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:
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:
Hypsilophodon