Living alone or moving in herds? A holistic approach highlights complexity in the social lifestyle of Cretaceous ankylosaurs

Botfalvai, Gábor and Prondvai, Edina and Ősi, Attila (2019) Living alone or moving in herds? A holistic approach highlights complexity in the social lifestyle of Cretaceous ankylosaurs. Cretaceous Research. (In Press)

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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 the group could have been gregarious. In this review, we assess taphonomical history, ontogenetic composition of monotaxic MDAs, defence system and other comparative anatomical attributes, and inferred habitat preference; aspects that 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 Iharkút 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 Iharkút 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.

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