly as a cementlike mixture of water, fresh volcanic ash, and eroded soil, says Paul Barrett of the Natural History Museum in London.

All of the house cat-size psittacosaurs were intact, and they were lying in the same orientation in an area measuring about 0.5 square meter, suggesting that the youngsters were trapped together, he notes. But individual body measurements indicate that members of the group would have ranged in age from about 18 months to 3 years, says Barrett.

Psittacosaurs didn't have horns and broad skull frills like those on Triceratops, a distant relative that lived about 60 million years later. However, the new finding hints that psittacosaurs traveled in groups that included individuals from several different clutches of eggs, says Barrett. Previously, researchers had discovered a fossilized nest that included almost three dozen psittacosaur hatchlings, possibly from several mothers. Together, this evidence of gregarious behaviors suggests that ceratopsians developed complex social behaviors long before they possessed extreme cranial ornamentations, says Barrett.--S.P.