Hypsilophodontidae
A historically used but now largely invalid dinosaur family.
Hypsilophodontidae is a family of ornithopod dinosaurs that was widely used in the past but is now generally considered invalid. It once grouped together many small, two-legged neornithischians from the Middle Jurassic to the Late Cretaceous, found across the globe. The family was first named in 1882 by Louis Dollo, at first containing only the genus *Hypsilophodon*. Over time, more genera were added. Its validity has been heavily debated: some phylogenetic studies from the 1990s and mid-2000s supported it as a natural group, while many others concluded it is an artificial assemblage. These latter studies argue that the family should include only the type genus, *Hypsilophodon*, with the other former members placed in groups like Thescelosauridae and Elasmaria.
In 2014, David Norman recovered a clade containing *Hypsilophodon*, Rhabdodontidae, and *Tenontosaurus*, which he called Hypsilophodontia. Under the PhyloCode, this clade is formally defined as the smallest ornithopod group that includes both *Hypsilophodon foxii* and *Tenontosaurus tilletti*, but not *Iguanodon bernissartensis*. However, nearly all other analyses from the same period place *Rhabdodontidae* and *Tenontosaurus* within Iguanodontia, while *Hypsilophodon* is typically recovered as a basal ornithopod outside Iguanodontia. The family Hypsilophodontidae itself was formally defined under the PhyloCode in 2021 by Daniel Madzia and colleagues as the largest clade containing *Hypsilophodon foxii* but excluding *Iguanodon bernissartensis* and *Rhabdodon priscus*.
In Linnaean usage, after Dollo’s initial naming, the family was expanded in 1911 by Karl von Zittel, who included *Hypsilophodon*, *Nanosaurus*, *Laosaurus*, and *Dryosaurus*. Zittel defined the group by features such as a lack of premaxillary teeth, a single row of maxillary teeth, neck vertebrae with flat or flat-fronted articulations, fused sacral vertebrae, a femur shorter than the tibia, five-fingered hands, and four-toed feet. In 1915, Charles Gilmore reclassified *Thescelosaurus*—originally placed in Camptosauridae—into Hypsilophodontidae, moving *Laosaurus*, *Nanosaurus*, and *Dryosaurus* into Laosauridae.
Quick Facts
- Taxon
- Hypsilophodontidae
Facts from the source article.
Lore & Background
Hypsilophodontidae was originally named by Louis Dollo in 1882 to include Hypsilophodon and other small ornithopods with a single row of teeth, four pedal digits, and a rhomboid sternum. For decades after its naming, the family only contained Hypsilophodon. In 1911, Karl von Zittel expanded it to include Nanosaurus, Laosaurus, and Dryosaurus, based on features such as lack of premaxilla teeth and a femur shorter than the tibia. Charles Gilmore in 1915 reclassified several genera into Laosauridae, leaving only Thescelosaurus and Hypsilophodon in Hypsilophodontidae, and revised the family's characteristics to include premaxilla teeth.
Reader's Guide
Hypsilophodontidae has been a central but controversial group in ornithopod systematics. Its traditional composition included many small, bipedal herbivores from the Jurassic and Cretaceous, but phylogenetic analyses since the 1990s have increasingly shown it to be paraphyletic or polyphyletic. The 2014 analysis by Norman recovered a clade called Hypsilophodontia containing Hypsilophodon, Rhabdodontidae, and Tenontosaurus, while other studies placed these taxa within Iguanodontia. The PhyloCode formally defines Hypsilophodontidae as the largest clade containing Hypsilophodon foxii but not Iguanodon bernissartensis or Rhabdodon priscus. Modern usage varies, with some researchers restricting the family to Hypsilophodon and Vectidromeus, while others recover a broader clade as sister to Iguanodontia. The group's legacy lies in its historical role as a catch-all for small ornithopods, and its ongoing redefinition reflects advances in dinosaur phylogenetics.
Did You Know?
- Hypsilophodontidae was named by Louis Dollo in 1882, originally including only Hypsilophodon.
- The family was historically diagnosed by features such as a single row of teeth, four pedal digits, and a rhomboid sternum.
More in Dinosaur families 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced