New evidence that dinosaurs formed groups throughout their lives

For many years palaeontologists have accrued information about the behaviour and habits of the extinct dinosaurs. There is some excellent information from skeletons of animals that died together, footprints and nest sites that showed that large numbers of individuals of a single species came together for at least part of their lives.

One of the young Protoceratops preserved alongside three other 
animals of the same age [Credit: Dave Hone]

However, going beyond that is obviously pretty tricky – sure, a group of a dozen (or even a hundred) animals having died together in some way tells you they were in a group, but how natural and normal was this? Even animals that are antagonistic to one another will gather together to migrate, breed, or if food is plentiful (like grizzly bears fishing) or may be forced into one place by something like a drought. A group together in the fossil record could easily be a result of a one-off event, or at least represent a pretty rare part of their behaviour. What you generally want, but so rarely get, is multiple sets of individuals together, but this is something that is possible for an animal called Protoceratops.

This was a genus of small dinosaur from the ceratopsian group – the horned dinosaurs that include famous species like Triceratops, although notably Protoceratops has the large frill at the back of the head, but no real horns on the face. It was a small herbivore (big adults were little more than two metres in total length) that lived in the arid regions of what are now northern China and Mongolia around 80m years ago. These animals were numerous and the shifting sands buried many animals from these deserts and now this is a major area where numerous fossils are found, with Protoceratops one of the most numerous.

There are records of groups of adults of these animals together and there is also a cluster of some very young juveniles from Mongolia that might be from a nest. This alone is quite leading, but the new specimen described today by myself and my co-authors fills the age gap wonderfully. We have a large block of sandstone with the remains of four juvenile Protoceratops in it. These are about twice the size of the animals in the young group, and about half the size of the largest ones known together, so this really helps create a series of specimens with animals together of about the same size.

Group of four juvenile Protoceratops preserved together
[Credit: Dave Hone]

The specimens are beautifully preserved and while it is not exactly clear what killed and buried them, they were apparently able to move at least a little initially before being overwhelmed as they have managed to climb on top of one another. This does suggest that they did not quite die instantly, but it is likely that it took only a few moments and importantly means that this is not an accumulation of separate individuals (bodies could build up over months or years in a river say) but a single event that trapped them all so they were together when they died.

So, we can be pretty confident that Protoceratops did cluster together in groups throughout their lives and this is the first such record for a non-avian dinosaur. Importantly, these groups we have are not of mixed ages, but only ever of animals of about the same size and age. The little ones stuck together and so too did the adults, but they did not mix. It would not be a surprise to find an adult with very young juveniles (there is good evidence of parental care of hatchlings at least in some of the dinosaurs) but it does seem that the juveniles were left to their own devices.

In fact, adults might well have stuck clear of young animals or even kept them away. For a start they would be in competition for limited resources in an arid environment like those where Protoceratops was present, but more importantly, they would draw in predators. Most carnivores target juveniles for a wide variety of reasons, and juveniles are more vulnerable to predation than adults. This may help explain why juvenile dinosaurs formed groups (not just Protoceratops, but actually many of the known groups of dinosaurs are all juveniles) as predators would take a heavy toll and staying in a group helps you keep an eye out and means that if you are attacked, someone else may take the hit.

Collectively it is difficult to be able to pick out individual species of dinosaurs as those that were forming groups and especially those that stayed together for extended periods and had strongly developed social interactions. Even when groups are found as in Protoceratops, that does not mean they spent their whole lives in groups – plenty may have been solitary for large parts, or even almost all, of their lives. Overall there are enough mass mortality sites and trackways to suggest dinosaurs were often in groups and this means at least some species were probably spending most of their lives like this, even if it is hard to determine exactly which ones.

Now though, for the first time, we have strong evidence for this being a major part of the life of at least one ancient dinosaur and this allows us to assemble a better idea of the patterns of behaviour of an animal that has been gone for 70m years.

References

Hone DWE, Farke AA, Watabe M, Shigeru S, Tsogtbaatar K (2014) A New Mass Mortality of Juvenile Protoceratops and Size-Segregated Aggregation Behaviour in Juvenile Non-Avian Dinosaurs. PLoS ONE 9(11): e113306. doi:10.1371/journal.pone.0113306

Author: Dr Dave Hone | Source: The Guardian [November 26, 2014]