'Vegetarian Meat Eaters'

From left to right: Therizinosaurus cheloniformis, Limusaurus inextricabilis, Incisivosaurus gauthieri, and Ornithomimus velox; the one leaping into the air is Troodon formosus; the one perched on Therizinosaurus cheloniformis' head is Ophisthocomus hoatzin

Although commonly known as "meat-eating dinosaurs", theropods actually comprise a number of dinosaurs that eat plants, making these dinosaurs "vegetarian meat eaters".

The most cited of these "vegetarian meat eaters" are the therizinosaurs. Most therizinosaurs were almost doubtlessly plant eaters, even though they had giant claws, which were probably for warding off predators and pulling branches towards their mouths. This has led some to nickname them "sloth dinosaurs", as the recently extinct ground sloths seem to have had a similar lifestyle. Their legs were short and not good for running, their teeth were small and blunt, and they seem to have had long intestines to house micro-organisms and digest plant material.

Also well known for (possibly) eating plants are the ornithomimosaurs, or ostrich dinosaurs. It isn't easy to tell what these enigmatic theropods ate because they either have very small teeth or none. However, they are often compared to ostriches and may well have eaten similar foods: plants, and occasionally small animals. Some ornithomimosaurs have been found with stones in their gut, which indicate that they probably did eat at least some plants, because many plant-eating dinosaurs are known to have eaten stones to help grind vegetation.

Another group of theropods which may have been plant eaters are the oviraptorosaurs. The diet of the oviraptorosaurs are something of a mystery, because, like the ornithomimosaurs, many of them don't have teeth, and these show various adaptations for a predatory lifestyle. In fact, a lizard was found fossilized in the stomach of Oviraptor philoceratops. However, those oviraptorosaurs that did have teeth seem to have eaten mostly - or perhaps even only - plants. Incisivosaurus gauthieri for example had rodent-like incisor teeth that show wear patterns similar to those of plant-eating dinosaurs.

Troodonts were a group of small dinosaurs closely related to birds and dromaeosaurids (and, together with the latter, are popularly dubbed "raptor dinosaurs). They had features that hint at both plant- and meat-eating. They had long legs and excellent senses of sight and hearing for tracking down small prey, but they also had short wings that were little good for hunting and their teeth were generally not blade-like and serrated but leaf-shaped and bumpy.

Needless to say, many modern birds eat plants, though few exclusively so. One exclusively plant-eating bird is Ophisthocomus hoatzin, a medium-sized bird found in the swampy forests of South America. It eats mainly fruit, flowers, and leaves, and is unique in having a crop that serves as a rumen, a cavity that is filled with plant-digesting bacteria.

Among the strangest of the plant-eating theropods was Limusaurus inextricabilis. All the other theropods discussed above belong to the coelurosaurs, or fuzzy theropods. Limusaurus inextricabilis, however, was a ceratosaur, and not particularly closely related to the coelurosaurs. All other known ceratosaurs seem to have been meat eaters, but Limusaurus inextricabilis had a beak that was short, blunt, and weak - in other words, little use for eating meat.