Euoplocephalus

Euoplocephalus was a large, plant-eating ankylosaurid dinosaur from the Late Cretaceous of Canada. It had only one named species, *Euoplocephalus tutus*. The first fossil was found in Alberta in 1897. It was originally named *Stereocephalus* in 1902, but that name was already taken by an insect, so it was changed in 1910. Many more ankylosaurid remains from the Campanian of North America were later found and often assigned to separate genera. In 1971, Walter Coombs argued they all belonged to *Euoplocephalus*, making it one of the best-known dinosaurs. More recently, experts have reversed that view, limiting genuine *Euoplocephalus* finds to about a dozen specimens. These include several nearly complete skeletons, so its body shape is still well understood.

The animal reached about 5.3 meters (17 feet) in length and weighed around 2 metric tons (2.2 short tons). Its body was low, flat, and wide, supported by four sturdy legs. The head had a short, drooping snout and a horny beak for biting off plants, which were digested in a large gut. Like other ankylosaurids, it was heavily covered in bony armor plates, including rows of large, high-ridged oval scutes. Two bone rings protected the neck. It could actively defend itself against predators such as *Albertosaurus*, *Daspletosaurus*, and *Gorgosaurus* using a heavy club at the end of its tail.

Among ankylosaurids, only *Ankylosaurus*, and possibly *Tarchia* and *Cedarpelta*, were larger. *Euoplocephalus* had a very broad, flat torso about four feet high on short legs. Its skull can be told from most other ankylosaurids by details like the bony sculpturing in front of the eyes, the form of the palpebral bones (which may have acted as bony eyelids), the shallow nasal vestibule, the inward curve of the upper tooth rows, and the small teeth lacking true cingula with variable fluting. However, these traits are shared with close relatives, some once included in the genus. In 1990, Coombs and Teresa Maryańska said *Euoplocephalus* could be identified by four traits: the premaxillae (front snout bones) are not covered by dermal ossifications; the external nostrils are slit-like, face forward, and are divided by a vertical bone sheet; the beak is as wide as or wider than the distance between the rear upper cheek tooth rows; and the foot has three digits with metatarsals and toes.

In 2013, Victoria Arbour and Phil Currie gave a more precise diagnosis. Compared to *Anodontosaurus* and *Scolosaurus*, *Euoplocephalus* lacks round osteoderms at the base of the squamosal and quadratojugal horns. Unlike *Anodontosaurus*, it lacks small osteoderms at the bases of the large osteoderms on the first cervical half-ring, but in top view it has large rounded osteoderms on the sides of the tail club. It differs from *Dyoplosaurus* in having sacral ribs that point straight outward. It differs from *Scolosaurus* in having keeled osteoderms with round or oval bases on the top and sides of the first cervical half-ring and a shorter rear blade of the ilium. It differs from *Ankylosaurus* in having forward-pointing external nostrils and lacking a continuous keel between the squamosal horn and the supraorbitals.

The skull, seen from above, looks like a truncated equilateral triangle and is slightly wider than long. The largest known skull (specimen AMNH 5403) is 411 millimeters long and 478 millimeters wide. Most skull bones are fused, and two openings normally present in dinosaurs—the antorbital fenestra and the upper temporal fenestra—are closed. There are 19 to 24 teeth in each upper jaw. The front snout bone (premaxilla) is toothless. The teeth are very small, at most 7.5 millimeters tall and wide. The strongly drooping snout is blunt, wide, and high, filled with complex air passages and sinuses whose form and function are not fully understood. Each side has two external nostrils. The lower jaw has a very concave upper rim with 21 teeth, and a short low extension at the front to contact the wide predentary (the bony core of the lower horny beak), which fit inside the upper beak.

The neck is moderately short, as in most quadrupedal ornithischians. 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 its rejected synonyms (*Dyoplosaurus*, *Scolosaurus*) often show a club with two large vertical spikes. This mistake came from a restoration of *Scolosaurus* by Franz Nopcsa; the specimen he used had an incomplete tail that stopped just past a pair of conical spikes now known to be halfway along its length. He restored the tail as ending right after those spines. Other artists combined the spikes with the tail club, making the error worse. The narrow distal half of the tail is stiffened by bundles of ossified tendons. The vertebral column has at least seven cervical vertebrae and at least eleven "free" dorsal vertebrae.