In contrast, nodosaurids had:

1 laterally placed nares;

2 lacked horns; and

3 more rounded profiles to their skulls (Fig. 12.4).

Skull interiors were also markedly different. Ankylosaurid nares led into complicated and sinuous nasal chambers, whereas nodosaurids only had a single tube associated with each naris that connected with the throat.

At the other end of these dinosaurs, the tails were also different in the two clades. Ankylosaurids had long processes of the distal caudal vertebrae that reinforced the tail. This provided a handle for a bony club, composed of two pairs of large and small osteoderms (Fig. 12.5). Nodosaurids lacked such modifications to their

FIGURE 12.4 Late nodosaurid skull, showing typical traits for a skull of its clade: laterally placed nares, hornless, and a rounded shape. Compare with skull of Cargoyleosaurus in Figure 12.6. College of Eastern Utah Prehistoric Museum, Pice, Utah.
FIGURE 12.5 Tail club of the Late Cretaceous ankylosaurid Ankylosaurus, composed of paired osteoderms. Denver Museum Science and Nature, Denver, Colorado.
FIGURE 12.6 Gargoyleosaurus parkpini, a Late Jurassic ankylosaurid from the Morrison Formation of Wyoming, USA. Denver Museum of Science and Nature, Denver, Colorado.

caudal vertebrae and as a result lacked clubs. Some nodosaurids did carry a large spike that projected horizontally from their sides, which would have deterred most predators from random biting. An interesting observation made about some of these spikes is that they show muscle-attachment sites at their bases and could articulate with one another. These features support the hypothesis that the spikes could have been moved in a scissor-like motion. Such a function would have had a deleterious effect on any living flesh caught between them, such as phalanges on a theropod manus.

Ankylosaurs made their earliest known appearance in the fossil record in the Middle Jurassic, represented by the basal ankylosaur Sarcolestes of England, which was succeeded by the Late Jurassic Dracopelta of Portugal and Mymoorapelta and Gargoyleosaurus of western North America (Fig. 12.6). The earliest known and most basal nodosaurid is the Early Cretaceous Cedarpelta of Utah; no Jurassic nodosaurids are so far known. Cedarpelta differs from all other nodosaurids because of its general paucity of cranial osteoderms, meaning that later members of its lineage were more adorned. The large number of Early Cretaceous ankylosaurs bespeaks of their later diversification: Hylaeosaurus and Polacanthus of western Europe; Gastonia, Pawpawsaurus, Sauropelta, and Silvisaurus of western North America; and Shamosaurus of Mongolia. The Early Cretaceous Minmi of Australia was once considered a transitional form between ankylosaurids and nodosaurids, but is now classified as a primitive ankylosaurid. This confusion is understandable in the light of how this genus is known only from two partial skeletons, of which only one has a skull. Indeed, the fragmentary nature of the single specimens that define many ankylosaur species means that their designations as "ankylosaurid" or "nodosaurid" have been subject to revisions with each new find and more detailed cladistic analyses.

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