Euoplocephalus

Euoplocephalus (meaning "well armored head") was one of the largest of the ankylosaurian dinosaurs at about the size of a small elephant. It is also the ankylosaurian with the best fossil record, so its stiff shell of spiked armor, low-slung body, and great club-like tail are well documented.

Description

Among the ankylosaurids, Euoplocephalus, was exceeded in size by only Tarchia and Ankylosaurus. Euoplocephalus was 6 meterss long, and weighed about 2 metric tonnes (2.2 tonss). While it was 2.4 meters (8 feet) wide, it was low-slung, with short, stout legs. The rear legs were larger than the front legs, and all four limbs were tipped with hoof-like claws. A trackway of an ankylosaur found in 1996 at Sucre, Bolivia indicates that Euoplocephalus could probably move fairly quickly.

Like all ankylosaurids, it has a flat, thick, triangular skull with little room left over for a brain. The mouth is a horny beak, and the teeth are small, weak and peg-like. It has a short neck.

Armed and armored

The entire head and body of the Euoplocephalus are covered with bands of armor, which allowed a surprisingly amount of flexibility. It was the first ankylosaurid discovered with armored shutters that it could slide down to cover its eyelids.

Each narrow band of armor was composed of a thick oval plate embedded in the thick surrounding skin, which was studded with short, horny spikes (dermal scutes, like those of crocodiles), which were about 10 to 15 centimeters (4 to 6 inches) long. In addition to the spines running down its back, Euoplocephalus had large horns growing from the back of its head.

It also had a bony club at the end of its rigid tail, which it carried in a raised position. The tail was muscular, so the club could be swung from side to side for defense.

Internally, many bones were fused together to provide support for the heavy armor. The backbone (the dorsal vertebrae) is merged with the ribs, and several of the back bones in front of the hips (presacral vertebrae) were also fused together into a rod. The tail is made from hardened tissues, which are fused to the tail bones (known as caudal vertebrae).

Environment

Euoplocephalus lived between 65 and 70 million years ago, in the Campanian and Maastrichian agess of the late Cretaceous period.

Euoplocephalus was plant eating (herbivorous) dinosaur. It had a complicated nose design (nasal structure), which indicates that it probably had a good sense of smell, and flexible legs which it might have used for digging. The stiff, low-slung dinosaur had poor teeth (weak dentition), so it must have grazed on fleshy low-lying vegetation, and shallow tubers.

All the skeletons discovered have been isolated, so the conventional wisdom is that all ankylosaurids were solitary. However the discovery of a herd of 22 young Pinacosaurus was announced in 1988, which indicates that Euoplocephalus may have displayed herd behavior, at least as a juvenile.

Only the underbelly of the Euoplocephalus was unarmored. Like a porcupine, flipping it over may have been the only way to harm it. A survey of dinosaur bones in Alberta, Canada supports this, showing many bite marks on dinosaurs like the unarmored Hadrosaurus, but none on the ankylosaurids. There is some speculation, however, that the incredible biting power of the Tyrannosaurus may have ultimately led to the species' extinction. This may have been risky because a swing of the tail could cause massive damage if it hit a predator in the stomach, or knocked its legs out.

Classification and history

Paleontologist Lawrence Morris Lambe discovered the first specimen (the holotype) 1902, and proposed the name Stereocephalus. Unfortunately, the name was already assigned to an insect (the name was preoccupied), so he changed it to Euoplocephalus in 1910. This new name has been misspelled more than a dozen different ways in formal scientific literature. It was also once thought to be the same genera as Ankylosaurus.

The original species is now known as Euoplocephalus tutus, and a second species, Euoplocephalus acutosquameus (originally named Dyoplosaurus), was discovered by William Arthur Parks in 1924. While the two species have clubs with different shapes, they may actually be the same species. Specimens now assigned to the Tarchia and Ankylosaurus genera were also considered to be species of Euoplocephalus at one point.

Fossils from more than 40 individuals have been discovered in Alberta, Canada and Montana in the United States, making Euoplocephalus the best known ankylosaurid. This includes 15 skulls, teeth, and one almost-complete skeleton, found with the armor still attached. The tail club is particularly common.

Etymology

The name Euoplocephalus is derived from the Greek eu, meaning "good" or "well", hoplon meaning "weapon" or "shield", and kephale meaning "head".

Cladogram

A simplified version of one possible evolutionary tree (a cladogram):