Dinosaurs indecline long before mass extinction?
The findings, published online today 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.
“Few issues in the history of paleontology have fueled as much research and popular fascination as the extinction of non-avian dinosaurs,” said lead author Steve Brusatte, a Columbia University graduate student who is affiliated with the AMNH’s Division of Paleontology. “Did sudden volcanic eruptions or an asteroid impact strike down dinosaurs during their prime? We found that it was probably much more complex than that, and maybe not the sudden catastrophe that is often portrayed.”
Brusatte, Richard Butler of the GeoBio-Center ofLMU Munich, Albert Prieto-Márquez from the Bavarian State Collection for Palaeontology, and Mark Norell, chair of the AMNH’s Division of Paleontology, are the first to look at dinosaur extinction based on “morphological disparity”—the variability of body structure within particular groups of dinosaurs. Previous research was based almost exclusively on estimates of changes in the number of dinosaur species over time. However, it can be very difficult to do this accurately.
“Accurate counts of species in the fossil record are notoriously difficult to compile,” Butler said. “Palaeontologists have not been able to study all periods of time equally, so apparent changes in the numbers of species recognized through time might merely represent differences in the amount of rock and number of fossils found and studied. Morphological disparity offers a different way to look at the problem.”
By looking at the change in biodiversity within a given dinosaur group over time, researchers can create a rough snapshot of the animals’ overall well-being. This is because groups that show an increase in variability might have been evolving into more species, giving them an ecological edge. On the other hand, decreasing variability might be a warning sign of extinction in the long term.
The researchers 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.“People often think of dinosaurs as being monolithic – we say ‘The dinosaurs did this and the dinosaurs did that,’” Butler said. “But dinosaurs were hugely diverse. There were hundreds of species living in the Late Cretaceous and these differed enormously in diet, shape, and size. Different groups were probably evolving in different ways and the results of our study show that very clearly.”
The researchers 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.“These disparity calculations paint a more nuanced picture of the final 12 million years of dinosaur history,” Brusatte said. “Contrary to how things are often perceived, the Late Cretaceous wasn’t a static ‘lost world’ that was violently interrupted by an asteroid impact. Some dinosaurs were undergoing dramatic changes during this time, and the large herbivores seem to have been mired in a long-term decline, at least in North America.”
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. Therefore, the authors say, the North American record might not be representative of a global pattern, if one exists. They also note that there’s no way to tell whether a declining dinosaur group would have survived if the asteroid had not struck Earth.
“As palaeontologists continue to discover new dinosaur forms at a faster pace each year, our understanding of their biodiversity and morphological variability through time will become further refined,” Prieto-Márquezsaid. “This will likely have a substantial impact in regions like Europe, where the fossil record for many dinosaurian groups like hadrosaurs has been less well sampled relative to other continents like North America or Asia.”
Funding for this study was provided by the German Research Foundation’s Emmy NoetherProgramme; the Alexander von Humboldt Foundation; the National Science Foundation, through the Division of Earth Sciences of the AMNH, the Division of Biological Infrastructure of the AMNH, a Graduate Research Fellowship, and a Doctoral Dissertation Improvement Grant; the Charlotte and Walter Kohler Charitable Trust; the American Museum of Natural History; and Columbia University. American Museum of Natural History
Dinosaurmorphologicaldiversityandthe end Cretaceousextinction
Brusatte, SL, Butler, RJ, Prieto-Márquez, A &Norell, MA
Nature Communications, 1 May