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505 Ankylosaurs Kenneth Carpenter 24 Ankylosaurs were short-limbed, four-legged, armor-plated dinosaurs, with a long, wide body (Fig. 24.1). It is a body encased in armor or osteoderms that characterizes the group (see also Plates 27–29). The most prominent osteoderms consisted of keeled or flat plates of bone embedded in the skin (hence the term “osteoderm”–bone skin). These osteoderms are sometimes supplemented with spines or spikes on the body and tail. In at least one group, there was also a bone club on the end of the tail. At one time it was thought that the rough texture of the skulls was due to armor fusing to the bone surface. However, recent studies have shown that in most (but not all) cases, the texture is due to remodeling of the skull surface by the overlying skin or scales (Vickaryous et al. 2001; Carpenter et al. 2001). The taxonomy of ankylosaurs remains confusing because so many of the species are named from fragmentary material. The Ankylosauria can be divided into three families, the Polacanthidae (informally called polacanthids ), the Nodosauridae (or nodosaurids), and the Ankylosauridae (or 24.1. Top and side view of Saichania showing the characteristic features of ankylosaurs, including the wide body and extensive covering of body armor, or osteoderms, embedded in the skin. Kenneth Carpenter 506 ankylosaurids), which is subdivided into the subfamilies Shamosaurinae (shamosaurines) and Ankylosaurinae (ankylosaurines). This threefold family division is not universally accepted (e.g., Vickaryous et al. 2004) despite features in the skull, scapula, and osteoderms that unify the various species into the three families (Fig 24.2). Skull The ankylosaur skull is greatly modified as compared to that of other dinosaurs . Whereas in most dinosaurs the skull is deeper than wide, in ankylosaurs , this is reversed (Fig. 24.3) due to lateral expansion or bulging of the maxillary bones above the tooth row (more on this below). As a general rule of thumb, the skulls of polacanthids are triangular, being as wide, or nearly as wide, as long in a manner similar to ankylosaurids (Fig. 24.3A, C). Skeletal Features 24.2. Representatives of the three families of ankylosaurs: A, Polacanthidae (Gargoyleosaurus); B, Nodosauridae (Sauropelta); C, Ankylosauridae (Euoplocephalus). Scales in m. 3.59.236.219] Project MUSE (2024-04-24 12:52 GMT) Ankylosaurs 507 In contrast, the skulls of nodosaurids (Fig. 24.3B) are proportionally longer and less wide (although still wider than typical in dinosaurs). The surface of the skull in many ankylosaurs is rough, or knobby. At one time it was thought that this texture was due to thin sheets of osteoderms fusing to the skull surface. But recent histological studies, where thin sections are examined microscopically, have shown that in many examples the texturing is the result of remodeling of the skull surface by skin, with each bump and knob being the site of a former scale (Carpenter et al. 2001; Vickaryous et al. 2001). The scale pattern is especially noticeable on the skull of Edmontonia, where the textured surface consists of large, slightly raised regions that are symmetrically arranged (see Fig. 24.3B). In contrast, the scale pattern on polacanthid and ankylosaurid skulls is typically smaller, more numerous, and asymmetrical (Fig. 24.3A, C). Ankylosaur skulls also typically have horns of sorts projecting from the rear corners. These structures 24.3. Representative skulls of the three families in top, side, and bottom views: A, Polacanthidae (Gargoyleosaurus); B, Nodosauridae (Edmontonia); C, Ankylosauridae (Ankylosaurus). Abbreviations: c–choana; cp–cheek plate; en–external nares; jqjh–jugalquadratojugal horn; ltf–lateral temporal fenestra; m–mandible; o–orbit; oc–occipital condyle; pmb–premaxillary beak; posqh–postorbitalsquamosal horn; pt–pterygoid (damaged in A); suo–supraorbital; tr–tooth row. Scales in cm. Kenneth Carpenter 508 are actually outgrowths of the postorbital and/or squamosal (upper horn), and of the jugal and/or quadratojugal (lower horn). This remodeling and outgrowths from the skull bones have obliterated most of the cranial sutures in adults. In juveniles, such as of Pinacosaurus, or subadults, such as of Cedarpelta, the cranial sutures remain distinct, and these are important for determining skull bone homologies. Of the five major skull openings typically seen in the dinosaurs (external nares, antorbital fenestra, supratemporal fenestra, and lateral temporal fenestra), ankylosaurs have only three: external nares, orbit and lateral temporal fenestra (Fig. 24.3). The loss of the antorbital and supratemporal fenestrae occurs by expansion of the adjacent bones closing the openings (a dimple may mark the antorbital fenestra in some ankylosaurids). The...

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