Geography helps solve dinosaur evolution puzzle

The birth of mountain ranges, like the Rockies, substantially changed the rate of dinosaur speciation

The number of dinosaur species in the Americas increased following tectonic activity that led to the creation of a mountain range, a study has suggested.

Fifteen million years before the mass extinction that wiped out the giant reptiles, the number of species increased in what is now North America.

Researchers said that the birth of the mountains probably sped up diversification.

The findings have been published in the journal PLoS One.

In their research, Terry Gates from Ohio University and colleagues showed that as the "Sevier" mountain range grew on the west of what is now America, which caused the east of the continent to "flex" downwards creating a shallow sea as water from the ocean flooded the land.

This new ocean resulted in America being split into three islands, of which the western one, known as Laramidia, was densely populated by ornithischian dinosaurs (bird-hipped dinosaurs).

Many of these species were adorned with ornaments for sexual displays, such as horns and crests.

"We know from lots of evidence that animals that have these types of features speciate at a faster rate than other animals," explained Dr Gates.

"If you combine that with the fact that they are isolated within their ecosystems, what you have is a great recipe for lots of new dinosaur species."

The isolation of the dinosaurs and their subsequent diversification makes sense in light of knowledge about how island isolation in modern animals leads to high levels of speciation.

Birth of the Rockies

This period of rapid diversification ended as the Rocky Mountains began to form towards the end of the Cretaceous Period (145.5-65.5 million years ago).

"When these started to form, you no longer had this beautiful paradise... Instead what happened was that the slow rising hills altered the weather patterns," Dr Gates observed.

This ecological change that accompanied the geological upheaval, and the increase in land area due to the vast seaway receding, led to a slowdown in the number of species evolving.

The reasons why dinosaur species had been so numerous, and then became less so before the Cretaceous mass extinction, had previously remained elusive.

It was only through talking to structural geologists rather than palaeontologists that Dr Gates began to develop ideas.

"I was working on a different project and I realised 'there's a lot of useful information over here' and the pieces started falling together," he told BBC News.

"The people that studied the structural geology side of the story never talked to the palaeontologists and the palaeontologists never thought to talk to them. I see this paper as an excellent example of multidisciplinary research," he said.

This research has implications for understanding Earth's history in addition to explaining the evolution of dinosaurs.

"The radiation of these dinosaurs begins slightly before the geologic record indicates the beginning of mountain building in the area," Dr Laura Porro of the University of Chicago, who was not involved in the study, told BBC News.

"We expect animals to respond to environmental changes before these events leave a geological trace," she said.

"So not only are these long ago mountain-building events impacting the animals, but changes in the animals can also help us pinpoint the timing of the tectonic events."