could function as an opposable hallux, Abel stated that Hypsilophodon was a fully arboreal animal and even that an arboreal lifestyle was primitive for the dinosaurs as a whole. Though this hypothesis was doubted by Nopcsa, it was adopted by the Danish researcher Gerhard Heilmann who in 1916 proposed that a quadrupedal Hypsilophodon lived like the modern tree-kangaroo Dendrolagus. In 1926 Heilmann had again changed his mind, denying that the first toe was opposable because the first metatarsal was firmly connected to the second, but in 1927 Abel refused to accept this. In this he was in 1936 supported by Swinton who claimed that even a forward pointing first metatarsal might carry a movable toe. As Swinton was a very influential populariser of dinosaurs, this remained the accepted view for over three decades, most books typically illustrating Hypsilophodon sitting on a tree branch. However, Peter M. Galton in 1969 performed a more accurate analysis of the musculo-skeletal structure, showing that the body posture was horizontal. In 1971 Galton in detail refuted Abel's arguments, showing that the first toe had been incorrectly reconstructed and that neither the curvature of the claws, nor the level of mobility of the shoulder girdle or the tail could be seen as adaptations for climbing, concluding that Hypsilophodon was a bipedal running form. This convinced the paleontological community that Hypsilophodon remained firmly on the ground. The level of parental care in this dinosaur has not been defined, nests not having been found, although neatly arranged nests are known from related species, suggesting that some care was taken before hatching. The Hypsilophodon fossils were probably accumulated in a single mass mortality event, so it has been considered likely that the animals moved in large groups. For these reasons, the hypsilophodonts, particularly Hypsilophodon, have often been referred to as the "deer of the Mesozoic". Some indications about the reproductive habits are provided by the possibility of sexual dimorphism: Galton considered it likely that exemplars with five instead of six sacral vertebrae — with some specimens the vertebra that should normally count as the first of the sacrum has a rib not touching the pelvis — represented female individuals. References Bibliography Ornithischian genera Barremian life Early Cretaceous dinosaurs of Europe Cretaceous England Fossils of England Fossil taxa described in 1869 Taxa named by Thomas Henry Huxley