Euoplocephalus

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

thumb|left|Size of specimen AMNH 5405 compared with a human.

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 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 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. 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 Palaeoscincus tutus. Today however, Palaeoscincus is considered to be a nomen dubium based on indeterminate ankylosaurian teeth. In 1964, Euoplocephalus was by Oskar Kuhn referred to Ankylosaurus, as a Ankylosaurus tutus.

The genus name Euoplocephalus, meaning "well-armed head", is derived from the Greek words eu (εὖ) meaning "well", hoplo~ (ὁπλο~) meaning "armed", and kephale (κεφαλή) meaning "head". This name has been misspelled more than a dozen different ways in formal scientific literature. The specific name tutus means "safely protected" in Latin. The only valid species known today is Euoplocephalus tutus.

Referred material

thumb|left|Skull of specimen UALVP 31

During the early twentieth century many more ankylosaurid fossils were uncovered in North America. Some were referred to Euoplocephalus, others named as separate genera. In 1971 however, Walter Coombs submitted a dissertation containing a landmark re-appraisal of North American ankylosaurs. He noted that, among the many specimens similar to Euoplocephalus, their skulls varied so much that either every known specimen must be a new species, or they all represented individual variation within a single species: Euoplocephalus tutus. The fossils now referred to this species contained more than forty individuals discovered in Alberta, Canada and Montana in the United States, which would have made Euoplocephalus the best known ankylosaurid. This included fifteen skulls, teeth, and a few almost-complete skeletons, found with the armor still attached.

The synonymy of all Campanian North-American ankylosaurids was followed for several decades, until scientists from the University of Alberta began to re-examine the fossils. A 2009 study found that Dyoplosaurus is in fact a valid taxon, and identified unique characteristics that differentiated it from Euoplocephalus, including its triangular claws. The validity of Anodontosaurus was accepted in two subsequent studies. The first, published by Paul Penkalski and William T. Blows in 2013, re-validated Scolosaurus as well. The second study, by Penkalski (2013), named and described Oohkotokia from Montana on the basis of remains that were originally thought to be referable to Euoplocephalus.

Palaeoscincus asper, "the rough one", is now considered to be Euoplocephalus. It is a dubious tooth taxon from the late Campanian Dinosaur Park Formation of Alberta, named by Lambe in 1902. It consists of a single tooth, specimen NMC 1349.

In 2013 Arbour limited the specimens that could be reliably referred to Euoplocephalus to the lowest thirty metres of the Dinosaur Park Formation. The material would in that case, apart from the holotype, consist of partial skeletons with skull AMNH 5337, AMNH 5403, AMNH 5404, AMNH 5405, ROM 1930 and UALVP 31; partial skeleton lacking the skull AMNH 5406; CMN 842, a cervical half-ring; CMN 8876, a skull, TMP 1979.14.74, a fragmentary skull; and UALVP 47977, a skull roof piece. The hands, feet and tail, including the club, are therefore not completely known. However, many of those specimens have now been reassigned to other, new taxa, including Scolosaurus and Platypelta, and others.

Classification

thumb|upright|Referred skull NHMUK R4947

In 1910, Lambe assigned Euoplocephalus to the Stegosauria, a group then encompassing all armoured dinosaur forms and thus having a much wider range than the present concept. In 1917, Charles Whitney Gilmore assigned it to the Ankylosauridae. Today, Euoplocephalus is still seen as an ankylosaurid, but as a member of the Ankylosauria, not the Stegosauria. It is likely also a member of the derived subgroup Ankylosaurinae. The recent splitting of the ankylosaurid Campanian material of North America has complicated the issue of the direct affinities of Euoplocephalus. Penkalski (2013) performed a small phylogenetic analysis of some ankylosaurine specimens. The only Anodontosaurus specimen that was included in this analysis was its holotype. Anodontosaurus was placed in a polytomy with the holotype of Euoplocephalus and some specimens that are referred to it, while Oohkotokia was placed in a clade with Dyoplosaurus, and specimens that are thought to represent either Dyoplosaurus or Scolosaurus.

The results of an earlier analysis of the ankylosaurid tree by Thompson et al. (2011), is shown by this cladogram.

Paleoecology

Euoplocephalus, following the synonymizations proposed by Coombs (1971), was thought to exist for far longer, and was a member of more distinct faunas, than any of its contemporaries, as these fossils dated to between 76.5 and 67 million years ago in the Campanian-Maastrichtian ages of the late Cretaceous period, and came from the Dinosaur Park and Horseshoe Canyon Formations of Alberta, Two Medicine Formation of Montana, and possibly from the Oldman Formation of Alberta. However, recent studies referred all Horseshoe Canyon Formation specimens to Anodontosaurus, Based on the form of the humerus-shoulder articulation and the arrangement of the protracting muscles of the upper arm, it appears that the upper arm sloped away from the body. Coombs and Maryanska (1990) observed that Euoplocephalus specimens are usually discovered as isolated elements or partial skeletons, which suggested that this animal engaged in solitary habits and was usually either solitary or participated in small group clusters.

The armor of Euoplocephalus may have had a keratinous covering, or it may have floated in the skin, as is seen in modern crocodiles. In addition to protection, the heavily vascularized armor may have had a role in thermoregulation. Because only the distal half of the tail was stiffened by tendons, the anterior half could still move freely from side to side. The ossified tendons would have transmitted the force of the swing to the club and reinforced the supporting vertebrae. A 2009 study concluded that "large ankylosaurian clubs could generate sufficient force to break bone during impacts, while average and small ones could not". The tail club could be swung low, toward the fragile metatarsals or shin bones of attacking theropods.

Senses and airflow

Euoplocephalus had relatively small eyes, but this does not necessarily mean that it had restricted vision. The complex respiratory passages observed in the skull suggest that Euoplocephalus had a good sense of smell, although in 1978 an examination of casts of the endocranium did not show an enlarged olfactory region of the brain. Teresa Maryanska, who has worked extensively on Mongolian ankylosaurids, suggested that the respiratory passages were primarily used to perform a mammal-like treatment of inhaled air, based on the presence and arrangement of specialized bones,

A 2011 study found that the nasal passages of Euoplocephalus were looped and complex; possibly an adaptation for heat and water balance and vocal resonance, and researchers discovered an enlarged and vascularised chamber at the back of the nasal tract, which was considered by the authors to be an adaptation to improve its sense of smell. The researchers also managed to reconstruct the dinosaur's inner ear and concluded that it was capable of hearing at low frequencies. They suggested that this may have been an adaption to hearing low-toned resonant sounds produced by the nasal passages.

Diet

Euoplocephalus, like other ankylosaurians, is thought to have been a herbivore.

However, later research indicated that forward and sideways jaw movement was possible, the skull being able to withstand considerable forces. A study conducted in 2014 found that the ankylosaurs were capable of eating of tough fibrous plant material, though not to the same degree as their nodosaur relatives or the ceratopsians and hadrosaurs.