The remaining major group of armoured dinosaurs, the ankylosaurs, consisted of two families - Nodosauridae and Ankylosauridae. Ankylosaurs were characterised by a great variety of spikes and armour plating. Some genera possessed cranial horns, and the ankylosaurids were endowed with massive tail clubs (Fig. 90), which were probably used both in defence against predators and to combat rivals. In addition, the broad, flattened and armoured skulls might have been employed in head-to-head pushing (Sampson 1997). In contrast,the earlier and more primitive nodosaurids had narrower skulls and lacked tail clubs, but many of them had enlarged spikes which might have interlocked with those of opponents in dominance contests (Coombs 1990). Their bodies were also covered with bony plates from head to tail. Well-known genera include the Lower Cretaceous Hylaeosaurus, whose remains were described by Gideon Mantell in 1832, and its contemporary Polacanthus (Fig. 115), also from southern England. Because the fossils are incomplete, the arrangement of the spines is not known and it is possible that the two animals are the same. Sauropelta and Silvisaurus date from the Lower Cretaceous of North America, Nodosaurus (Fig. 115) and Panoplosaurus (Fig. 115) from the Upper. The diminutive Struthiosaurus (length ca. 2 m) might well have been an island form. Many large animals have evolved dwarf species when confined to islands where food supplies are limited. In contrast, Sauropelta was ca. 7.6 m long and weighed about 3 tonnes (Palmer 1999).
In addition to their formidable spines, the ankylosaurs were encased in thick, defensive, body armour. Rows of horny plates with raised keels ran from the head to the tip of the tail in Sauropelta. Silvisaurus was covered with thick bony
plates, while the armour of Nodosaurus consisted of narrow rectangular plates alternating with wider plates studded with bony nodes. Whereas most genera probably squatted, retracted their legs, and relied upon their armour for defence against predators, Panoplosaurus (Fig. 115), although well armoured, could have charged and struck the enemy with its large shoulder spikes. Its forelegs were particularly strongly built and well endowed with muscles, suggesting that this dinosaur was quite manoeuverable. The shoulders and hips of the ankylosaurs were well modified to bear the great weight of armour, while the legs were strong, with broad hoofed feet (Palmer 1999).
Ankylosaurid armour and spines were even heavier and more formidable than those of the nodosaurids. Dyoplosaurus (Fig. 89) from the Upper Cretaceous of Canada had a toothless beak, small cheek teeth, an armour-plated body, and a tail ending in a bony club remarkably like that of Euoplocephalus (Fig. 90; Sect. 9.2.2). Indeed, it is quite possible that once again two sets of fossils belong to the same species!
The amount of bone was much reduced in most nodosaurid skulls, but the head armour of the ankylosaurids was extensively developed. In Talarurus (Fig. 116) from Mongolia, the skull broadened out from the toothless beak and small cheek teeth of the mouth to a pair of hefty, bony spikes at the rear. Another pair of spikes projected from the side of the head. The massive head of Saicharnia, also from the Upper Cretaceous of Mongolia, was armoured with bony nodules, while the skull of Euoplocephalus from North America had a skull wider than any other part of its body. Its snout was blunt and, as usual, the front of the mouth consisted of a toothless beak. Massive armour, pointed spines, and a tail ending in a great bony club, were the main distinguishing features of the Ankylosauridae.
Many of the nodosaurids had toothless beaks, but Silvisaurus retained teeth in its upper jaws. Ankylosaur teeth were small in relation to the size of the skull. They were laterally compressed and leaf-shaped. Those of nodosaurids were slightly larger, both absolutely and relative to skull size. Whereas the ankylosaurids would have cropped low-level vegetation indiscriminately, the nodosaurids, with their narrow snouts, may have been more selective in their choice of food.
A complex of chambers and air passages was characteristic of both ankylosaurids and nodosaurids. Like the crests of hadrosaurs (Sect. 9.3.1), these might have been used to produce sounds for social communication and agonistic threat. The nasal passages of the nodosaurids were relatively simple paired tubes leading from the nostrils to the back of the throat,but those of the ankylosaurids followed an S-shaped course through the head. On either side, there were additional branches or sinuses that may have had several functions in addition to sound production. They might have improved the sense of smell, or served to filter, warm, and moisten inspired air. In 1977, Teresa Maryanska discovered bones within the nasal passages of Saichania and Pinacosaurus that resembled the turbinal bones in the noses of mammals. Covered by moist membranes, these bones would have had just those regulatory functions (Norman 1985).
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