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. The specific name honours collector Samuel Beckles who discovered the material of Echinodon and many other taxa from across England, while the genus name translates as "prickly tooth" in reference to the dental anatomy of the taxon.
Originally, Echinodon was considered to be a type of herbivorous lizard, though this was quickly revised to an intermediate ornithischian. It was referred to the clade Stegosauria based on general dental anatomy and incorrectly referred armour that was later identified as a turtle's. Echinodon was then referred to the early ornithischian family Fabrosauridae, which was later identified as an artificial group with Echinodon reassigned to Heterodontosauridae. 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.
All specimens of Echinodon have been found in the Purbeck Group of Dorset, which has been variably considered to be from the latest Jurassic or the earliest Cretaceous. Current studies accept an Early Cretaceous Berriasian age, making Echinodon both the youngest and the smallest heterodontosaurid. Other dinosaurs it lived alongside include the ornithopod Owenodon and the theropod Nuthetes, which are both also fragmentary. 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.
History of discovery
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. These fossils, including many teeth as well as maxilla and dentary bones of the upper and lower jaws, were found alongside shells and plant fossils in the Purbeck Beds. These were first described in a monograph published in 1861 written by Sir Richard Owen, a British palaeontologist who also described fossils of Iguanodon and Megalosaurus. Owen gave the name Echinodon becklesii for the fossils, which he considered to be part of the lizard clade Lacertilia. While the specific name honoured Beckles for his discovery of the fossils and allowing Owen to study his collection of Purbeck fossils, the generic name was derived from the Ancient Greek , 'hedgehog', and , 'tooth', which Owen combined as "prickly tooth" to describe the anatomy of the along the sides of the teeth. Owen had corresponded with British palaeontologist Hugh Falconer, who had suggested the name "Sauraechinodon", but as the shortened form Echinodon was not preoccupied, Owen chose to use the abbreviated form as the name for his new animal. Falconer issued a correction in 1861, specifying that he proposed the name "Sauraechmodon" instead of "Sauraechinodon".
Although originally described as a lacertilian by Owen, he revised his classification in 1874 to group Echinodon with Scelidosaurus and Iguanodon in a clade he called Prionodontia, which was within the larger clade Dinosauria. 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. The series of specimens designated as types by Owen were purchased by the British Museum of Natural History (BMNH now NHMUK) in 1876 and form part of the Beckles Collection, bearing specimen numbers NHMUK 48209 to 48215. Lydekker also specified that Echinodon was found in the Middle Purbeck Beds, an informal unit of the modern Purbeck Limestone Group, the formal name for the historic Purbeck Beds. British palaeontologist Peter Galton narrowed down the depositional locality of Echinodon further in 1978 to the freshwater "Dirt Bed", also known as the "Mammal Pit" that was excavated by Beckles in 1857. However, no evidence exists to link Echinodon to any specific bed in the Lulworth Formation of the Purbeck Group. Galton also referred the specimen NHMUK 48229 to Echinodon, a fragmentary dentary with teeth, and the only further referrals to the genus includes isolated teeth also from the Purbeck beds.