Dinosaur diaries: were ankylosaurs socially distancing?

Fossil bones are great for telling us how ancient animals looked, but not how they behaved. Whether dinosaurs maintained their social distance or moved in herds has long been an area of debate.

Evidence for herding behaviour in large herbivorous dinosaurs has previously been found in the form of trackways and something known as Mass Death Assemblages (MDAs). These are sites where the bones of large numbers of dinosaurs are found in a single location and can contain the remains of thousands of individuals.

If these bones are all from dinosaurs of the same species, it's possible they all died around the same time and so were perhaps living together as a herd.

A team led by Gábor Botfalvai have recently published a paper investigating the evidence for social structures amongst one particular group of dinosaurs: the armoured, plant-eating ankylosaurs.

Typically, ankylosaur fossils are found as individual specimens.

Their tank-like bodies and short legs were likely inefficient for long-distance walking in herds suggesting that they lived a solitary lifestyle with a limited home range, similar to that of modern-day rhinoceros.

This solitary lifestyle is supported by the fact that the dinosaurs were covered in extensive body armour that was probably used in part for defence. This indicates that at least for heavily armoured adult ankylosaurs, the animals were capable of fending off predators on their own.

But the remains of ankylosaurs have also been found in Mass Death Assemblages.

The team studied the fossils from six ankylosaur MDAs found in the Cretaceous rocks of the USA, Hungary and Mongolia (145 to 66 million years ago). They looked specifically at the age of the individuals buried at each site, their armour and body plan and the types of habitat they died in. In addition to the fossils, the team also studied how modern large herbivores live and socialise.

The Mongolian MDA consisted of juvenile individuals of one species, Pinacosaurus, indicating that young animals were gathering together, possibly for protection. The MDAs of the USA contained small groups of adult Gastonia, whilst the Hungarian MDA was also made up of mature ankylosaurs. This raises the possibility these groups were segregated by age.

By looking at all areas of how the dinosaurs died, the team concluded that although many ankylosaurs may well have led solitary lives, at least some species gathered into groups at certain times in their life.

No common traits for sociality were identified by the team, indicating perhaps a specific set of circumstances led each group to form. This suggests that perhaps ankylosaurs had a more complex social structure than previously understood.

So, whilst many ankylosaurs practiced social distancing, some very likely did move in herds.

Dinosaur skulls evolved faster than those of birds

Dinosaurs came in a huge variety of shapes and sizes – and nowhere is this clearer than when looking at their heads.

The skulls of dinosaurs range from the heavy, robust jaws of Tyrannosaurus through to the delicate snout of Diplodocus. Whereas duckbilled hadrosaurs like Edmontosaurus had hundreds of teeth, Gallimimus and its relatives had none at all.

Dinosaurs also sported a host of fancy headgear, such as the horns of ceratopsians such as Triceratops, domes of pachycephalosaurs, and crests of hadrosaurs like Parasaurolophus and theropods such as Dilophosaurus. These were likely used in social display or fights over the pecking order.

By contrast, the skulls of birds, the only living group of dinosaurs, lack many of these features. A new study led by Ryan Felice, at University College London, has used a combination of techniques to unpick these evolutionary patterns.

Felice and colleagues used x-ray and laser scanning techniques to create three-dimensional computer models of the skulls of 36 non-avian dinosaurs and 355 birds.

They then digitally placed a series of coordinates on these skulls that allowed them to measure their shape, and to capture how individual regions – such as the jaw joint or snout – varied between species. This allowed them to calculate how quickly regions of the skull were evolving within different dinosaur lineages, including birds.

The results showed that non-avian dinosaurs had incredibly varied skulls, which were evolving more quickly than those of their bird descendants.

The fastest rates of evolution were seen in the bones of the skull roof and the jaw joint. Rapid evolution of the skull roof was associated with the development of the elaborate horns, crests, and domes common in non-avian dinosaurs. The variance in the jaw joint reflects the ways in which dinosaurs adapted their skulls to tackle a wide range of diets, ranging from heavy bone-crunching in tyrannosaurids through to the complex chewing motions that hadrosaurs used to grind their plant food.

In birds, meanwhile, rates of evolution slowed down across most of the skull. Compared to other dinosaurs, birds were both smaller but also evolved relatively larger brains and eyes. These are essential for flight but packing them into such a small space appears to have constrained the evolutionary flexibility across much of the skull. These constraints mean that, rather than showing off with large horns or head-crests, birds use their feathers to perform social displays.

Rapid rates of evolution, however, are still seen in the beaks of birds. Rather than varying multiple parts of the skull as non-avian dinosaurs did, birds have instead focused on adapting the shape of their beaks towards different diets and behaviours.

These results help us to understand the great diversity of dinosaur fossils we see, and how they dominated so many ecological niches throughout the Mesozoic (252-66 million years ago), specialising their adaptable skulls towards a range of lifestyles and tasks. It also helps us to understand the diversity of living birds, and the processes that have shaped the evolution of the distinctive forms we still see around us today.