A new paper:
We use palaeontological and biological data to infer social lifestyle in ankylosaurs.
Six ankylosaurian mass death assemblages are known indicating gregarious lifestyle.
No common set of traits for gregariousness could be identified in these ankylosaurs.
Most likely a specific set of drivers led to group formation in each taxon.
This diversity indicates an unexpectedly complex social structuring in ankylosaurs.
Abstract
Gregarious behaviour of large bodied herbivorous dinosaurs, such as ceratopsians, hadrosaurs and sauropods, has received much attention due to their iconic mass death assemblages (MDAs). Yet, social lifestyle of ankylosaurs, a highly specialized group of armoured herbivores that flourished predominantly during the Cretaceous Period, remains largely ambiguous. Whereas most ankylosaurs are found as isolated individuals, which may suggest a dominantly solitary lifestyle, the few examples of ankylosaur MDAs indicate that some members of this clade could have been gregarious. In this review, we assess taphonomic history, ontogenetic composition of the MDAs, defence system and other comparative anatomical attributes, and inferred habitat characteristics of ankylosaurs; aspects that may indicate and/or influence group formation in extant herbivores and can also be studied in fossils. We show that the ankylosaurian gross anatomy, such as their heavy armour, barrel-shaped body and usually stocky limbs, combined with the rarity of their MDAs and multiple parallel trackways, all suggest a solitary adult life with efficient anti-predator defence system, limited agility, and confined foraging range. However, characteristics of the known MDAs of Pinacosaurus, Gastonia, and the IharkÃt nodosaurids evaluated in this study imply that at least some ankylosaurs formed groups. Nevertheless, we found no common and consistent set of features to explain why these particular ankylosaurs were gregarious. While inefficient anti-predator defence along with likely higher agility of juvenile Pinacosaurus living in open habitats could account for their gregarious behaviour, such ontogenetic, anatomical and habitat features are not combined either in Gastonia or in the IharkÃt nodosaurid MDAs. Instead, members of each MDA likely had their own specific conditions driving them to form relatively small herds, indicating a more complex social structuring in ankylosaurs than previously acknowledged. Studying morphological and functional disparity within Ankylosauria may help explain the repertoire of their social behaviour. Our holistic approach shows that combining palaeontological and biological information is essential and can provide new insights into the behavioural ecology of long extinct vertebrates.