The first remains of Hysilophodon ("high-ridged tooth") were recovered in the early days of paleontology, in 1849. However, at the time the bones were thought to belong to a young Iguanadon. It wasn't until 1870 that paleontologist T. H. Huxley was able to publish a full description of Hysilophodon as we know it today. He had been provided with a number of skeletons by the Rev. William Fox, after whom the first species of Hysilophodon was named.
Since then, three near-complete and over twenty minor finds have been made, especially on the Isle of Wight, south of England. Other finds have been made in southern England, Portugal and south Dakota.
Hysilophodon was among the smallest of the dinosaurs. While not quite so small as for example Compsognathus, Hysilophodon was only around 2.3 metres in length, and would have reached approximately waist height on a modern man. It weighed around 70kg. It ran on two legs and was herbivorous. It appears that it was able to hunt for itself on hatching - there was no parental care after the eggs had hatched, although a neatly-arranged nest has been found, suggesting that some care was taken before hatching.
Despite existing in the last of the periods in which dinosaurs ruled the earth, the Cretaceous, Hysilophodon had a number of remarkably primitive features. For example, although it had a beak like most ornithischians, it still had pointed triangular teeth in the front of the jaw. Most herbivorous dinosaurs had, by this stage, become sufficiently specialised that the teeth had been altogether lost. Also, there were five digits on each hand, and four on each foot - most dinosaurs had lost these redundant features by the Cretaceous period.
In fact, the group Hypsilophodontia remained remarkably static from the late Jurassic to the end of the Cretaceous. This was because their basic structure was perfectly adapted to their life style and selective pressure, we assume, was low. The entire body was built for speed while running: the light-weight, minimised skeleton, low, aerodynamic posture, long legs and stiff tail for balance all allowed it to travel remarkably fast for its size. Most Hypsilophodontids, like Valdosaurus remained small, although one (Tenontosaurus) grew to 6.5 metres long! Due to its small size, Hysilophodon would have eaten low-growing vegetation and evaded likely predators of the time - such as Baryonyx and Megalosaurus by sprinting away in the thick forests that covered much of the landscape at the time.
Hysilophodon had twenty-eight to thirty teeth in the front of its jaw, and it seems these were self-sharpening. It could also store food in its cheek pouches and consume it later, like a modern hamster.
In 1882, some paleontologists suggested that, like a modern tree kangaroo, Hysilophodon was able to climb trees in order to seek shelter. This was the accepted view for almost a century. However, Peter M. Galton finally performed more accurate analysis of the skeleto-muscular structure in 1974 and convinced most paleontologists that Hysilophodon remained firmly on the ground.
There is only one known species of Hysilophodon, Huxley's original H. foxii
. Galton and Jensen named another species, H. wielandi
in 1979, but it now seems likely this was just an variant individual within H. foxii