How One Scientist Explains the Success of so Many Plant-eating Dinosaurs

For years, scientists have wondered how so many large, plant-eating species of dinosaurs could co-exist successfully over such long period of time - a mystery one researchers says he has solved.

Jordan Mallon, a post-doctoral fellow at the Canadian Museum of Nature, decided to tackle the question by measuring and analyzing characteristics of nearly 100 dinosaur skulls recovered from the Dinosaur Park Formation in Alberta, Canada.

The results of the research, published in the journal PLOS ONE, indicate that these megaherbivores, all weighing greater than 2,200 pounds (1,000 kilograms), had differing skull characteristics that would have allowed them to specialize in eating different types of vegetation.

Mallon's findings support a concept known as niche partitioning, which dates to the 19th-century studies of Charles Darwin, though it didn't fully come into its own until the 1950s with the development of the science of ecology.

The Dinosaur Park Formation is between 76.5 and 75 million years old and is known for its rich concentration of dinosaur remains, having yielded nearly 20 species of megaherbivores from the Late Cretaceous period. Of these, six species would have coexisted at any one time, including two types of ankylosaurs (tank-like armoured dinosaurs), two types of hadrosaurs (duck-billed dinosaurs), and two types of ceratopsids (horn-faced dinosaurs).

Although Earth is still home to megaherbivores, the selection is much smaller and includes elephants, giraffes, hippos and rhinos.

"Today's megaherbivore communities are not nearly as diverse as those from the Late Cretaceous of Alberta, and most other fossil communities also pale by comparison. So the question is: how does an environment support so many of these large herbivores at once?" Mallon asked.

In coming to his conclusion, Mallon tested two competing hypotheses: availability of food was not a limiting factor in species survival either because plants were superabundant or the dinosaurs' metabolisms were relatively low, so the environment could support more species relative to a fauna comprised entirely of high-metabolic animals.

Only in the second hypothesis would niche partitioning really become relevant since, even with slow metabolisms, there weren't that many plants to go around, which would have forced the species to share available food sources by specializing on different types of vegetation.

"If niche partitioning was in effect, then you would expect to see various dietary adaptations among the coexisting dinosaur species, " Mallon explained. "So you would look for differences in the shapes of the skull, in the teeth and in the beaks that might reflect adaptations for feeding on diverse plants or plant parts."

For each of the nearly 100 dinosaur skulls he studied, Mallon measured 12 characteristics that are known to relate to diet in modern animals, including depth of the jaw, angle of the beak, size of muscle insertions and length of the tooth row. "We can apply those same functional and mechanical principles to dinosaurs to see what they might tell us about niche partitioning," he explains.

As was expected, differences were found between the three major groups (ankylosaurs, hadrosaurs and ceratopsids). More striking, however, were the subtle yet significant differences within each of the three groups that were probably related to feeding.

For example, the palaeontologist suggests that ankylosaurs probably specialized in eating ferns, because they stood low to the ground and their wide beaks would have allowed them to feed efficiently on abundant, relatively low-nutrient plants. However, within this group, the family known as nodosaurids (clubless ankylosaurs) had more efficient jaw mechanics that might have enabled them to consume tougher plants.

In contrast, ceratopsids had skulls that suggest they were adapted to feeding on mid-sized shrubs, while the taller hadrosaurs would have fed on anything within reach.

Although different species came and went, the same ecological roles were filled over the 1.5 million year span of the Dinosaur Park Formation.

"This tells us that niche partitioning was a viable strategy for the coexistence of these animals," Mallon said. "The study provides further evidence to explain why dinosaurs were one of the most successful groups of animals to live on this planet."