Dinosaur digestion

We can learn about what dinosaurs ate and how they digested their meals by looking at their fossil skeletons, examining stomach contents preserved in some specimens and by peering into their fossilised dung.

Dinosaurs evolved from small carnivorous reptiles during the Triassic. Some retained this ancestral preference for meat; some evolved adaptations to eat plants, and others were omnivores, eating a combination of plant and animal foods.

Plant eaters

Herbivorous dinosaurs include the long-necked sauropods, the beaked ornithischians and some theropods. Leaves aren’t easy to digest - their cells are surrounded by a tough material called cellulose. Herbivorous dinosaurs probably relied on bacteria in their gut to break down leaves sufficiently to release nutrients.

Sauropod dinosaurs were gigantic; they had relatively tiny heads, long necks and tails and massive, barrel-shaped torsos supported by four legs. Their teeth were simple, only good for stripping foliage which they swallowed without chewing. The mouthful of food travelled down their long oesophagus (the muscular tube connecting the mouth to the stomach) where it entered a chamber called the gizzard. Modern dinosaur relatives (crocodiles and birds) have gizzards so we can be confident that dinosaurs had them as well.
 
Masses of pebbles have been found within the bodies of some herbivore dinosaurs, suggesting they were used in digestion as ‘gizzard stones’, or gastroliths. This suggests some dinosaurs used a ‘gastric mill’, where muscular contractions of the gut mashed the stones against the plant material, forming an easily digestible pulp. Birds, the descendants of dinosaurs, still use this approach to digestion as an alternative to chewing.

After the plant material had been pulverised to a pulp, it then passed further along into a long gut which incorporated one or more fermentation chambers called caecum. These contained helpful bacteria which fed on the cellulose of the plant matter and allowed nutrients to be extracted. The food was further broken down by acids and enzymes, travelling through the intestines where nutrients were absorbed into the blood stream, the waste product of this process eventually exiting the body as dung.

Herbivore droppings were irregularly shaped and rich in partially digested plant matter. The droppings often fell apart due to decomposition or scavenging but in some rare conditions they were preserved as fossils, called coprolites.

Unlike sauropods, another group of dinosaur herbivores called the ornithischians chewed their food. No reptile or bird alive today chews their food this way, but many mammals do. The ornithischians nipped at vegetation with a horny beak, behind which were batteries of teeth that ground against each other, while muscular cheeks prevented food from dropping out of the mouth. By chewing their food, the process of digestion became much more efficient.

Different ornithischians had different feeding and digestion strategies. Armoured stegosaurs and ankylosaurs had inefficient chewing mechanisms and wide bodies to allow for fermentation chambers in their belly. Smaller ornithischians supplemented their chewing with gastric mills, with masses of gastroliths found within their fossil ribcages. Hadrosaurs had cropping bills coupled with highly efficient grinding teeth and a more streamlined body shape, suggesting they probably had reduced need for fermentation as part of digestion.

Artist’s reconstruction of the Hadrosaur Edmontosaurus, an efficient herbivore with a duck bill, chewing teeth and muscular cheeks.
Source: Museum Victoria

Meat eaters

Digesting meat is much simpler than digesting plants - meat lacks tough fibres and is highly nutritious. Carnivorous theropod dinosaurs had much shorter digestive tracts than herbivores, relying on powerful stomach acids to break down their meals. A relatively short intestine was enough to absorb the nutrients. Without the need of huge gut but with the need to catch their food, meat-eating dinosaurs did not have large bulky bodies like herbivores.

The fearsome jaws of carnivorous dinosaurs were not for chewing. They were filled with blade-like teeth that acted like shears, slicing out chunks of flesh that were swallowed whole. Some predatory theropods had specialised teeth that gave them a powerful, bone-crushing bite, such as those of Tyrannosaurus and Tarbosaurus. Like carnivores today, meat-eating dinosaurs probably ate less frequently than herbivores, but had huge helpings when they did.

The carnivorous theropod dinosaur Tarbosaurus, a close relative of the famous Tyrannosaurus. The teeth of these dinosaurs helped tear chunks of flesh out of their prey.
Source: Museum Victoria

The dung of carnivores was more compressed and dense. Coprolites from meat-eaters are rich in calcium phosphate from the bones of their victims. The largest known dinosaur dropping is 44cm long and is thought to have come from Tyrannosaurus rex. The dropping was 50% bone fragments, a testament to the powerful bite of this famous dinosaur.