Echinodon is a genus of heterodontosaurid dinosaur that lived during the earliest Cretaceous of southern England and possibly western France in the Berriasian epoch.

The first specimens were jaw bones named Echinodon becklesii by Sir Richard Owen in 1861, and since their original description only additional teeth have been discovered.

While the family was originally considered to be closest to more derived ornithopods, it was eventually reidentified as the most basal group of ornithischians, making Echinodon a taxon descended from many genera from the Early Jurassic, with a ghost lineage of 50 million years of unpreserved evolution.

An abundance of small mammals also lived alongside Echinodon, and the sediments show that the Purbeck Group was a variably lagoonal environment initially similar to the modern Mediterranean but became wetter over time.

Multiple specimens of jaw bones were discovered by Samuel Beckles high on a cliff in Durdlestone Bay on the Isle of Purbeck in southern England.

[3] In 1888 British palaeontologist Richard Lydekker followed previous classification of Echinodon as a dinosaur based on the anatomy of its teeth, describing them as similar to Scelidosaurus although not referring them to a more specific clade than Dinosauria indeterminate.

[5] British palaeontologist Peter Galton narrowed down the depositional locality of Echinodon further in 1978 to the freshwater "Dirt Bed",[6] also known as the "Mammal Pit" that was excavated by Beckles in 1857.

[8] In the late 1970s and early 1980s, excavations of the Natural History Museum of Los Angeles County uncovered many small ornithischian fossils in the Fruita Paleontological Area of Colorado.

[10] The known material of Echinodon is limited to bones of the skull, but the multiple specimens include at least some of the premaxilla, maxilla, lacrimal, jugal, palatine, ectopterygoid and dentary along with most teeth of both the upper and lower jaws.

A premaxillary foramen is present near the anterior margin of the individual bone, and the fossa it is nested within is more similar in shape to Heterodontosaurus than more derived Hypsilophodon.

[13] Isolated dermal armour found in the Purbeck Beds was referred to Echinodon on the basis of a stegosaurian classification by Justin Delair in 1959,[14] although these have since been reassigned to solemydid turtles.

[17] Galton followed up on this classification with the naming of the genus Lesothosaurus in 1978, and along with its description reviewed the anatomy of multiple basal ornithischian genera, including Echinodon, Nanosaurus and Fabrosaurus.

As these were what was suggested to unite the genus with "fabrosaurs", Sereno considered Echinodon to be a heterodontosaurid based on the presence of canines and arched gaps in the front of the tooth row.

[11] Norman and Barrett redescribed Echinodon in 2002 and supported the heterodontosaurid classification but instead referred it to the clade based on the lacking foramina on tooth-bearing bones and possessing denticles restricted to the top third of the crown.

[19] The family, either excluding Echinodon as in Thulborn's 1971 study, or including the genus as in Galton's and Sereno's work, was originally considered to be a group of basal ornithopods more derived than ankylosaurs and stegosaurs.

[16][17][6][18][11] Further research, including the extensive phylogenetic analysis of British palaeontologist Richard J. Butler and colleagues in 2008 supported heterodontosaurids as the most basal ornithischians instead.

[20] Following additional analysis by South American palaeontologist Diego Pol and colleagues in 2011, Echinodon was resolved as a basal ornithischian, yet not within the family Heterodontosauridae.

The clade including the Late Jurassic and Early Cretaceous Echinodon, Fruitadens and Tianyulong was poorly supported based on few dental features.

Echinodon, Fruitadens, Tianyulong and the undescribed Kayenta heterodontosaurid all bear low-crowned teeth unlike the Gondwanan forms, as well as a lobular cingulum and a prominent anterior groove leading to a foramen on the lateral surface of the dentary.

Their results placed the Echinodon as the sister taxon to the taxa typically classified within Pachycephalosauria, followed by Tianyulong, while all other heterodontosaurs studied formed a group at the origins of the clade.

[19] The hypothesis of omnivory was also supported by Norman and colleagues in 2011 under the interpretation that the canines and premaxillary teeth lacked wear from cropping vegetation.

[5] The Cinder Beds have, at times, been considered the Jurassic-Cretaceous Boundary, which would result in the entire Lulworth Formation being latest Jurassic, Tithonian, in age.

[13] Beyond ornithischians, the Lulworth Formation also contains the theropod Nuthetes, amphibians, turtles, lizards, snakes, mammals and crocodilians, and varieties of invertebrates.

[30] The genera Becklesius, Dorsetisaurus, Durotrigia, Paramacellodus, Pseudosaurillus, Parasaurillus, Purbicella, Saurillus, Parviraptor and three unnamed tooth morphologies represent the known squamates,[31] and fossils referred to the rhynchocephalians Homoeosaurus and Opisthias have also been found.