(since named
Othnielosaurus and later
Nanosaurus) Hypsilophodontidae was named originally in
1882 by
Louis Dollo, as a family to include
Hypsilophodon and other small ornithopods with a single row of
teeth, four
pedal digits, and a
rhomboid sternum. For several decades after its naming the family only included
Hypsilophodon. In
1911 Karl von Zittel published a textbook on vertebrate classifications, in which he included multiple genera in "Hypsilophodontidae" (
sic for Hypsilophodontidae), including
Hypsilophodon,
Nanosaurus,
Laosaurus and
Dryosaurus. Zittel considered the family to unite all taxa that lacked
premaxilla teeth, had a single row of
maxilla teeth, neck
vertebrae which have flat articulations or a flat front and round back, fused
sacral vertebrae, a
femur shorter than the
tibia, 5 fingered
manus' and 4 toed peds.
Thescelosaurus was named in 1913 by
Charles Gilmore, and its skeleton was described in detail by the same author in 1915. Gilmore had originally classified
Thescelosaurus within
Camptosauridae, but in the 1915 description he determined that it shared far more features with Hypsilophodontidae. He reclassified
Laosaurus,
Nanosaurus and
Dryosaurus in the family Laosauridae, leaving only
Thescelosaurus and
Hypsilophodon in Hypsilophodontidae. The characteristics of the family were also re-analysed, and Gilmore showed that the
premaxilla actually had teeth, a characteristic of the family; the 3rd manus digit had 4
phalanges; the femur was either shorter or longer than the tibia; and dorsal
ribs had only a single articulation point. '' in display as preserved 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.
Peter M. Galton in
1972 re-studied the relationships of taxa within
Ornithischia.
Thescelosaurus was removed from Hypsilophodontidae because of its short limbs, meaning it was probably not
cursorial, unlike all other hypsilophodonts. The presence of premaxilla teeth, once used to diagnose the group, was found to be present in unrelated taxa like
Heterodontosaurus,
Protoceratops and
Silvisaurus. Galton made Hypsilophodontidae paraphyletic, as he considered
Thescelosaurus to be a hypsilophodont, but excluded it from the family Hypsilophodontidae. The phylogenetic hypothesis of Galton is shown below. Taxa considered hypsilophodontids are enclosed by green. }} == Cladistic usage ==