An ancient mystery solved

By Sean Willett

A pair of University of Calgary paleontologists may have solved one of the ancient world’s biggest mysteries. With research published earlier this month in the science journal PLOS ONE, Jordan Mallon and Jason Anderson have given a new answer to a difficult question: how were numerous species of large herbivorous dinosaurs able to co-exist in western North America 75 million years ago?

During the late Cretaceous period, a large part of North America was a long, thin island known as Laramidia. Fossil records from Alberta, Montana and Utah show that Laramidia boasted an impressively high number of massive plant-eating dinosaurs, many of which lived together in the same time and place. While some research has been done to explain how large carnivorous dinosaurs shared their habitats, the reason as to why these “megaherbivores” were able to live together remained unclear.

“This question has never been explored in detail in respect to herbivores, especially with the big herbivores,” Mallon said. “It’s an important question to ask because you get upwards of six, seven or eight megaherbivore species in the same area and that sort of diversity is virtually unrivalled in the fossil record.”

Several theories explaining this diversity have already been put forward. Some paleontologists have suggested that resources during the Cretaceous period were essentially unlimited, meaning that there was more than enough plant material for the megaherbivores to freely share. Another theory suggests that resources were limited, but dinosaurs had too slow of a metabolism for competition to matter. Neither of these theories have had much evidence supporting them, so Mallon and Anderson put forward one of their own. 

“No one had really looked in depth at niche partitioning, which is like the other side of the fence,” Mallon said. “It’s the idea that resources were limiting and that dinosaurs had to find some way of sharing those resources by specializing on different plants and diets. If that’s the case then you would expect to see all of the different dinosaurs that lived at the same time to exhibit features that would allow them to specialize on different plants.”

To test their hypothesis, Mallon and Anderson looked at the skulls of 82 different individual dinosaurs from 17 different species and six subfamilies, all of which were found in the same Albertan fossil formation. Traits that would indicate specialized diets — features such as jaw shape, skull size and tooth structure — were analyzed and recorded, allowing the paleontologists to deduce and compare the diets of the different dinosaur groups. 

The pair found that their hypothesis was well supported, and that the different megaherbivore subfamilies had a range of different adaptations that allowed each group to specialize on vegetation of a certain toughness or height. 

Mallon and Anderson’s research also showed that these differences weren’t present between similar species, which helps to explain why certain dinosaur species only appear briefly in the fossil record, or were very rare.

“For example, usually you will only get one lambeosaurine living alongside one hadrosaurine, rarely do you get two lambeosaurines living alongside one another,” said Mallon. “Given our results you might explain that to be the case because there are no dietary differences within lambeosaurines. If you did have a second lambeosaurine species show up on the scene, they might not be able to hack out a niche. They would be in competition with the already established lambeosaurines and wouldn’t be able to cut the mustard there.”

Mallon hopes that his research will be used as a model for other paleontologists studying the dinosaurs of Laramidia, in order to better understand how these enormous animals co-existed in such a limited space.

“Hopefully this will encourage people to test my ideas outside of Alberta, using other fossil ecosystems in places like Utah and Montana,” said Mallon. “I would love it if their findings supported mine, but hopefully they can at least test my findings to see how well the Alberta dinosaur model reflects other ecosystems.”

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