Genus of ankylosaurid dinosaur from the Late Cretaceous period
Euoplocephalus ( ) is a genus of large herbivorous ankylosaurid dinosaurs, living during the Late Cretaceous of Canada. It has only one named species, Euoplocephalus tutus.
The first fossil of "Euoplocephalus" was found in 1897 in Alberta. In 1902, it was named "Stereocephalus", but that name had already been given to an insect, so it was changed in 1910. Later, many more ankylosaurid remains were found from the Campanian of North America and often made separate genera. In 1971, Walter Coombs concluded that they all belonged to "Euoplocephalus", which then make it one of the best-known dinosaurs. Recently, however, experts have come to the opposite conclusion, limiting the authentic finds of "Euoplocephalus" to about a dozen specimens. These include a number of almost complete skeletons, so nevertheless much is known about the build of the animal.
"Euoplocephalus" reached in length and in body mass. Its body was low-slung and very flat and wide, standing on four sturdy legs. Its head had a short drooping snout with a horny beak to bite off plants that were digested in the large gut. Like other ankylosaurids, "Euoplocephalus" was largely covered by bony armor plates, among them rows of large high-ridged oval scutes. The neck was protected by two bone rings. It could also actively defend itself against predators like "Albertosaurus", "Daspletosaurus", and "Gorgosaurus" using a heavy club at the end of its tail.
Description.
Size.
Among the ankylosaurids, "Euoplocephalus" was exceeded in size only by "Ankylosaurus", and perhaps "Tarchia" and "Cedarpelta". "Euoplocephalus" was about long and weighed about . Like other ankylosaurids, it had a very broad and flat low-slung torso, about four feet high, positioned on four short legs.
Distinguishing traits.
The skull of "Euoplocephalus" can be distinguished from most other ankylosaurids by several anatomical details, including: the pattern of bony sculpturing in the region in front of the eyes; the form of the palpebral bones (small bones over the eyes), which may have served as bony eyelids; the shallowness of the nasal vestibule at the entrance of the nasal cavity; the medial curve of the tooth rows in the upper jaw; and the teeth, which are relatively small, lacking true cingula, and having variable fluting of the denticles. However, these traits are shared with a number of closely related forms, some of which in the past have been included in the genus. Combining such forms, Walter Coombs and Teresa Maryańska in 1990 stated that "Euoplocephalus" could be distinguished based on four traits. The premaxillae, the front snout bones, are not covered by dermal ossifications. The external bony nostrils are slit-like, face to the front and are each divided by a vertical bone sheet or septum. The beak has a width equal to or greater than the distance between the rear maxillary, upper cheek, teeth rows. The foot has three digits, metatarsals with toes.
In 2013, Victoria Arbour and Phil Currie provided a differential diagnosis, setting "Euoplocephalus" apart from its nearest relatives. When compared with "Anodontosaurus" and "Scolosaurus", "Euoplocephalus" lacks round osteoderms at the base of the squamosal and quadratojugal horns. Compared with "Anodontosaurus" it lacks small osteoderms at the bases of the large osteoderms on the first cervical half-ring, but, contrary to that genus, does in top view have large rounded osteoderms at the sides of the tail club. It differs from "Dyoplosaurus" in possessing sacral ribs that perpendicularly point outwards. It differs from "Scolosaurus" in possessing keeled osteoderms with a round or oval base on the top and sides of the first cervical half-ring and having a shorter rear blade of the ilium. "Euoplocephalus" differs from "Ankylosaurus" in possessing anteriorly directed external nostrils and in lacking a continuous keel between the squamosal horn and the supraorbitals.
Skeleton.
The skull of "Euoplocephalus" resembles a truncated equilateral triangle when viewed from above, and is slightly wider than it is long. The largest known skull, that of specimen AMNH 5403, has a length of 411 millimetres and a width of 478 millimetres. Most skull elements are completely fused and two skull openings normally present with dinosaurs, the antorbital fenestra and the upper temporal fenestra, have closed. The skull has nineteen to twenty-four teeth in each upper jaw. The frontmost snout bone, the premaxilla, is toothless. The teeth are very small, with a maximal height and width of just 7.5 millimetres. The strongly drooping snout is blunt, wide and high, and filled with very complex air passages and sinuses, the form and function of which are not yet completely understood. Each side has two external nostrils. The lower jaw has a very concave upper rim with twenty-one teeth. At its front a short low extension is present, to provide contact with the wide predentary, the bony core of the lower horny beak, that fitted within the upper beak of the snout.
As in most quadrupedal ornithischians, its neck is moderately short. The scapula is massive and robust, and the very robust forelimbs are shorter than the hindlimbs. The tail is long and ends in a bony club. Old restorations of "Euoplocephalus" and rejected synonyms ("Dyoplosaurus", "Scolosaurus") often show a club with two large vertical spikes. This is an error based on a restoration of "Scolosaurus" by Franz Nopcsa; the specimen he used had an incomplete tail that stopped just beyond the pair of conical spikes now known to have been positioned halfway along its length. He restored the tail as ending just after the spines. Other artists combined the spikes with the tail club, compounding the inaccuracy. The narrow distal half of the tail is stiffened by bundles of ossified tendons.
The vertebral column of "Euoplocephalus" is made up of at least seven cervical vertebrae, at least eleven "free" dorsal vertebrae, typically four sacrodorsals forming a fused "sacral rod" in front of the sacrum proper, three "true" sacral vertebrae, and between one and four caudosacrals. Like with other ankylosaurians, the last four dorsal vertebrae and the first caudal vertebra are thus fused to the sacrum, forming a reinforced synsacrum of at least eight vertebrae, the spines of which form a fused supraneural plate, also incorporating the zygapophyses. There are at least twenty-one caudal vertebrae; the total number of caudal vertebrae is uncertain because approximately ten are fused to form part of the tail club, bringing the total to about thirty. This fusion is also seen in other ankylosaurids; it is possible that the extent of fusion is an age-related feature.
The humerus is very robust with strongly expanded upper and lower joints, combined with a narrow shaft. On the upper shaft an enormous deltopectoral crest is present of which the lower part does not gradually merge with the shaft but is warped to the front, forming a thick knob or lip. All this indicates a very heavy musculature. In the lower arm the robust ulna has a well-developed olecranon process. The wrist and hand bones are not well known. In the pelvis, the front blade of the ilium splayed out to the front, reaching all the way to the widest point of the belly to support the gut. This blade also forms a bone shelf at the rear side of the body. The rear blade of the ilium is shorter than the diameter of the hip socket it was located behind, meaning the leg is located at the rear end of the pelvis, near the tail base and much closer to the midline than the belly sides. The pubic bone is unknown. The ischium is a short, curved, vertically positioned bone strap. The thighbone is short, robust and straight with a low fourth trochanter positioned below the midpoint of the shaft. The robust shinbone is shorter than the thighbone. The foot is not well known but functionally tridactyl with hoof-shaped instead of sharp claws.
Armor.
The head and body of "Euoplocephalus" were covered with bony armor, except for parts of the limbs and possibly the distal tail. The armor consisted of osteoderms, skin ossifications that are not part of the skeleton proper. This armor was in 1982 extensively described by Kenneth Carpenter, who however, largely based himself on the very complete specimen NHMUK R5161, the holotype of "Scolosaurus", which genus no longer is seen as a synonym of "Euoplocephalus". When limited to the certain material of the latter genus, little is known of the exact configuration of the armor, with the exception of the head and the neck. The most informative specimen in this respect would then be ROM 1930, having conserved some osteoderms of the torso in their original position.
In any case, much of the armor was made up of small ossicles, bony round scutes with a diameter of less than five millimetres, of which often hundreds have been found with a single specimen. If the armor was configured in an identical way to that of "Scolosaurus", many of these small ossicles had fused into a kind of pavement, forming transverse bands on the body. The banded arrangement is thought to have permitted some freedom of movement. Four of these bands might have been present on the anterior half of the tail, three on the pelvis, perhaps fused into a single "sacral shield", and four across the front part of the torso. Inset in these bands were horizontal rows of larger oval, flat or keeled, scutes. Types of large scutes varied by body region. It might be that the scutes on the shoulder, near the midline of the body, were largest and tallest; ROM 1930 includes some osteoderms with a base length of fifteen centimeters. Little is known about the armor of the limbs. Large keeled plates were present on the upper arms as shown by specimen TMP 1997.132.01 conserving a round osteoderm near the humerus with a diameter of twenty centimeters and narrower spikes associated with the lower arm. The neck was protected by two bone rings, open at the underside, that are called "cervical half-rings". Earlier seen as a fusion of osteoderms, this was doubted by Arbour et al. in 2013, who pointed out that they formed a lower layer, possibly consisting of ossified cartilage, as indicated by a smooth surface and a woven bone texture. Each half-ring is constructed out of six rectangular concave plates, three per side. Each plate has a large keeled osteoderm on top, often not fused with it. With "Euoplocephalus", these neck osteoderms do not have smaller osteoderms at their bases, and their keels do not overhang their posterior edges.
The armor of the skull consists of a large number of ossicles, called "caputegulae" ("head tiles"), that have fused with the normal skull elements, largely fading their sutures. On the snout they form a chaotic and asymmetric mosaic. On the rear nasal region, at the midline a single hexagonal larger plate is present. A keeled plate per side, somewhat more to the back forms the snout rim. Behind the level of the eye sockets the "caputegulae" fuse into a single bone surface. The upper rim of the eye sockets is formed by two pyramid-shaped osteoderms pointing to the sides and rear. In addition, "Euoplocephalus" had two pyramid-shaped squamosal "horns" growing from the back corners of its head. Between them the nuchal crest is covered by two osteoderms per side. At the lower rear side of the skull, a quadratojugal horn is present, in the form of an enormous tongue-shaped osteoderm projecting below.
Discovery and species.
Canadian paleontologist Lawrence Morris Lambe discovered the first specimen on 18 August 1897 in the area of the present Dinosaur Provincial Park, in the valley of the Red Deer River, Alberta, Canada. In 1902, this fossil, CMN 210 (also NMC 210) was designated as the holotype specimen of the type species "Stereocephalus tutus". This specimen consists of the upper part of a cranium and a transverse series of five scutes that were part of a cervical half ring. The generic name was derived from Greek στερεός, "stereos", "solid", and κεφαλή, "kephalè", "head", which refers to the formidable armour. However, the genus name was already "preoccupied" — the name had already been given to an insect, the beetle "Stereocephalus" Lynch 1884 — so Lambe changed it to "Euoplocephalus" in 1910, with as "combinatio nova" (new combination name) "Euoplocephalus tutus". The type species remains "Stereocephalus tutus". In 1915, Edwin Hennig classified "E. tutus" under the genus "Palaeoscincus" Leidy 1856, coining a "Palaeoscincus tutus". Today however, "Palaeoscincus" is considered to be a