Hypsilophodontidae

 Hypsilophodontidae (or Hypsilophodontia) is a traditionally used family of ornithopoddinosaurs, generally considered invalid today. It historically included many small bodied bipedal neornithischian taxa from across the world, and spanning from the Middle Jurassicuntil the Late Cretaceous. This inclusive status was supported by some phylogeneticanalyses 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 like Thescelosauridae and Elasmaria.^{[4][5][6][7][8][9][10][11][12][13][14]} A 2014 analysis by Norman recovered a grouping of Hypsilophodon, Rhabdodontidae and Tenontosaurus, which he referred to as Hypsilophodontia.^[15] All other analyses from around the same time have instead found these latter taxa to be within Iguanodontia.^[11][16]

Hypsilophodonts Temporal range: Early Cretaceous, 130–125 Ma  PreꞒ Ꞓ O S D C P T J K Pg N ↓
Hypsilophodon skeleton
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Order:	†Ornithischia
Clade:	†Clypeodonta
Family:	†Hypsilophodontidae Dollo, 1882[1]
Subgroups
†Hypsilophodon †Tenontosaurus? †Rhabdodontidae?
Synonyms
Hypsilophodontinae Nopsca, 1928 Hypsilophodontia Cooper, 1985

Linnaean usageEdit

Skeleton of Laosaurus consors as drawn by Othniel Marsh (since named Othnielosaurus)

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.^[3] In 1911 Karl von Zittelpublished a textbook on vertebrate classifications, in which he included multiple genera in "Hypsilophontidae" (sic for Hypsilophodontidae^[17]), 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 sacralvertebrae, a femur shorter than the tibia, 5 fingered manus' and 4 toed peds.^[18]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.^[17]

Skeleton of Thescelosaurus edmontonensis 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.^[19] 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.^[20]

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.^[21]

Ornithischia

Fabrosaurus -> Echinodon       Heterodontosaurus     Ankylosauria     Stegosauria   Pisanosaurus     Hadrosauridae     Iguanodon -> Tenontosaurus           Ceratopsia     Stenopelix -> Psittacosaurus         Camptosaurus leedsi -> Dysalotosaurus -> Dryosaurus -> Wealden ornithopod       Camptosaurus       Yaverlandia -> Stegoceras -> Pachycephalosaurus     Laosaurus   Parksosaurus   Hypsilophodon   Laosaurus minimus     Thescelosaurus

This article uses material from the Wikipedia article  Metasyntactic variable, which is released under the  Creative Commons Attribution-ShareAlike 3.0 Unported License.