Discovery of a Triceratops bonebed: Triceratops long bone histology and implications on social behaviour

Abstract

Triceratops, an herbivorous dinosaur that lived during the Late-Cretaceous of North-America, is relatively well-known from a large number of fossil discoveries. Although these discoveries have yielded many different fossils, numerous aspects about Triceratops palaeobiology remain unknown. Ongoing fieldwork in the Lance Formation in Wyoming, USA by the national natural history museum of the Netherlands Naturalis, Leiden has yielded a multi-generation bonebed of at least six Triceratops individuals. The uniqueness of this site is characterized by the high amount of post-cranial material of different-sized individuals. Furthermore, the presence of indicators for different ontogenetic stages, such as epoccipitals and post-orbital horn-cores, suggests that the bonebed consists of sub-adults and adults. This assemblage provides the opportunity to document images on Triceratops long bone histology throughout parts of their ontogeny and allows for direct comparison with other ceratopsian taxa of which comparable histological data is available. Bone histology studies the microstructure of (fossil) bone tissue and observed histological markers can shed light on the growth and development of extinct animals. By combining CT-scans with the histological coring method (HCM), it was possible to do high-precision sampling on six Triceratops femora. The femoral histology yielded typical laminar bone tissue for all of the six specimens and the bone tissue and nature of vascularization was different than that of other available ceratopsians. Moreover, the long bone histology confirms the presence of different ontogenetic stages in this bonebed and distinctly shows the transition from sub-adults to adults on an osteohistological level, thereby mapping Triceratops growth patterns. This notion is further supported by evidence provided by the ontogeny and growth patterns of related ceratopsian taxa (e.g. Centrosaurus, Einosaurus, Kosmoceratops and Utahceratops). Interestingly, the femora did not contain clear lines of arrested growth (LAGs) in the cortex, even during μXRF element analysis. LAGs, indirect indicators for the age of an animal, typically form annually as a result of halted growth during unfavourable seasons. The lack of such histological markers is attributed to a latitudinal gradient that seems to determine the composition of histological markers and is recurrent in other ceratopsian –and other vertebrate– histological studies. This gradient arises through the variation of seasonal changes in climate among different latitudes. Additionally, the latitudinal gradient also implies that migratory behaviour was limited in Triceratops and that this species of dinosaur, and potentially other ceratopsians and dinosaur species, had their own niche in which they lived. As a result, conspecifics lived in close proximity to each other and intraspecific communication was common among Triceratops. Evidence provided by the bonebed taphonomy, dinosaur biogeography and the similar histological patterns among Triceratops all suggest a gregarious nature of these particular set of animals. Moreover, additional indicators of behavioural patterns (e.g. cranial ornamentation) strongly support the notion that Triceratops was a social animal contrary to current popular believe. Future research on Triceratops bone histology is needed to fully evaluate certain hypotheses on growth and ontogeny as well as the latitudinal gradient hypothesis.