What is Hypsilophodontidae?

The largest clade containing Hypsilophodon foxii (Huxley, 1869) but not Iguanodon bernissartensis Boulenger (Beneden, 1881) and Rhabdodon priscus (Matheron, 1869). At one point, palaeontologists looked at Hypsilophodontidae as the nexus from which many of the major ornithischian groups sprung, and almost all 2-legged "bird-hipped" dinosaurs that weren't iguanodonts — from the Middle Jurassic to Late Cretaceous of Asia, Australia, Europe, New Zealand, North America and South America — were assigned here.

But, over recent years, its members have been shipped off to various other spots on the ornithischian family tree. Now, the only "hypsilophodont" left that palaeontologists could agree on is the group anchor Hypsilophodon, which would render Hypsilophodontidae — a family group with no family members — moot.

Despite that, some palaeontologists would still casually mention a "hypsilophontont-grade" or "hypsil-grade" critter from time to time. But what they were referring to is a small (1-2 meter), bipedal, swift-running, herbivorous ornithischian that's hard to classify with certainty, which is pretty much what hypsilophodonts have always been.

However, Hypsilophontontidae came roaring back into fashion in 2021 with the release of the "Phylocode" — a formal set of rules governing phylogenetic nomenclature — and was given a polished definition. Whether palaeontologists use it or not remains to be seen. Click here to search Dinochecker for hypsilophodonts. Hypsilophodontidae is derived from "Hypsilophodon" (the group anchor) and the Greek "idae" (family). Hypsilophodon itself is derived from "Hypsilophus" — a name stemming from Greek and proposed by Fitzinger for a type of Iguana with a "hypsi" (high) "lophos" (crest) running down its back, plus "odon" (tooth).