| Euoplocephalus Temporal range: Late Cretaceous, 76.5–67 Ma |
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| Skeleton of E. tutus, Senckenberg Museum | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | Dinosauria |
| Order: | †Ornithischia |
| Family: | †Ankylosauridae |
| Subfamily: | †Ankylosaurinae |
| Genus: | †Euoplocephalus Lambe, 1910 |
| Species: | †E. tutus |
| Binomial name | |
| Euoplocephalus tutus (Lambe, 1902 [originally Stereocephalus, preoccupied]) |
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| Synonyms[1] | |
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Euoplocephalus ( /juːˌɒplɵˈsɛfələs/ ew-op-lo-sef-ə-ləs; Greek: eu-/ευ- meaning 'well', hoplo-/οπλο- meaning 'armed' and kephale/κεφαλη meaning 'head', "well-armored head") was one of the largest genera of ankylosaurian dinosaurs, at about the size of a small elephant. It is also the ankylosaurian with the best fossil record, so its extensive spiked armor, low-slung body and great club-like tail are well documented.
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Among the ankylosaurids, Euoplocephalus was exceeded in size only by Tarchia and Ankylosaurus. Euoplocephalus was 6 metres (20 ft) long and weighed about 2 tonnes (2.2 short tons).[2] It was also 2.4 metres (7.9 ft) wide.
The skull of Euoplocephalus can be distinguished from 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),[3] which may have served as bony eyelids;[4] the shallowness of the nasal vestibule at the entrance of the nasal cavity;[3] the medial curve of the tooth rows in the upper jaw; and the teeth, which are relatively small, lack cingulums, and have variable fluting.[3] The skull resembles a truncated equilateral triangle when viewed from above, and is slightly wider than it is long.[3] As in most quadrupedal ornithischians, its neck is short, the scapula is massive and robust, and the forelimbs are shorter than the hindlimbs.[5] The tail is long and ends in a bony club. Old restorations of Euoplocephalus, synonyms (Scolosaurus) and rejected synonyms (Dyoplosaurus) 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.[5] The distal half of the tail has ossified tendons.
The vertebral column of Euoplocephalus is made up of seven cervical vertebrae, 13 dorsal vertebrae, three sacral vertebrae, and at least 21 caudal vertebrae; the total number of caudal vertebrae is uncertain because several are fused to form part of the tail club.[5] This fusion is also seen in other ankylosaurids; it is possible that the extent of fusion is an age-related feature.[6][7] Like other ankylosaurians,[1] the last four dorsal vertebrae and the first caudal vertebra are fused to the sacrum, forming a reinforced synsacrum.[5]
The head and body of Euoplocephalus were covered with bony armor, except for parts of the limbs and possibly the distal tail. Much of this armor was made up of small ossicles. Larger flat scutes, conical plates, and disc-shaped plates were arranged in transverse bands among this pavement of ossicles. Two bands protected the dorsal and lateral surfaces of the neck, four were present across the front part of the torso, three protected the pelvis, and four were present on the anterior half of the tail. The banded arrangement is thought to have permitted some freedom of movement.[5] Types of large scutes varied by body region. For example, the neck armor included large, crested plates, the shoulder region included tall plates near the midline of the body, the armor over the sacral region included large disc-shaped scutes, and tall conical scutes were present in the final band of the tail. Little is known about the armor of the limbs. Large keeled plates were present on the upper arms.[5] In addition, Euoplocephalus had two pyramid-shaped horns growing from the back of its head.
Paleontologist Lawrence Morris Lambe discovered the first specimen (the holotype) 1902 and proposed the name Stereocephalus.[8] However, the name was already preoccupied (the name had already been given to an insect), so he changed it to Euoplocephalus in 1910.[9] This new name has been misspelled in more than a dozen different ways, in formal scientific literature. It was also once thought to be the same genus as Ankylosaurus.
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 a few almost-complete skeletons, found with the armor still attached.[1] Individual armor plates are the most commonly found element from them.
In 1971, Coombs published a landmark re-appraisal of North American ankylosaurs. He noted that among the many specimens similar to Euoplocephalus (many of which had been assigned their own genera and species), 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 (E. tutus).[6] Starting from this assumption that there was only one species of ankylosaur during the Campanian stage of the Upper Cretaceous, Coombs synonymized the genera Anodontosaurus, Dyoplosaurus, and Scolosaurus with Euoplocephalus or E. tutus, creating a species that spanned nearly ten million years, or the entire Campanian.[10] This synonymy was followed for several decades,[1] until scientists from the University of Alberta began to re-examine the fossils. A recent study found that Dyoplosaurus is in fact a valid taxon, and identified unique characteristics that differentiated it from Euoplocephalus, including its triangular claws.[10]
Euoplocephalus as it is currently understood existed for far longer, and was a member of more distinct faunas, than any of its contemporaries (though it is possible that the fossils currently assigned to Euoplocephalus actually represent several different genera). Euoplocephalus fossils have been found in the Dinosaur Park and Horseshoe Canyon Formations of Alberta, as well as the Judith River Formation of Montana. Its fossils date to between 76.5 and 67 million years ago, in the Campanian-Maastrichtian ages of the late Cretaceous period.[10]
The tail club of ankylosaurids is often interpreted as a weapon. In Euoplocephalus, the presence of ossified tendons only with the distal half of the tail may support such a function.[11] 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.[5] The club was likely held just above the ground.[12] A 2009 study concluded that large ankylosaurian clubs could generate sufficient force to break bone during impacts, while average and small ones could not.[13] It has also concluded that that tail swinging behavior is feasible in ankylosaurids, but it remains unknown whether the tail was used for interspecific defense, intraspecific combat, or both.[13]
Euoplocephalus may have had cursorial abilities on par with those of the modern rhinoceros and hippopotamus.[14] 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.[15]
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.[5]
The palpebral bones over the eyes may have provided additional protection for the eyes. These bones were located in the eyelid musculature and were probably mobile enough to be moved over the eyes.[4] Euoplocephalus had relatively small eyes; this does not necessarily mean that it had restricted vision, but it appears that this animal had a better-developed sense of smell than vision.[5] The complex respiratory passages suggest that Euoplocephalus had a good sense of smell, although casts of the endocranium have so far not shown an enlarged olfactory region.[16] Teresa Maryanska, working with Mongolian ankylosaurids, has suggested that the respiratory passages were instead primarily used to perform mammal-like treatment of inhaled air, based on the presence and arrangement of specialized bones,[7] which Euoplocephalus also had.[17]
Eoplocephalus, like other ankylosaurians, is thought to have been a herbivore.[1] It had a broad muzzle, which could indicate that it was a non-selective feeder, perhaps similar to a hippopotamus. This would provide niche separation from contemporaneous nodosaurids with narrower muzzles.[5] Ankylosaurians have historically been thought of as feeding using simple up-and-down movements of the jaws, but Euoplocephalus appears to have been able to make more complex movements. Tooth wear and jaw articulations (within the lower jaw and at the lower jaw-quadrate joint) suggest that the lower jaws were pulled back during feeding, and also slightly pivoted inward. This action would have sheared food.[18]