Tremendous herds of dinosaurs hundreds of thousands strong once ruled the entire western interior of North America, according to a new statistical analysis of the fossil record.
71 to 65 million years ago, a sea cut into a large portion of the western United States, covering half of Texas and stretching as far as North Dakota. But that didn’t prevent dinosaurs from roaming the entire continent tip to toe, from the arctic circles to the equator, say a pair of McGill researchers.
Earlier studies had suggested that these tremendous herds of creatures lived in smaller, regional groupings, rarely leaving their home areas. The thinking was that the types of dinosaurs you'd find in the north were very distinct from those in the south, with very little overlap between species.
"It didn't make sense that these two- or three-ton animals were restricted to a small portion of the landmass at the time," said Matthew Vavrek, a PhD student at McGill University and coauthor of an article on dino diversity that appears in the April 19 issue of the Proceedings of the National Academy of Sciences.
The confusion arose from the challenges of interpreting events tens of millions of years ago based solely on the handful of fossils that have been recovered.
"If you took a 500-page book and tore out two pages, and then tried to retell the story, that's what we're trying to do with the fossil record. So we asked, 'how can we look at the numbers differently, taking into account the fact that we don't find that many dinosaur skeletons?'"
Professor Hans Larsson, a Canada research chair in macroevolution, and Vavrek used statistics to examine the alpha diversity, the number of species in an immediate area, and compare it with the beta diversity, the differences in species across two different areas. Their research shows low beta biodiversity among dinosaurs of the time, indicating little variability.
The same dinos roamed the entire continent, in other words.
This makes sense given the climate at the time, which was far less variable than today's climate. There was no ice and hardly any snow at the poles during the Maastrichtian stage, a 5-million year swath of the late Cretaceous period when temperatures were 10 to 15 degrees warmer everywhere.
"Due to the even distribution of temperatures, dinosaurs could roam freely across the entire continent -- just as with today's species," explained Vavrek.
"Deer, moose, brown bears, even grizzly bears -- all these things at one point had a pretty wide range across North America. It would have been the same with the dinosaurs."
"The findings give us an insight into what kind of ranges these types of communities may have had," Larsson explained. "We also demonstrate that after more than a century of collecting dinosaurs in North America, we should expect to find about 16 types, on average, in any one region of western North America just before their mass extinction."
So were there other giant herds of dinosaurs roaming around? Did they mark their boundaries somehow, however tremendously large those borders were? And when they encountered each other, did they battle it out for domination?
"It's not an incorrect speculation," mused Vavrek.
"The nice thing about being in paleontology: It's really hard for anyone to tell you you're wrong."