Dinosaurs | Egypt | Mesopotamia |Human Ancestors

Hypsilophodon foxii

Hypsilophodon foxii ('Fox's high ridged tooth') was named after both Rev. William Fox, an amateur collector who found many of the early fossils, and the shape of the cheek teeth. Initially, Hypsilophodon was thought to be an Iguanodon and was described as such by Sir Richard Owen. But shortly thereafter, Sir Thomas Huxley studied the remains and realized that there were too many differences for it to be an Iguanodon. The shape of the teeth was one element used by Huxley to reach his conclusions. The animal was reclassified and renamed.

Discovery
Hypsilophodon fossils were discovered in England in the mid 1800s and first formally described in 1882 by James Hulke. And what a description it was! According to Hulke, Hypsilophodon was arboreal (lived in trees) and was ill suited to life on the ground. He felt that the creature's long fingers and toes were better for climbing among tree branches than scrambling about in the dirt. In fact, many early pictures show Hypsilophodon perched on tree limbs like a bird. Only this "bird" was 2-4 meters (6-13 ft.) long, had a tail, and no feathers!

Recent re-evaluations of the fossil material have changed our view on the "tree-dwelling" Hypsilophodon. It is now thought that Hypsilophodon was a terrestrial creature and moved about on its two hind legs. Its long tail stretched out stiffly behind as a balance to the forward leaning front of its body. More than likely it was a fast runner. It would need to be. The Lower Cretaceous world of Hypsilophodon was a dangerous place and the hunting ground of Megalosaurus, a large Allosaurus-like predator. Excavations at a single site on the Isle of Wight have yielded over 20, nearly complete and beautifully preserved skeletons. This site supports the notion that Hypsilophodon moved about in herds.

Taxonomy
Hypsilophodon skulls are very similar to Heterodontosaurus skulls, but without the large canines. Both are ornithopods. The Ornithopoda are one of the three main branches on the ornithischian side of the dinosaur family tree : the Thyreophora and Marginocephalia are the other two. Ornithopods are best known for the larger 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). Heterodontosaurs and dryosaurs are two less well known groups. Hypsilophodonts are the last of the five and are a likely representation of the transition between the small, early heterodontosaurs and the later, larger hadrosaurs.

Anatomy
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. Hypsilophodon continued the evolution of grinding by the addition of joints in the skull that allowed the upper battery of teeth to rotate sideways (outwards) during chewing. As the upper teeth moved across the lower teeth, food was ground between the two surfaces. This must have provided a clear advantage that allowed ornithopods to thrive and diversify right up to the end of the Cretaceous.

[ Dinosauria -> Ornithischia -> Ornithopoda -> Hypsilophodontidae ]

[ Dinosauria Main Page | InHandMuseum ]