the middle rib shaft and overlapping the front edge of the subsequent rib. Such plates are better known from Talenkauen and Thescelosaurus, and were probably cartilaginous in origin. Phylogeny Huxley originally assigned Hypsilophodon to the Iguanodontidae. In 1882 Louis Dollo named a separate Hypsilophodontidae. By the middle of the twentieth century that had become the accepted classification but in the early twenty-first century it became clear through cladistic analysis that hypsilophodontids formed an unnatural, paraphyletic group of successive off-shoots from throughout Neornithischia. Hypsilophodon in the modern view thus simply is a basal ornithopod. In 2017, Daniel Madzia, Clint Boyd, and Martin Mazuch removed Hypsilophodon itself from Ornithopoda altogether, placing it as the sister group to the Cerapoda, in a more basal position; several other "hypsilophodontids" have undergone similar reclassifications. The following cladogram is reproduced from this study: Paleobiology Due to its small size, Hypsilophodon fed on low-growing vegetation, in view of the pointed snout most likely preferring high quality plant material, such as young shoots and roots, in the manner of modern deer. The structure of its skull, with the teeth set far back into the jaw, strongly suggests that it had cheeks, an advanced feature that would have facilitated the chewing of food. There were twenty-three to twenty-seven maxillary and dentary teeth with vertical ridges in the animal's upper and lower jaws which, due to the fact that the tooth row of the lower jaw, its teeth curving outwards, fitted within that of the upper jaw, with its teeth curving inwards, appear to have been self-sharpening, the occlusion wearing down the teeth and providing for a simple chewing mechanism. As in almost all dinosaurs and certainly all the ornithischians, the teeth were continuously replaced in an alternate arrangement, with the two replacement waves moving from the back to the front of the jaw. The Zahnreihen-spacing, the average distance in tooth position between teeth of the same eruption stage, was rather low with Hypsilophodon, about 2,3. Such a dentition would have allowed to process relatively tough plants. Early paleontologists modelled the body of this small, bipedal, herbivorous dinosaur in various ways. In 1882 Hulke suggested that Hypsilophodon was quadrupedal but also, in view of its grasping hand, able to climb rocks and trees in order to seek shelter. In 1912 this line of thought was further pursued by Austrian paleontologist Othenio Abel. Concluding that the first toe of the foot