Dinosaurs in decline long before mass extinction?

The study, carried out by a team including Dr. Richard Butler of the GeoBio-Center together with Stephen Brusatte and Dr. Mark Norell of the AMNH and Albert Prieto-Márquez from the Bavarian State Collection for Palaeontology, is the first to look at dinosaur extinction based on “morphological disparity”—the variability of body structure within particular groups of dinosaurs, which provides an estimate of changes in biodiversity through time. The findings, published online in Nature Communications, suggest that in general, large-bodied, “bulk-feeding” herbivores were declining during the last 12 million years of the Cretaceous. But carnivorous dinosaurs and mid-sized herbivores were not. In some cases, geographic location might have been a factor in the animals’ biological success.

Disparity trends in four dinosaur groups during the final 12 million years of the Cretaceous (North American species only). Time (from 77-65 million years ago) is shown on the x axis. The y axis shows the disparity metric: sum of variances derived from anatomical character databases. The error bars indicate whether comparisons between time intervals are significant or not (overlap of error bars means non-significance, no overlap means significance). Overall, the large-bodied bulk-feeding ceratopsids and hadrosauroids underwent a marked long-term decline, but the carnivorous coelurosaurs and small herbivorous pachycephalosaurs were stable. (AMNH/S. Brusatte)

The research team calculated morphological disparity for seven major dinosaur groups using databases that include wide-ranging characteristics about the intricate skeletal structure of nearly 150 different species. They found that hadrosaurs and ceratopsids, two groups of large-bodied, bulk-feeding herbivores (i.e. animals that did not feed selectively) may have experienced a decline in biodiversity in the 12 million years before the dinosaurs ultimately went extinct. In contrast, small herbivores (ankylosaurs and pachycephalosaurs), carnivorous dinosaurs (tyrannosaurs and coelurosaurs), and enormous herbivores without advanced chewing abilities (sauropods) remained relatively stable, or even slightly increased, in biodiversity. As a complication, hadrosaurs showed different levels of disparity in different locations: While declining in North America, the disparity of this dinosaur group seems to have been increasing in Asia during the latest Cretaceous.

Overall, the results suggest a more nuanced picture of the final 12 million years of dinosaur history. Although large herbivores appear to have been declining within North America, this pattern is not seen in other groups or geographic regions. In North America, extreme fluctuations of the inland Western Interior Sea and mountain building might have affected the evolution of dinosaurs in distinct ways from species on other continents. The research team suggests that, therefore, the intensively studied North American record might not be representative of a global pattern, if such a global pattern exists.

Funding: Richard Butler is funded by a DFG Emmy-Noether Programme award.

Paper: Brusatte, SL, Butler, RJ, Prieto-Márquez, A & Norell, MA. 2012. Dinosaur morphological diversity and the end Cretaceous extinction. Nature Communications. Doi: 10.1038/ncomms1815