7 ago 2012
"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," researcher Albert Prieto-Márquez at the Bavarian State Collection for Paleontology and Geology in Munich, Germany, said in a statement. "We hypothesize that such isolation facilitated rapid speciation and increased diversity in these animals."
New species of duck-billed and horned dinosaurs were being born at an explosive rate of every few hundred thousand years during the brief time when the two mountain ranges and the seaway coexisted. Isolated populations often evolve new features more rapidly, Gates said.
Eventually, the continued rise of the Rocky Mountains kept the sea away from the continent's interior. This change opened up a vast territory for these dinosaurs to roam. This, in turn, reduced how fast new species evolved in the region to every few million years, the researchers suggest.
"Our data suggests that changing geography contributed to the pattern we see in western North America," Gates said.
During the times of isolation, a number of species of giant duck-billed dinosaurs "roamed a much smaller area than you might think given that many were larger than elephants," Gates said. It may be possible these dinosaurs evolved to eat specialized plants found only in certain regions, explaining why they lived in relatively tight confines.
Dinosaur diversity dip
Researchers had suggested that dinosaurs were declining before their mass extinction, due to a dip in diversity in the years leading up to the calamity.
"The major question I've been thinking about for 10 years was, 'Were dinosaurs really declining before they went extinct?'" Gates told LiveScience. "It turns out the time period of dinosaur diversity we were looking at, the Campanian, was a bit of an anomaly. It saw three converging geologic structures all coming together to form perfect conditions for a dinosaur species boom. Everyone was using this time as a baseline for dinosaur diversity, when it should have been seen as an anomaly, and the decrease in diversity later on was really a return to the status quo."
The mountain and seaway changes not only influenced dinosaur diversity in North America, but they also may have had effects elsewhere in the world. For instance, the rise of the predecessor to the Rocky Mountains created a barrier, meaning that only species living in the southern part of Laramidia could get to South America, and only species living north of the mountains could reach Asia across modern-day Alaska.
"These giant herbivores were truly invasive species that seemingly came to dominate these other continents," Gates said.
Gates and his colleagues are now exploring the western United States to better understand patterns of dinosaur evolution and diversity there, as well as how other groups of animals, such as mammals and amphibians, might have been affected by these geological changes. They detailed their findings online yesterday in the journal PLoS ONE.
