The young Protoceratops did not meet a swift end. The four of them struggled against the sand that trapped them, trying to push their heads up into the air. But they were hopelessly stuck. Hidden by the entombing sand, they died together.
This is the tale of four juvenile Protoceratops that died over 70 million years ago in a desert that once existed where Mongolia’s own Gobi Desert resides today. They’re not the first of their kind to be found like this. Palaeontologists have uncovered aggregations of infant Protoceratops, as well as adults, but that’s what makes this block full of juveniles so special. Described by University of Bristol palaeontologist David Hone and colleagues in PLoS One, the unfortunate dinosaurs fill a gap in our knowledge of dinosaur social behaviour.
Hone and coauthors were able to tell that the Protoceratops were juveniles from a few skull clues. The bones of the dinosaurs’ skulls had not fully fused, their eyes were relatively large for their skull, and their frills were proportionally small. They weren’t babies, but they were still growing. And all four of Protoceratops in the block belonged to the same life stage, which seems to be a common occurrence for these dinosaurs.
Of course, dinosaurs that were buried together did not necessarily live together. There are many reasons why bones may become buried in the same place, from social interactions to the way bodies are transported after death. But the completeness and positioning of the Protoceratops skeletons suggests that they were all rapidly buried at the same time – probably by an intense sandstorm or by a dune that collapsed on top of them. The little horned dinosaurs were likely travelling as a group when they perished.
So what does this mean for Protoceratops behaviour? Hone and colleagues urge caution in interpreting the find. There are plenty of reasons for individuals of a species to group together, from family groupings to aggregations of animals driven together by scarce resources. Looking at animals today, the researchers point out, there are some species that may live in groups for part of their lives but not for others, so saying this or that dinosaur lived in social groups for it’s whole life stretches the evidence too far.
For Protoceratops, however, palaeontologists have groups representing single life stages from infancy to adulthood. This indicates that these dinosaurs were regularly gregarious, or formed small groups. Whether there were dominance hierarchies or more complex interactions among the dinosaurs is impossible to say given the current evidence. The bones will only take us so far. But from the time they hatched, Protoceratops appear to have formed groups with other dinosaurs of their same age. Perhaps, in the land of Velociraptor, it was best not to travel alone.
For more, read David Hone’s post about the paper at Archosaur Musings.
Hone, D., Farke, A., Watabe, M., Shigeru, S., Tsogtbaatar, K. 2014. A new mass mortality of juvenile Protoceratops and size-segregated aggregation behavior in juvenile non-avian dinosaurs. PLoS One. 9 (11): e11306. doi: 10.1371/journal.pone.0113306