| Hypsilophodontidae | |
|---|---|
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| Hypsilophodon skeleton | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Clypeodonta |
| Family: | †Hypsilophodontidae Dollo, 1882[1] |
| Subgroups | |
| Synonyms | |
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Hypsilophodontidae (orHypsilophodontia) is a traditionally usedfamily ofornithopoddinosaurs, generally considered invalid today. It historically included many small bodied bipedalneornithischian taxa from around the world, and spanning from theMiddle Jurassic until theLate Cretaceous. This inclusive status was supported by somephylogenetic analyses from the 1990s and mid 2000s,[2][3] 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 likeThescelosauridae andElasmaria.[4][5][6][7][8] A 2014 analysis by Norman recovered a grouping ofHypsilophodon,Rhabdodontidae andTenontosaurus, which he referred to asHypsilophodontia.[9] That clade is formally defined in thePhyloCode as "the smallest clade withinOrnithopoda containingHypsilophodon foxii andTenontosaurus tilletti provided it does not includeIguanodon bernissartensis".[10] All other analyses from around the same time have instead found these latter taxa to be withinIguanodontia.[7][11] The familyHypsilophodontidae is formally defined in thePhyloCode by Daniel Madzia and colleagues in 2021 as "the largest clade containingHypsilophodon foxii, but notIguanodon bernissartensis andRhabdodon priscus".[10]
Hypsilophodontidae was named originally in1882 byLouis Dollo, as a family to includeHypsilophodon and other small ornithopods with a single row ofteeth, fourpedal digits, and arhomboidsternum. For several decades after its naming the family only includedHypsilophodon.[3] In1911Karl von Zittel published a textbook on vertebrate classifications, in which he included multiple genera in "Hypsilophodontidae" (sic for Hypsilophodontidae[12]), includingHypsilophodon,Nanosaurus,Laosaurus andDryosaurus. Zittel considered the family to unite all taxa that lackedpremaxilla teeth, had a single row ofmaxilla teeth, neckvertebrae which have flat articulations or a flat front and round back, fusedsacral vertebrae, afemur shorter than thetibia, 5 fingeredmanus' and 4 toed peds.[13]Thescelosaurus was named in 1913 byCharles Gilmore, and its skeleton was described in detail by the same author in 1915. Gilmore had originally classifiedThescelosaurus withinCamptosauridae, but in the 1915 description he determined that it shared far more features with Hypsilophodontidae. He reclassifiedLaosaurus,Nanosaurus andDryosaurus in the family Laosauridae, leaving onlyThescelosaurus andHypsilophodon in Hypsilophodontidae. The characteristics of the family were also re-analysed, and Gilmore showed that thepremaxilla actually had teeth, a characteristic of the family; the 3rd manus digit had 4phalanges; the femur was either shorter or longer than the tibia; and dorsalribs had only a single articulation point.[12]
The first expansive analysis on the relationships of Hypsilophodontidae was that of Swinton in1936, during a redescription ofHypsilophodon from new specimens. The possible hypsilophodontsGeranosaurus andStenopelix were removed from the clade (then the subfamily Hypsilophodontinae), and considered to be intermediate basal ornithopods, as there were no features linking them toHypsilophodon.Thescelosaurus was considered within the family, because of the large number of shared features, as well asDysalotosaurus, from theKimmeridgian ofTanzania.Laosaurus andDryosaurus 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 andIguanodontidae, and were probably synonyms of each other as well.[14]Charles M. Sternberg (1940) considered there to be multiple genera within the family, all sharing fullyenamelled teeth, divided into twosubfamilies, Hypsilophodontinae andThescelosaurinae. Within Hypsilophodontinae–grouped by a longerscapula, thinnerforelimb and femora shorter than tibiae–Sternberg includedHypsilophodon,Dysalotosaurus, andParksosaurus (renaming ofThescelosaurus warreni). OnlyThescelosaurus was included in Thescelosaurinae, as it had a tibia shorter than the femur.[15]
Peter M. Galton in1972 re-studied the relationships of taxa withinOrnithischia.Thescelosaurus was removed from Hypsilophodontidae because of its short limbs, meaning it was probably notcursorial, unlike all other hypsilophodonts. The presence of premaxilla teeth, once used to diagnose the group, was found to be present in unrelated taxa likeHeterodontosaurus,Protoceratops andSilvisaurus. Galton made Hypsilophodontidae paraphyletic, as he consideredThescelosaurus 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.[16]
In 1992David Weishampel and Ronald Heinrich reviewed the systematics and phylogenetics of Hypsilophodontidae. Hypsilophodontidae was supported as a monophyletic clade that encompassed "thescelosaurids",Hypsilophodon andYandusaurus. The family was diagnosed by the absence of ridges that end asdenticles in teeth (reversed inHypsilophodon); presence of a single central ridge on dentary teeth; ossified sternal plates on torso ribs; and a straight and unexpanded shape of theprepubis. Their resulting cladogram is reproduced below:[3]
The followingcladogram of hypsilophodont relationships depicts theparaphyletic 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.[11] Ornithischia, Ornithopoda, and Iguanodontia were not designated in their result, and so are left out here. Additional ornithopods beyondTenontosaurus are omitted. Dinosaurs traditionally described as hypsilophodonts are found fromAgilisaurus orHexinlusaurus toHypsilophodon orGasparinisaura.
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An alternate phylogeny, by Norman in 2014, resolved a monophyletic Hypsilophodontia (the family Hypsilophodontidae was not used because of its history), composed ofHypsilophodon grouped withRhabdodontidae andTenontosaurus.[9] In one analysis in her 2022 review ofiguanodontian phylogenetic relationships, Karen E. Poole recovered a large Hypsilophodontidae as thesister taxon of Iguanodontia, which consisted of several "traditional" hypsilophodontids, as well asThescelosauridae. The Bayesian topology of her phylogenetic analyses is shown in thecladogram below:[17]
In 2023, Longrich et al. describedVectidromeus as a new genus of hypsilophodontid. Although they did not perform a phylogenetic analysis, they suggested that, since other taxa previously assigned to Hypsilophodontidae had subsequently been moved to other groups,Vectidromeus andHypsilophodon remained as the only members of the clade.[18] A comprehensive phylogenetic framework focused on the evolution of early ornithischians published by Fonseca and colleagues in 2025 failed to recognizeVectidromeus as a close relative ofHypsilophodon, instead notingdryosaurid affinities for the taxon. As in previous proposals, the researchers' analyses recovered Hypsilophodontidae as the earliest-diverging clade within the Ornithopoda, as the sister taxon to Iguanodontia. It comprises twoBarremian-aged European genera,Gideonmantellia and the eponymousHypsilophodon.[19]
