Echinodon

''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.

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. These remains, collected from sandstones at the base of the Brushy Basin Member of the Morrison Formation, were approximately 150.2-150.3 million years old, and were initially described as intermediate fabrosaur remains by their collector George Calliston in 1984. Three years later Calliston revised his description and referred the material, including jaw bones, vertebrae and most of the hindlimb, to ''Echinodon'' sp., an assignment supported by Galton in 2002, although in 2006 he reconsidered the material and noticed differences in tooth anatomy from ''Echinodon'' proper. In 2009, these fossils were given their own genus, ''Fruitadens haagarorum'', a related but distinct taxon from ''Echinodon''.

In 2021 two premaxillary teeth that were attributed to ''Echinodon'' were reported from the Angeac-Charente bonebed of France, which is coeval with the Purbeck deposits.Ronan Allain, Romain Vullo, Lee Rozada, Jérémy Anquetin, Renaud Bourgeais, et al.
Vertebrate paleobiodiversity of the Early Cretaceous (Berriasian) Angeac-Charente Lagerstätte (southwestern France): implications for continental faunal turnover at the J/K boundary
Geodiversitas, Museum National d’Histoire Naturelle Paris, In press. ffhal-03264773f

Description

The known material of ''Echinodon'' is limited to bones of the skull, but the multiple specimens include at least some of the , , , , , and along with most of both the upper and lower jaws. Based on proportions of the related genus ''Heterodontosaurus'', the skull of ''Echinodon'' would have been long, which is comparable to ''Tianyulong'' at but less than adults of ''Fruitadens'' at long, making ''Echinodon'' the smallest presumably adult heterodontosaur and one of the smallest non-avian dinosaurs.

The main body of both premaxillae are preserved in the lectotype specimen of ''Echinodon'', although fractured and crushed. A premaxillary is present near the anterior margin of the individual bone, and the it is nested within is more similar in shape to ''Heterodontosaurus'' than more derived ''Hypsilophodon''. The surface of the front of the bone along the toothrow is textured and lacks teeth, distinguishing the bone from that of ''Lesothosaurus'' which bears teeth along its entire length. The premaxilla bore only three teeth, lacking denticles (serrations) present in most basal ornithischians, although all teeth are subequal in size with the third being the largest.

Three maxillae are preserved among the material of ''Echinodon'', preserving almost all the bone including most regions of contact with other cranial bones. The maxillae are slightly flattened, which minimises the strength of the buccal emargination, a diagnostic character of ornithischians where the maxillary tooth row and bone directly above it is inset from the outer edge of the bone. The presence of an arched diastema in ''Echinodon'' is a topic of disagreement. While Galton and American palaeontologist Paul Sereno interpret a diastema as present as in ''Heterodontosaurus'', British palaeontologists David B. Norman and Paul M. Barrett