The term "dinosaur" is sometimes used disparagingly in reference to something that is unsuccessful. That annoys me because dinosaurs were one of the most-successful groups of animals to ever appear on our planet.
The term dinosaur is sometimes used disparagingly in reference to something that is unsuccessful. That annoys me because dinosaurs were one of the most-successful groups of animals to ever appear on our planet.
Paleontologists have discovered approximately 1,000 species that dominated terrestrial ecosystems for almost 150 million years. Hardly a failure.
They also were widely diverse. Sure, they came in many sizes — from creatures no larger than pigeons to the huge multi-ton brontosaurs. But they also included two-legged types, four-legged types, herbivores, carnivores, omnivores, forest-dwellers, desert-dwellers, night-hunters and others whose lifestyle might not resemble anything living today.
That diversity, however, creates a paleontological mystery. A major tenet of ecological relationships is that, in general, smaller areas of land can support fewer species than larger areas. Dinosaurs were land-dwellers, so the amount of land they had to live on was constrained by sea level.
During the Late Cretaceous, about 73 million years ago, the sea level was so high that ocean water covered what are now the Great Plains, connecting the Arctic Ocean to the Gulf of Mexico and splitting North America into two island continents.
The western island, where so many dinosaur fossils are found, is called Laramidia. The most-diverse dinosaur fauna of Laramidia come from a rock layer called the Dinosaur Park Formation of Alberta, Canada, which has produced 33 species of herbivorous dinosaurs.
That is a huge number. The most-diverse large-animal fauna today occur in sub-Saharan Africa, which has an area of 9 million square miles. Laramidia was 1.5 million to 3?million square miles.
How did such a small land area support so many species? There are two main hypotheses.
The first is that plants were not the limiting factor, either because dinosaurs were cold-blooded (unlikely), higher levels of carbon dioxide increased plant productivity, or predation pressure from carnivorous tyrannosaurids and dromaeosaurids (raptors) suppressed herbivore population densities below levels where plants were the limiting factor.
The second hypothesis suggests that so many species of herbivore were able to coexist because of what is called “niche partitioning,” in which different species ate different plants or different parts of the same plants.
Two recent journal articles tested that possibility by looking at the feeding heights and skull shapes of the herbivorous dinosaurs from the Dinosaur Park Formation.
For feeding heights, two groups were found. One group, consisting mainly of ceratopsians (horned dinosaurs) and ankylosaurs (the armored forms) fed within about 3 feet of the ground. The other group, composed mainly of hadrosaurs (so-called duck-billed dinosaurs), could feed as high as 16 feet above the ground.
However, two groups don’t explain the diversity.
Looking at skull shapes proved a little more promising. Ankylosaurs and hadrosaurs had wide muzzles compared with the narrow-nosed ceratopsians, implying different diets. And within the hadrosaurs, different species had different size skulls, again implying possibly different diets.
However, within those large groups there were no obvious differences to suggest that they ate different plants. So the mystery remains of how such a small area could have supported so many species.
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.