How do paleontologists know what a particular species of dinosaur ate during the Mesozoic era?

We grow up absorbing explicit illustrations from picture books of sauropods nibbling leaves and tyrannosaurs ripping flesh, and yet some paleontologists maintain that these images are largely conjecture.

We can speculate about what the dinosaurs ate, but in fact no one knows for sure, just as we can only imagine what color the dinosaurs were and what sounds they made. Our guesswork about what dinosaurs consumed is based on parallels with mammals we know today, yet dinosaurs were of course extremely different from any creature we have ever known.

Bearing this in mind, we can look at some different species and simply theorize about their diet, largely by peering into their mouths at the size and shape of their teeth.

Take Diplodocus, for instance, which had stubby, pencil-like teeth jutting slightly forward from the jaw and located mostly at the front of the mouth. It would seem impossible for this slim-limbed creature to have chomped down on an armored dinosaur or even gnawed down a tree. Instead, it probably used its teeth like a sieve to obtain tiny organisms from fresh water and, since excavated teeth of this species show signs of wear, it may also have combed or cropped twigs and leaves.

Tracks show that the huge, peaceful, herbivorous sauropods (“lizard feet”) like Diplodocus, Apatosaurus, and Brachiosaurus may have traveled in herds. With their long necks they could have grazed easily among the treetops, some forty feet high. They would have spent most of their waking hours munching, because an eighty-ton brachiosaur, if cold-blooded, would have required some three hundred pounds of vegetation a day to sustain itself, if warm-blooded it would have had to consume a ton of greens each day.

Since its teeth were little help in breaking down foodstuff, one theory goes that the sauropods swallowed rocks, which ground food to a pulp inside a gizzard. Thousands of such stones, known as gastroliths, have been found along with the bones of these reptiles.

While the sauropods may have held sway over the forest canopy, other dinosaurs were equipped to feed on the lower tiers of woods and in swamps.

Iguanodon had numerous sharp cheek teeth for consuming twigs and leaves. It roamed warm swampy areas and probably ate tough marsh plants such as horsetails. Horned dinosaurs such as Triceratops had a strong beak for tearing down and stripping tough, woody vegetation. Its many sharp teeth meshed like scissors to masticate spiky palm fronds, magnolias, and fibrous plants. A long, thick muscle that stretched from the lower jaw to the frill at the back of the neck added strength to its jaws.

A very different creature, whose teeth are a notable indicator of life-style, is the well-known Tyrannosaurus rex. This animal’s stupendous teeth measured seven and a quarter inches in length and, like steak knives, were perfectly designed to slice flesh. They were narrow and serrated, curving slightly inward to clench hold of prey.

Some scientists believe that this enormous dinosaur, which weighed about seven tons, was one of the most ferocious hunters that ever lived, bringing down even tough horned and armored dinosaurs. Others maintain that Tyrannosaurus was too large and clumsy to tackle live prey and that it merely lumbered about in search of carrion.

Other carnosaurs, or “flesh lizards,” such as the allosaurids and megalosaurids, also had saw-edged fangs, powerful jaws, and two- or three-fingered hands which could have been used for grasping prey. Probably the earlier species, which were smaller and more active than the later ones, were nimble enough to run down a browsing plant-eater.

There has been just one instance in which paleontologists uncovered the actual stomach contents of a dinosaur. In 1922 an Anatosaurus was found with a belly full of twigs and pine needles.

At the time the finding was ignored because the scientific community believed that anatosaurs and other duck-billed dinosaurs ate like ducks, that is, aquatic plants. Over four decades later John H. Ostrom of Yale University reintroduced the find and demonstrated that the anatosaur, and other hadrosaurs, were not aquatic. It so happens that the teeth of the anatosaur would have been well suited for chewing its dinner.

Behind the bill of a hadrosaur lay rows of packed teeth, and beneath them, new teeth growing up to replace those that wore out. Hadrosaurs munched on hard dry-land fare, such as branches and seeds, and over the course of a lifetime would have grown and put to work some two thousand teeth.

One hopes that paleontologists will be fortunate enough to find other species of dinosaurs mummified with a full stomach, but until they do, much of what we suppose about these unique creatures must hark from the land of make-believe.