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 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 2022 two premaxillary teeth from the Angeac-Charente bonebed of France were assigned to an indeterminate species of Echinodon. The teeth showed features interpreted as being most similar to Echinodon among ornithischians in the lack of denticles, and their assignment to the genus was used to support a Berriasian age for the bonebed.
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 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 concluded in 2002 that, based on the images of Owen prior to later damage of the fossil NHMUK 48209, a diastema was absent. Sereno reiterated in 2012 that a diastema was present and arched based on the maxilla NHMUK 48211. Nine teeth are present in the maxilla of Echinodon, the first being an enlarged and slender canine similar to those seen in the premaxilla of Lycorhinus and Heterodontosaurus. Owen also figured a partial tooth in front of the caniniform, but it has since been lost in damage to the material. While Norman and Barrett used this as evidence for a second caniniform smaller than the one behind, Galton in 1978 and Sereno in 2012 identified only one canine present. The first post-caniniform tooth of Echinodon is the largest, although only slightly taller than those following, which are all the same size. There is a round prominence along the middle of the tooth crowns, but there are no prominent ridges present on the crown. Eight to ten denticles are present on each side of the tooth crowns.
Little of the lacrimal, jugal, and palatine are preserved, although the margin of the can be identified in the fragment of the lacrimal. The ectopterygoid bone of the is partially complete and preserved in articulation with the maxilla of NHMUK 48210. Lack of preservation limits the anatomical details that can be identified beyond its resembling the bone known in other ornithischians.
The bone is not preserved in Echinodon, but its presence can be confirmed by the morphology of the anterior end of the known dentaries. Like in other heterodontosaurids, the predentary was only loosely articulated with the dentary in life, lacking lateral and ventral processes. The dentary was deep for a basal ornithischian, its height at mid-length being 30% of the total length. The bone tapers anteriorly along its length, although the margins are subparallel under the middle of the tooth row. A row of are present along the margin of the buccal emargination, as in the maxilla. The process is prominent, unlike other basal ornithischians, although the contact between the dentaries is V-shaped as in basal ornithischians. There were 11 teeth in the dentary, the first two of which were specialised compared to most ornithischians. While specialised anterior teeth were not discussed by Galton in 1978, and were considered absent by Norman and Barrett in 2002, Sereno described the first two of Echinodon as differing in size from following teeth in 2012. The first alveolus was extremely reduced, indicating a small peg-like first dentary tooth as in Lycorhinus, and the second was significantly enlarged indicating a caniniform larger than the one in the maxilla. The regular dentary teeth were slightly taller than those of the maxilla and bore denticles along the top half of the crown instead of the top 25%. As in the maxilla, the crowns has a medial bulge, eight to ten denticles on either side of the tip, and symmetrical.