Discussions by Yann Oliver
Up: Dinosaurs lifestyles
The plant-eating dinosaurs show various feeding strategies, and sometimes very well-developed anatomical features helping them chewing or digesting, not found in any other animal.
Sauropods were tall enough to browse on high trees. Some of them could even rear to catch the highest vegetation. It is possible that some sauropods had had a trunk, which would even more increase their food-collecting capabilities.
Stegosaurs could rear as well to catch high plants.
Ceratopians, ornithopods and ankylosaurs were low-feeders. They were not in direct competition with each other: ornithopods had a wide snout and efficient jaws, and could ate every plant, whereas ceratopians had very narrow, strong beaks and probably ate more choosily among tough plants.
At the end of the Jurassic, the high-feeding dinosaurs (sauropods, stegosaurs) were replaced as dominant herbivore by the low-feeding forms. It has been suggested that this change may have helped the newly appeared, fast growing, flower plants spread out.
Most ornithischian dinosaurs show both fleshy cheeks to retain the food while chewing, and a precise occlusion pattern between the upper and lower jaw, like mammals. On the other hand, theropods and sauropods lacked cheeks.
Hadrosaurs probably had the most efficient jaw of any plant-eating animal, with up to 2,000 teeth. Worn-out teeth were quickly replaced by new ones, as in all dinosaurs.
Ceratopians developed a very powerful beak. To provide strong anchor points for the jaw muscles is probably the main function of the big frill they had on the back of their heads. They could have bitten in and chewed almost every plant.
On the other hand, ankylosaurs had very weak teeth.
Some theropods like ornithomimosaurs, oviraptors, segnosaurs completely lacked teeth. Their diet is quite mysterious: molluscs, eggs, fruit, insects, small reptiles?
All other theropods had very sharp teeth. Some (allosaurs for instance) show very flexible skull articulations, probably to swallow big pieces of meat.
Gizzard stones are unambiguously known from sauropods, psittacosaurs, ornithomimosaurs and Caudipteryx: polished stones inside or near the belly, of a type sometimes not found kilometres around. They were never found among ceratopians or hadrosaurs, which had no need for them due to their chewing facilities.
The backward-oriented pubis of ornithischians may have helped them increase the length of their intestine, which is needed in every plant-eating species.
Dinosaurs with very deep (sauropods) or very wide (ankylosaurs) bellies probably had several stomachal sacs with numerous bacteria, like today\\\'s ruminants. This is the more probable that these dinosaurs were precisely those who lacked evolved jaws and had the weakest teeth, but apparently did survive.