The first expansive analysis on the relationships of Hypsilophodontidae was that of Swinton in 1936, during a redescription of Hypsilophodon from new specimens. The possible hypsilophodonts Geranosaurus and Stenopelix were removed from the clade (then the subfamily Hypsilophodontinae), and considered to be intermediate basal ornithopods, as there were no features linking them to Hypsilophodon. Thescelosaurus was considered within the family, because of the large number of shared features, as well as Dysalotosaurus, from the Kimmeridgian of Tanzania. Laosaurus and Dryosaurus were not considered hypsilophodonts because of their lack of distinguishable features, as Swinton concluded that they were probably in the family Laosauridae, intermediate between Hypsilophodontidae and Iguanodontidae, and were probably synonyms of each other as well. Charles M. Sternberg (1940) considered there to be multiple genera within the family, all sharing fully enamelled teeth, divided into two subfamilies, Hypsilophodontinae and Thescelosaurinae. Within Hypsilophodontinae–grouped by a longer scapula, thinner forelimb and femora shorter than tibiae–Sternberg included Hypsilophodon, Dysalotosaurus, and Parksosaurus (renaming of Thescelosaurus warreni). Only Thescelosaurus was included in Thescelosaurinae, as it had a tibia shorter than the femur.
