Tuesday, April 19, 2016

Decline In Dinosaur Diversity 40 Million Years Before Asteroid Impact

This is based on modeling of speciation and extinction rates.

Manabu Sakamotoa, Michael J. Benton,and Chris Vendittia

Whether dinosaurs were in a long-term decline or whether they were reigning strong right up to their final disappearance at the Cretaceous–Paleogene (K-Pg) mass extinction event 66 Mya has been debated for decades with no clear resolution. The dispute has continued unresolved because of a lack of statistical rigor and appropriate evolutionary framework. Here, for the first time to our knowledge, we apply a Bayesian phylogenetic approach to model the evolutionary dynamics of speciation and extinction through time in Mesozoic dinosaurs, properly taking account of previously ignored statistical violations. We find overwhelming support for a long-term decline across all dinosaurs and within all three dinosaurian subclades (Ornithischia, Sauropodomorpha, and Theropoda), where speciation rate slowed down through time and was ultimately exceeded by extinction rate tens of millions of years before the K-Pg boundary. The only exceptions to this general pattern are the morphologically specialized herbivores, the Hadrosauriformes and Ceratopsidae, which show rapid species proliferations throughout the Late Cretaceous instead. Our results highlight that, despite some heterogeneity in speciation dynamics, dinosaurs showed a marked reduction in their ability to replace extinct species with new ones, making them vulnerable to extinction and unable to respond quickly to and recover from the final catastrophic event.

The paper is under a paywall so I won't comment on the details of the analysis. Ed Yong has written a good summary of the paper in The Atlantic.

I heard this research being covered on BBC News and out came the inevitable statement.. " dinosaurs may have been on their way out"... meaning even if there had been no asteroid impact the dinosaurs would have gone extinct soon. I am not  sure the researchers actually think this way. The dinosaurs were a very long lived group, originating in the Triassic about 230 million  years ago, with all the non-avian groups going  extinct 66 million years ago. In that time frame there were boom and  bust  phases. There were earlier episodes of rapid diversification , then decline and then recovery. The overall pattern seems to be a late Triassic to mid Jurassic phase of diversification, followed by a slowing down or small declines and small expansions of different groups thereafter.

Who knows, maybe the late Cretaceous decline would have given way to another period of recovery and diversification. Or, the branches of the dinosaur tree may have gotten pruned with many groups going extinct, but with some groups continuing to prosper. Dinosaurs were not one entity. They were a varied group.  The extinction of all non-avian dinosaurs 66 million years ago may create an illusion that their evolutionary fates were always co-dependent, but that need not be so. This analysis shows that even when many dinosaurs groups were declining in the late Cretaceous, a few herbivorous groups were diversifying.  All this came to an abrupt end because of the asteroid impact. Only the avian lineage of dinosaurs survived.

The one conclusion that can be definitely drawn from the dinosaur story is that random events and chance have played a huge role in shaping the history of life.

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