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Heterdonsaurus tucki

Say the word dinosaur and people think "huge," fierce" and even "terrifying." But not all dinosaurs were "huge," "fierce" and "terrifying." Elephants, lions and rhinos aren't the only animals found on the plains of East Africa, many smaller, less well known creatures live in the shadow of those that dominate our perception of African wildlife. The same is true with dinosaurs. Heterodontosaurus was an important small dinosaur from the Early Jurassic. This fleet-footed herbivore was just over a meter long, about the size of a golden retriever. Scientists think that they were the gazelles and antelopes of their day, with little protection from predators other than their speed and watchfulness. They were bipedal (walked and ran on their two hind legs) and counted on their ability to outrun predators for survival. Whether or not they congregated in herds is not known. Their five-fingered hands seem to be adapted for manipulating vegetation during foraging.

Taxonomy

Ornithopods are one of the three main branches on the ornithischian side of the dinosaur family tree : Thyreophora and Marginocephalia are the other two. Ornithopods are a very diverse group that is best know for its larger members: iguanodontids and hadrosaurs. All ornithopod dinosaurs are distinguished from the other ornithischians by several evolutionary novelties: the teeth at the front of the mouth (premaxillary teeth) are set lower than the teeth at the back of the mouth (maxillary teeth); the hinge point for the lower jaw is set well below the level of the lower tooth rows; and they have a distinctive arrangement in the way the bones of the face (premaxillary, maxillary and nasals) are connected. All the ornithopods are further divided into five families. As we said, the two best know are the Iguanodontidae (the name for all the types of Iguanodon when taken together) and Hadrosauridae (all hadrosaurs). Hypsilophodon and dryosaurs are two less well known groups. Heterodontosaurs are the last of the five and are most likely ancestral to all the others.

Anatomy
Heterodontosaurids ('mixed tooth reptiles') are an unusual group of ornithopod dinosaurs. Their teeth set them apart from others. Look at the skull and you'll see three distinct types of teeth. The chisel-shaped cheek teeth run along the side of the mouth and are similar to those found in other ornithopod dinosaurs of the period. Just in front of the cheek teeth look for the upper and lower canines. Only some of the excavated fossils have these teeth, which leads scientists to think that they are a sex related trait. Most likely, males used these dangerous-looking teeth for display or defense. The small pointed teeth at the very front of the mouth are incisors that would have nipped vegetation as the animal browsed. Most dinosaurs have only one type of tooth; sometimes in combination with a beak-like tip at the end of the jaws. Later, some bird-like coelurosaurs lost their teeth altogether and relied solely on beaks. The three distinct types found in Heterodontosaurus are unique among dinosaurs.

The development of cheeks and a grinding movement with the teeth may have provided all Ornithopods with an evolutionary advantage over other plant eaters (like the giant sauropods) that swallowed vegetation without chewing and used a bird-like gizzard to grind their food. Mammals move their lower jaws from side to side and up and down to grind foods between top an bottom teeth. Reptiles, including dinosaurs, cannot move their jaws that way. Heterodontosaurs show the earliest signs of this grinding ability in dinosaurs. The cheeks keep food in the mouth until swallowed. There is still disagreement among scientists on exactly how Heterodontosaurs moved their jaws to achieve this grinding action. However they did it, it seems to have provided a clear advantage that allowed Ornithopods to thrive and diversify right up to the end of the Cretaceous.

[ Dinosauria -> Ornithischia -> Ornithopoda -> Heterodontosauridae ]

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