Wednesday, 6 August 2014

An unexpected ornithischian

We know surprisingly little about the early evolutionary history of the ornithischian dinosaurs (Irmis et al. 2007). Indeed, the global record of Late Triassic ornithischians consists of only three partial specimens and all of these suffer from incompleteness and have suffered controversy over their exact geological ages and phylogenetic positions. The situation improves in the Early Jurassic with rich faunas from southern Africa and other material from Europe and North America. Nonetheless, few of these sites are well dated, meaning that they cannot place precise constraints on the timing of key events in Ornithischia.

Today, my colleagues and I were excited to announce the discovery of a new early ornithischian that sheds some much needed light on the early stages of their evolutionary history – Laquintasaura venezuelae. Laquintasaura was a small biped, around 1 m in total length, and is known on the basis of at least four individuals that were found together in a rich bonebed (Barrett et al. 2014). The name is based on the La Quinta Formation, from which the material comes, and the country of Venezuela, honouring the fact that Laquintasaura is the first Venezuelan dinosaur and, more broadly, the first to be named from the whole of northern South America. The first specimens were found in the early 1990s and continuing fieldwork by Marcelo Sánchez-Villagra and collaborators has led to the accumulation of a large collection of material, numbering many hundreds of individual bones. Marcelo and I first started working on this material together when he joined the staff of the Natural History Museum in the mid-2000s, but the pressures of other work and the need to find additional diagnostic material led to a long incubation period for the paper.

Laquintasaura provides us with several insights. Critically, we were able to use high-precision radiometric dating techniques to provide a maximum age for the bonebed of 200.9 Ma. This places the deposition of the bonebed within 1 million years of the end-Triassic extinction event, which has a number of implications. It could mean that ornithischians went through the extinction relatively unscathed and/or bounced back very quickly after the extinction. It might also indicate that ornithischians, which were previously rare, benefitted from some kind of ecological release in the wake of the extinction that allowed them to diversify and increase in abundance in a way that wasn’t possible prior to the extinction. Maybe key competitors or predators were wiped out or reduced in number, although such ideas are difficult to test.

The occurrence of at least four individuals in the bonebed – and probably many more (some material remains unprepared and there is still potential for further discoveries at the site) – is intriguing. It hints at the possibility that these little dinosaurs were living in a social group at the time they died. Histological work indicates that the smallest individual we found was around three years old, while the largest was 10–12 years in age. Could this have been a small multigenerational herd? If so, it would be a very early example of ornithischian sociality. More work is needed on the taphonomy of the site to establish exactly how it was formed to confirm or reject this proposal.

The provenance of the new dinosaur is also interesting. Very few dinosaur localities are known from so close to the palaeoequator – indeed it has been suggested that arid climatic belts either side of the equator might have prevented large animals from colonising this region. At a stroke, Laquintasaura shows that dinosaurs were capable of living in this area, whose climate is thought to have experienced seasonal semi-arid and moist intervals.

A quick word on the lovely reconstruction of Laquintasaura produced by Mark Witton. As some of you may know, I have a pet idea that many basal dinosaurs were omnivores, and I wanted this new image to show something other than the usual docile herbivore munching on a fern. The teeth of Laquintasaura are perhaps its most unusual feature as they possess many ‘carnivore-like’ features (they are long, slender and slightly recurved) – these features suggest to me that they were used not only for plants, but also for small animal prey (at least some of the time).

This paper was a big team effort and I’d like to thank all of the other team members: Richard Butler and Randy Irmis for their ornithischian expertise; Roland Mundil for conducting the radiometric analyses; Torsten Scheyer for working out just how old each individual was; and especially Marcelo for his invitation to work on this exciting material, sorting out all of the logistics, and for his constant reminders that I had to get on and finish the work. In addition, we also relied on the efforts of many preparators at several institutions (Scott Moore-Fay, J. Carillo and U. Oberli) and Mark Witton and Scott Hartman's artistic talents. Now on to the longer-term project that will be the full description …

Barrett, P. M. et al. 2014. A palaeoequatorial ornithischian and new constraints on early dinosaur diversification. Proceedings of the Royal Society B 281, 20141147. (doi:10.1098/rspb.2014.1147)

Irmis, R. B. et al. 2007. Early ornithischian dinosaurs: the Triassic record. Historical Biology 19, 3–22. (doi:10.1080/08912960600719988)

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