Monday, 28 July 2014

Out with a bang!


Dinosaur extinction has always excited the imagination – what forces could possibly account for the disappearance of such a dominant and charismatic group? More than 100 different theories have been published to account for the extinction, ranging from the interference of meddling aliens, to collisions between the Earth and roving lumps of galactic antimatter and the possibility of some malign dinosaur pandemic flu. Almost all of these theories have been shown to be lacking in evidence (or just plain crazy). However, several events at the end of the Cretaceous Period have stood the test of scientific scrutiny. These are: the impact of a large meteorite (around 10 km in diameter); the climatic effects of the extensive volcanic eruptions that formed the stacked lavas of the Deccan Traps in India; and a series of more general, long-term global environmental changes, including sea-level rises and climatic shifts. Recently, a multi-disciplinary team reviewed the evidence for the asteroid impact, concluding that the timing of the impact, whose crater is centered on the small town of Chicxulub on the Yucatan Peninsula of Mexico, was the perfect smoking gun to drive the extinction (Schulter et al. 2010). However, dissenting voices (my own included) were quick to reply, noting that the longer-term effects of volcanism and environmental change might have been largely to blame (Archibald et al. 2010).

In order to get a fresh perspective on what happened to the dinosaurs, my colleagues Steve Brusatte and Richard Butler assembled a team of palaeontologists to re-examine the final stages of dinosaur evolution. Together, we took a cold, hard look at the diversity of Late Cretaceous dinosaurs during the last 20 million years of their reign, reanalysing the most recent data on dinosaur distributions and combining this with the latest information on climatic conditions and the timing of Deccan volcanism and the Chixuclub impact. There was much discussion and debate within the team until a consensus emerged: the end result of these deliberations was published online today in the journal Biological Reviews (Brusatte et al. 2014). 

Our new analyses find little support for long-term declines in dinosaur diversity and abundance in the lead-up to the extinction. Some herbivore groups in North America were suffering slightly, but this pattern was not repeated in other areas of the world. Instead, the extinction appears to have been geologically abrupt: an observation that rules out the more gradual declines predicted by extinction models reliant on slower mechanisms of global change. Moreover, the major pulses of Deccan volcanism do not correlate well with this sudden extinction. Consequently, we concluded that the abrupt global extinction of so many dinosaur lineages coincided best, and was most consistent with, the Chicxulub impact. Nevertheless, the stressed herbivore populations in North America may be hinting at the fact that at least some dinosaurs were under environmental strain prior to the impact. It seems reasonable to propose that longer-term mechanisms might have adversely affected at least some dinosaurs, making them more vulnerable to the cataclysmic effects of the impact.

It’s interesting to speculate what might have happened if the asteroid had not hit at this precise moment in time. Dinosaurs were successful and diverse, exploiting a wide range of niches from pole-to-pole. Although re-imagining historical events is always risky, it seems reasonable to speculate that had the asteroid arrived at a time when dinosaurs weren’t already stressed then some of them might have survived and even prospered until the present. Of course, it can be argued that this particular experiment has already been run – birds are nothing more than feathered, flying dwarf dinosaurs and are far more diverse than their extinct relatives. It could even be argued that the asteroid was just a blip in dinosaur evolution rather than a catastrophe: after all, the explosive radiation of avian dinosaurs was largely a post-impact event, even if the extinction took out all of their larger (and I’d suggest more exciting) non-avian relatives.

Archibald, J. D. et al. 2010. Cretaceous extinctions: multiple causes. Science 328, 973.

Brusatte, S. L. et al. 2014. The extinction of the dinosaurs. Biological Reviews. doi:10.1111/brv.12128

Schulte, P. et al. 2010. The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary. Science 327, 1214­–1218. doi; 10.1126/science.1177265

1 comment:

  1. When you say "even if the extinction event took out all of their larger (and I'd suggest more exciting) non-avian relatives", aren't you being sizeist and ornithopod-centric? (-;

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