It has long been believed that the dinosaurs went extinct after an asteroid hit the Earth.

Findings of a new study, however, challenge this view, as it suggests that the reign of these prehistoric giants may have already been shaky before the Chicxulub asteroid slammed into our planet about 66 million years ago.

The new research, which was published in the Proceedings of the National Academy of Sciences on Monday, claimed that species of the ancient reptiles were dying out faster than new ones emerge, and this pattern may have started at least 40 million years before an asteroid slammed into Mexico's Yucatan Peninsula, where it caused wildfires and smoke that blocked the sunlight and altered the climate.

With fewer species of dinosaurs and less variation in ecological niches and habitat requirements, the prehistoric animals would have been more susceptible to changes in the environment, which could have made survival more difficult, researchers of the study explained.

"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," study author Manabu Sakamoto, from the University of Reading in England, and colleagues wrote.

The researchers said that the dinosaurs may still be on their path to extinction whether or not the asteroid impact occurred.

"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," Sakamoto and study researcher Chris Venditti, also from the University of Reading in England, said.

For their research, the authors looked at three large dinosaur family trees to look for evidence when extinctions started to outpace the appearance of new species and found that for most of the dinosaur groups, extinction started to happen about 50 million years before the asteroid impact.

The researchers are not certain why the speciation rate of the dinosaurs slowed down. The drastic geological changes during the Cretaceous period could have been a factor, and prolonged volcanism may have pumped greenhouse gases, causing global climate change.

The supercontinent Pangea also started to break apart into separate continents, which could have led to dinosaurs eventually evolving into separate species.

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