Many years ago I recall seeing an arresting illustration by paleo-artist Mark Hallett in a magazine. It was of a group of Triceratops forming a protective circle to ward off a pair of Tyrannosaurus, but I would later learn there was a substantial problem with this picture. Even though Triceratops is one of the most common Late Cretaceous dinosaur fossils in North America and vast herds of other horned dinosaurs have been found, no direct evidence had been discovered that Triceratops traveled in social groups. A paper published in the latest issue of the Journal of Vertebrate Paleontology, however, provides the first evidence that this dinosaur may have been social for at least part of its life.
Written by Joshua Matthews, Stephen Brusatte, Scott Williams, and Michael Henderson, the new paper describes a Montana fossil site dating to the waning days of the Cretaceous. So far the team working the site has recovered numerous bones and bone fragments, over 98 percent of which appear to be from Triceratops. What makes this especially interesting is that among the fragments, the paleontologists found three left nasal bones. Since each individual Triceratops only had one left nasal bone, this means there are at least three individuals present at the site.
Unfortunately it does not appear that the entire skeletons of these dinosaurs are preserved at the site, but enough of the skeletal material was present to allow the scientists to determine that the Triceratops recovered so far were juveniles. The nasal bones are key to this conclusion. The right and left nasal bones of Triceratops fuse together as the animal approaches maturation. That the nasal bones the scientists found were still unfused indicates that the dinosaurs were still juveniles.
The authors of the paper note that more work remains to be done, but it appears that these juvenile Triceratops lived and died together. This is consistent with similar finds announced for other dinosaur species in the last few months, like Alamosaurus and Sinornithomimus. It is too early to know for sure, but perhaps juvenile Triceratops lived in groups and became more solitary as they reached maturity. This would explain why adult specimens have always been found alone, much like what was thought of the sauropod Alamosaurus. There is still plenty to uncover at the Montana dig site, though, and further discoveries may confirm or undermine this hypothesis.