Rise of mountains and seaway triggered North American dinosaur surge

Washington -  Scientists believe that the rise of the Rocky Mountains and the appearance of a major seaway that divided North America may have boosted the evolution of new dinosaur species.

The finding may explain patterns of evolution and migration of North American duck-billed and horned dinosaurs in the years leading up to their extinction 65 million years ago, said Terry Gates, a postdoctoral researcher with Ohio University’s Heritage College of Osteopathic Medicine who is lead author on the study.

“Over the past century, paleontologists have found a wide variety of dinosaurs in rocks dating to around 75 million years ago, but right before the asteroid hit at the end of the Cretaceous, there appeared to be fewer species in North America. The reason for this discrepancy in dinosaur diversity has never been adequately explained,” he said.

Gates and collaborators Albert Prieto-Marquez and Lindsay Zanno turned to the geologic record of western North America for possible answers. They examined trends in mountain and ocean formation during a period 80-70 million years ago, when there was an apparent explosion of dinosaur species, and later, when species became less diverse.

The record painted a picture of pronounced geological change.

During the early to middle Cretaceous, geological forces lifted the western United States, creating a huge mountain range (the Sevier Mountains) that extended in a line from the American southwest through Alberta, Canada. The area just to the east of the new mountain range flexed downward, creating a shallow North American seaway (known as the Western Interior Seaway) that flooded the continent from the Canadian arctic to the Gulf of Mexico.

This seaway cut the continent into three large islands to the north, east and west that were densely populated with dinosaurs. The dinosaurs of the west lived on an island known as Laramidia. Most fossil discoveries have been made in the area of the northern part of the island, in places such as Alberta, South Dakota and Montana, while dinosaurs have been found only recently in the former areas of southern Laramidia.

The new discoveries have helped illustrate how dinosaurs evolved on an island with changing geography. The rise of the Sevier Mountains and the growing seaway caused dinosaur habitat to shrink on Laramidia. Later, one of the tectonic plates under North America’s crust shifted position, building another mountain range—the Laramide Orogeny, or the infant stage of the modern-day Rocky Mountains—further east.

“At that time, it appears that geographic, as well as probably also ecological, barriers created by the rise of mountain ranges and the seaway caused isolation of the northern and southern populations of the crested duck-billed and horned plant-eating dinosaurs. We hypothesize that such isolation facilitated rapid speciation and increased diversity in these animals,” explained Prieto-Marquez.

The new species of duck-billed and horned dinosaurs were being born at an astounding rate of every few hundred thousand years during the brief time when the two mountain ranges and the seaway coexisted, Gates said.

Eventually, however, the continued rise of the Rocky Mountains would evict the seaway from the continent’s interior. Gates and his colleagues argue that this second geological change opened up a wide territory for duck-billed and horned dinosaurs to roam, that, in turn, reduced how fast new species evolved in the region to every few million years.

“Our data suggests that changing geography contributed to the pattern we see in western North America, but also that this pattern is unique to this region and should not be blindly extrapolated to infer global diversity leading up to the K-T extinction event,” Zanno noted.

The findings have been published in the journal PLOS ONE.