Ankylosauria Ankylosauridae and Nodosauridae

Ankylosauria derives its name from the encasement of its members in dermal armor (ankylos = "fused," sauros = "lizard"), which is why many paleontologists describe them as "tanks." This military allusion is apt in terms of the function of the armor, as certainly defense may have been one function, but its use for offense and species recognition is also possible. The following characters help to identify members of Ankylosauria (Fig. 12.2):

■ A broad, laterally compressed skull with armor covering the cranial sutures and the supratemporal fenestra.

■ Deeply inset cheek teeth, with cingulum on dentary and maxillary teeth.

Ankylosauridae
FIGURE 12.2 Defining character traits of Clade Ankylosauria: broad, armored skull with deeply inset cheek teeth; synsacrum; horizontal ilium; closed acetabulum; and body armor.

■ Fusion of the precaudal third and fourth dorsal vertebrae with the sacrum into a structure called a synsacrum.

■ A horizontally oriented ilium (evident from a lateral view) and a reduced pubis.

■ Separation of the pubis from the acetabulum, with a closing of the acetabulum so that it is more like a cup for fitting the femur, rather than an open hole.

Most of these features reflect adaptations that provided for protection but also allowed an ankylosaur's body to move about freely, even though armored. For example, the synsacrum provided support in the axial skeleton for body armor that covered the dorsal surface. This trait appeared in combination with a secondarily closed acetabulum. The latter is a unique feature in dinosaurs, which are partially defined by having an open acetabulum as a primitive trait (Chapters 1 and 5). The horizontally-oriented ilium also provided attachments for the muscles needed to accommodate a body laden with osteoderms. Furthermore, ankylosaur limb bones are thicker in proportion to limb lengths than most dinosaur limb bones. This means that their limbs were adapted for upright and compact ankylosaur bodies, which had great weights relative to volume. Ankylosaur fore limbs and hind limbs ended in a broad manus and pes, each with stout toes terminated by unguals (hooves). Unlike some other major dinosaur clades, digit counts for each foot differed for some ankylosaurs. For example, a five-toed manus is known for the Early Cretaceous Shamosaurus and Sauropelta, as well as the Late Cretaceous Talarurus and Pinacosaurus, but this same number has not been documented conclusively in other ankylosaurs. Numbers of digits in the pes of ankylosaurs range from five (Sauropelta) to four (Nodosaurus, Talarurus) to three (Euoplocephalus). Consequently, ankylosaur tracks are difficult to distinguish from similarly-sized stegosaur or cer-atopsian tracks on the basis of manus-pes digit numbers alone (Chapter 14).

Ankylosauridae and Nodosauridae, the ' two clades comprising Ankylosauria, are difficult to distinguish from one another.

Ankylosaurus Osteoderm
FIGURE 12.3 Closely spaced osteoderms of a typical Late Jurassic ankylosaur, which likely provided some excellent protection against predators from the same time. College of Eastern Utah Prehistoric Museum, Price, Utah.

Ankylosaurs can be classified into two clades by looking at either end of one. For example, most skulls of ankylosaurids have:

1 nares that face anteriorly;

2 horns at the posterior dorsal and ventral corners; and

3 a triangular profile when viewed by looking down on the dorsal surface.

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