Dinosaurs were in decline long before the Chicxulub asteroid finished them off

Sixty-five million years ago, a massive asteroid slammed into Earth, causing tsunamis, earthquakes, fires, a global winter, and the end of the age of the dinosaurs.

But what if the asteroid had glided safely past our planet? Would dinosaurs still be here today?

New research suggests the answer is probably not. Instead, scientists have found evidence that dinosaurs were in the midst of a long, slow decline that began millions of years before the asteroid struck.

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In a study published Monday in the Proceedings of the National Academy of Sciences, researchers report that at least 40 million years before the Chicxulub asteroid landed in Mexico, dinosaur species were going extinct at a higher rate than new ones were coming into existence. With fewer species -- and less variation in habitat requirements and ecological niches -- dinosaurs would be more susceptible to environmental changes, the authors write.

"There is no doubt that the Chicxulub impact was the final nail in the dinosaurs' coffin -- with the exception of birds," authors Manabu Sakamoto, a post-doctoral researcher at the University of Reading in England, and Chris Venditti, professor of evolutionary biology at the university, said in an email. "However, we can speculate that if the trajectory of dinosaurs continued as it was at that time, dinosaurs would eventually have become impoverished in terms of species numbers -- and may have gone extinct all together."

The question of whether dinosaurs were already in decline before the asteroid put an abrupt end to their reign has been debated among paleontologists for decades. Recent attempts to address this question relied mostly on counting the number of fossils in the different geological time bins, Sakamoto said.

"The dispute has continued unresolved because of a lack of statistical rigor, and appropriate evolutionary framework," the authors write.

In this paper, the authors turned to phylogenic trees, which show how species are related to each other, as the basis of their statistical analysis. This allowed them to study extinction and speciation rates in the three clades of dinosaurs through time. 

The researchers found that speciation was in a long-term decline across all dinosaurs, and was exceeded by extinction rates between 48 million and 53 million years before the Chicxulub event.

The authors are not sure what caused the speciation rate to slow down, but they have a few ideas. They explain that the Cretaceous period (145.5 million to 65.5 million years ago) was a time of drastic geological changes. The global climate was cooling down, there was prolonged volcanic activity and the continents were breaking apart. 

"Any combination of these processes could have affected dinosaur speciation," Sakamoto and Venditti said.

Alternatively, it is possible that competition from mammals -- which were just small, rodent-like creatures at the time -- had something to do with it.

"Recent studies show evidence that mammals were on the rise prior to the [asteroid event], so this scenario would be consistent with our findings," they said. 

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