Hypsilophodonts

Hypsilophodontidae (or Hypsilophodontia) is a traditionally used family of ornithopod dinosaurs, generally considered invalid today. It historically included many small bodied bipedal neornithischian taxa from around the world, and spanning from the Middle Jurassic until the Late Cretaceous. This inclusive status was supported by some phylogenetic analyses from the 1990s and mid 2000s, although there have also been many finding that the family is an unnatural grouping which should only include the type genus, '' Hypsilophodon'', with the other genera being within clades like Thescelosauridae and Elasmaria. A 2014 analysis by Norman recovered a grouping of ''Hypsilophodon'', Rhabdodontidae and '' Tenontosaurus'', which he referred to as Hypsilophodontia. All other analyses from around the same time have instead found these latter taxa to be within Iguanodontia.

Linnaean usage

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.

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

Question of monophyly

In 1992 David Weishampel and Ronald Heinrich reviewed the systematics and phylogenetics of Hypsilophodontidae. Hypsilophodontidae was supported as a monophyletic clade that encompassed "thescelosaurids", ''Hypsilophodon'' and ''Yandusaurus''. The family was diagnosed by the absence of ridges that end as denticles in teeth (reversed in ''Hypsilophodon''); presence of a single central ridge on dentary teeth; ossified sternal plates on torso ribs; and a straight and unexpanded shape of the prepubis. Their resulting cladogram is reproduced below:

The following cladogram of hypsilophodont relationships depicts the paraphyletic hypotheses; the "natural Hypsilophodontidae" hypothesis has been falling out of favor since the mid-late 1990s. It is after Brown ''et al.'' (2013), the most recent analysis of hypsilophodonts. Ornithischia, Ornithopoda, and Iguanodontia were not designated in their result, and so are left out here. Additional ornithopods beyond ''Tenontosaurus'' are omitted. Dinosaurs traditionally described as hypsilophodonts are found from ''Agilisaurus'' or ''Hexinlusaurus'' to ''Hypsilophodon'' or ''Gasparinisaura''.

Norman's Hypsilophodontia

A more recent alternate phylogeny, by Norman in 2014, resolved a monophyletic Hypsilophodontia (the family Hypsilophodontidae was not used because of its history). ''Hypsilophodon'' grouped with Rhabdodontidae and '' Tenontosaurus''.

References

{{Taxonbar, from=Q131437
Paraphyletic groups

Early Cretaceous first appearances
Late Cretaceous extinctions