Ankylosauria traits and diversity

The armored dinosaurs, or Ankylosauria, first appeared in the Middle Jurassic Epoch. The name Ankylosauria, meaning "fused lizard," refers to the fact that in the Late Cretaceous Ankylosau-rus, the first named ankylosaur, small armor plates appear fused to the skull, and some of the ribs are fused to the vertebrae in the back. Ankylosaurs were bulky, quadrupedal herbivores with highly specialized body armor. Some taxa were literally the size of tanks, reaching lengths of 33 feet (10 m). Armored dinosaurs are divided into two groups, the Ankylosauridae and the Nodosauridae.

Common traits shared by all ankylosaurs include skull armor; closed fenestrae in front of the eyes and on top of the skull; small heads with noninterlocking, spatula-shaped teeth; widely arched ribs that accentuated a broad body covered with extensive armor scutes and spikes; robust legs; and forelimbs that were shorter than the hind limbs. To accommodate their heavy body armor, the ribs and pelvic bones of ankylosaurs were broadened and strengthened to provide a firm base to which body armor and massive muscles could be affixed.

Ankylosaurs and nodosaurs were tallest at the hips. Their stocky legs were adapted for carrying their heavy weight. The limbs of

Ankylosauria
Euoplocephalus (foreground) and Edmontia

nodosaurs were a little lighter than those of ankylosaurs, but members of both clades were slow moving and probably not as fast as other quadrupedal ornithischians such as the horned dinosaurs.

The oldest known, unambiguous ankylosaur was Gargoylesau-rus (Late Jurassic, Wyoming), a small specimen known from a small skull and partial skeleton. At least two less well understood ankylosaur specimens, Dracopelta (Late Jurassic, Portugal) and Mymoorapelta (Late Jurassic, Colorado) suggest that the rise of armored dinosaurs was already in full swing during the heyday of the sauropods and stegosaurs. Definitive specimens of nodosaurids do not appear until the Early Cretaceous and include Sauropelta from Montana, Cedarpelta from Utah, and Pawpawsaurus from Texas. Both groups of armored dinosaurs outlasted the stegosaurs, which disappeared by the end of the Early Cretaceous, and both groups were still thriving near the time of the final extinction of the dinosaurs.

Remains of ankylosaurs have been found in both the Northern and Southern Hemispheres, but they were largely a Northern Hemisphere phenomenon, with extraordinary specimens coming from China, Mongolia, and western North America. A few specimens have also been found in Europe, South America, Australia, and Antarctica.

Ankylosauridae. These earliest members of the armored dinosaurs bore wide, triangular armored skulls and had massive clubs on the end of their tails. Head armor formed a mosaic of thick, bony plates that were fused to the skull. The dorsal surface of the head was completely armor covered, including bony studs around the eyes. Euoplocephalus (Late Cretaceous, Montana and Alberta) had the additional protection of a bony, retractable eyelid. Body armor consisted of small spines, bony knobs, long spikes, and flat and rounded scutes of various sizes. The side of the skull was protected by stout, bony prongs at the rear corners. The snout and skull of the most advanced ankylosaurs had a complex network of air chambers through which the animals breathed. These nasal passages improved their sense of smell and possibly acted as a resonating chamber for making sounds.

The armor plates of ankylosaurids and nodosaurids were normally arranged in bands along the neck, back, and tail. In some cases, the armor was embellished by knobs and spines, especially on the back. The space between individual scutes was filled with a matrix of smaller scutes or bony ossicles. The ankylosaur for which there is the most fossil evidence is Euoplocephalus ("well armored head"), a moderately large armored dinosaur that measured about 23 feet (7 m) long. The topside of this animal was completely protected by a bony mosaic of scutes from the tip of its nose to its bony tail club.

Ankylosaurus, from the Late Cretaceous of Montana and Alberta, is one of the largest and most famous ankylosaurs. It was certainly the largest known, measuring up to 33 feet (10 m) and probably weighing up to four tons (3.6 metric tons). The tail club alone,

Ankylosaurus tail club

consisting of a massive bony knob weighing more than 110 pounds (50 kg), was a lethal weapon if swung accurately at the head or legs of an attacking predator.

Several ankylosaurs, including Gastonia (Early Cretaceous, Utah) and Polacanthus (Early Cretaceous, England and Spain), are notable because they represent anatomic mosaics of ankylosaurid and nodosaurid traits. Gastonia had a heavily armored body and is known from the fossil remains of two complete and two partial skulls and five partial skeletons. Its body armor and skull most resemble those of other ankylosaurids. Gastonia, however, also had robust spikes lining its sides, and it lacked a tail club, two characteristics of the nodosaurids. It is possible that Gastonia and Polacanthus represent a third clade of Ankylosauria.

Nodosauridae. Nodosaurids differed from ankylosaurids in several respects. They did not have tail clubs, and their body armor

Body armor of the ankylosaur Gastonia

Sauropelta

often combined large spikes, particularly lining the sides of the body, with armored plates made up of smaller bony tiles or scutes. The head was narrower and not as heavily armored as in the ankylo-saurids. Nodosaurid skulls did not have bony horns at the back corners of the head as did the ankylosaurids. Nodosaurids had sturdy limbs like ankylosaurids and moved at a slow pace.

The nodosaurid Edmontonia had several bands of very large scutes crossing its back from side to side. The largest were over the shoulders and were six- or eight-sided, with sharp ridges down their centers. The back armor of the nodosaurid Sauropelta consisted of smaller knobby scutes than those of Edmontonia, but the scutes on Sauropelta were thicker and had a point or ridge.

Members of the heavily armored Ankylosauria must have been the most frustrating of all prey for a theropod. What the nodosaurids lacked in a tail club, they gained in the presence of more—and large—upward-pointing spikes and horny knobs. The shoulder areas of nodosaurids shows evidence of strong muscles, more so than in ankylosaurs. This suggests that nodosaurs actively defended themselves by lunging forward and swinging their shoulders back and forth, bringing into play an abundance of spikes and protective armor in the anterior part of the body. Their powerful gait, formidable armor, and threatening body spikes represented serious danger to any attacking predator. Having said that, it should be noted that as with antelope horns or deer antlers, while such body spikes can be used defensively, their primary function was probably for intraspecies displays or combat for mates and territory.

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