Dinosaurs' dominion ended more than 65 million years ago when a massive asteroid, roughly the size of Mount Everest, collided with Earth. It also started forest fires and sent vast quantities of particles into the atmosphere, leading to global cooling and wiping out much of the life on the planet.

Paleontologists have now found that about 24 million years before their extinction event, dinosaurs reached a point where more species were disappearing than were evolving. This resulted in a significant loss of biodiversity among the animals. Researchers now speculate that this loss of genetic diversity may have made the creatures more susceptible than normal to drastic environmental change.

This is not the first study to show that dinosaurs were facing significant challenges to their survival well before the final impact. As part of this new investigation, researchers carefully studied multiple branches of the dinosaur family tree, in an effort to record when individual species evolved and went extinct.

"Our study is the first to incorporate such phylogenetic [family tree] information when studying speciation and extinction in dinosaurs. This is what has allowed us to build a more nuanced and certain picture of dinosaur speciation than has ever before been possible," Manabu Sakamoto of the University of Reading in the UK, said.

As dinosaur species started to decline, early mammals began to thrive. Many biologists believe the rise of the warm-blooded animals have led to increased competition for resources, including food.

This period was also a time that experienced drastic effects of continental drift. With fewer small bodies of water to serve as barriers, dinosaurs may have roamed further than before, reducing diversity. Increased volcanic activity may have also played a role, as ash and dust in the air blocked sunlight, reducing vegetation and food producers. This could have impacted the entire food chain from algae to the fiercest carnivore.

When a mountain of rock smashed into Earth, ending the Cretaceous period, the impact wiped out dinosaurs, along with three-quarters of all other species on the planet. The blast and the fire-and-ice environmental effects that followed left behind birds as the only descendants of the once-prolific dinosaurs.

Nowadays, animal species around the world are going extinct at an incredible rate. Researchers believe that by studying how dinosaurs declined prior to their final extinction, they may be able to learn more about losses occurring in the modern age.

The decline of species was found over most varieties of dinosaurs, but to various degrees. Sauropods, known for their long necks, saw the greatest losses at the end of the age of dinosaurs. Next were theropods, such as the fearsome Tyrannosaurus rex.

The only two exceptions to the losses were Hadrosaurs, marked by sturdy, stout bodies and Ceratopsidae, a group which included Triceratops. Each of these groups witnessed a rise in the number of species, while other groups faltered.

Future research will continue to examine how details of the conditions which led to the decline in the variety of dinosaur species at the end of their age.

Investigation of the loss of dinosaur species prior to the Cretaceous-Tertiary impact was published in the journal Proceedings of the National Academy of Sciences.

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