According to a new research published online in the Proceedings of the National Academy of Sciences and which has been led by University of Reading scientists Dr. Manabu Sakamoto and Dr. Chris Venditti, dinosaurs were already in an evolutionary decline tens of millions of years before the Chicxulub impact that finally finished them off.
Torvosaurus gurneyi. Image credit: © Sergey Krasovskiy.
Whether dinosaurs were in decline before their final disappearance at the Cretaceous-Paleogene (K-Pg) mass extinction event 66 million years ago has been debated for decades with no clear resolution.
By using a statistical analysis in conjunction with information from the fossil record, Dr. Sakamoto, Dr. Venditti, and their colleague, Prof. Mike Benton of the University of Bristol, showed that dinosaur species were going extinct at a faster pace than new ones were emerging from 50 million years before the Chicxulub comet/asteroid hit.
The analyses demonstrate that while the decline in species numbers over time was effectively ubiquitous among all three dinosaur groups (Ornithischia, Sauropodomorpha, and Theropoda), their patterns of species loss were different. For instance, giant sauropod dinosaurs were in the fastest decline, whereas theropods, the group of dinosaurs that include T. rex, were in a more gradual decline.
“We find overwhelming support for a long-term decline across all dinosaurs and within all three dinosaurian subclades, 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 scientists said.
“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.”
“We were not expecting this result,” Dr. Sakamoto said. “While the comet/asteroid impact is still the prime candidate for the dinosaurs’ final disappearance, it is clear that they were already past their prime in an evolutionary sense.”
“All the evidence shows that the dinosaurs, which had already been around, dominating terrestrial ecosystems for 150 million years, somehow lost the ability to speciate fast enough,” Prof. Benton said.
“This was likely to have contributed to their inability to recover from the environmental crisis caused by the impact.”
According to the team, the observed decline in dinosaurs would have had implications for other groups of species.
“The decline of the dinosaurs would have left plenty of room for mammals, the group of species which humans are a member of, to flourish before the impact, priming them to replace dinosaurs as the dominant animals on Earth,” Dr. Venditti said.
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Manabu Sakamoto et al. Dinosaurs in decline tens of millions of years before their final extinction. PNAS, published online April 18, 2016; doi: 10.1073/pnas.1521478113
