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Introduction

The ankylosaurs, or armored-plated dinosaurs, first appeared in the Kimmeridgian (~156-152 my) of the Late Jurassic, and persisted 91 million years until they became extinct at the end of the Cretaceous (65 my). They had a global distribution, including Antarctica (Gasparini et al., 1996). As fossils, ankylosaurs are most abundant in upper Lower Cretaceous strata (Barremian to Albian, ~127-98.2 my) and in upper Upper Cretaceous strata (Campanian and Maastrichtian, ~83.5-65 my) (Carpenter, 1990, 1997a,b; Carpenter and Kirkland 1998; Carpenter et al, 1999). Not surprisingly, these intervals also mark their greatest taxonomic diversity. The specimens from the Upper Cretaceous include some exceptionally well preserved skeletons with much of the armor preserved in its original position (Carpenter, 1982; Maryanska, 1977).

Ankylosaurs from the Kimmeridgian of Colorado and Wyoming include two taxa, Gargoyleosaurus and Mymoorapelta from the dinosaur-rich Morrison Formation (Carpenter et al, 1998; Kirkland and Carpenter, 1994). These two taxa belong to the primitive ankylosaur family Polacanthidae (Carpenter 2001), first identified from the Lower Cretaceous of Europe (Mantell, 1833). One member, Gastonia, is perhaps the single most abundant ankylosaur known. It is represented by thousands of elements from three bone beds, as well as a few isolated occurrences, in the Cedar Mountain Formation of eastern Utah. One monospecific bonebed contains a minimum of 22 individuals (Carpenter, in preparation).

Minmi is an enigmatic ankylosaur from the Lower Cretaceous of Australia. It is more derived than Scelidosaurus in having a skull that is wider than tall, closed opening in front of the orbit and atop the skull, and a nearly closed hip socket. However, it lacks any of the derived characters that would place it in one of the three ankylosaur families (Carpenter, 2001).

Ankylosaurs underwent their greatest evolution during the Cretaceous (142-65 my), especially in the northern hemisphere. The Early Cretaceous was especially momentous for ankylosaur evolution in that two ankylosaur families, the Nodosauridae and Ankylosauridae, first appear in the fossil record (Carpenter 2001). Their synapomorphies, however, indicate a much longer evolutionary history, with ghost lineages possibly extending back to the Middle Jurassic (Carpenter 2001). The slight temporal overlap between the Nodosauridae and Polacanthidae in the Cedar Mountain Formation (Lower Cretaceous) raises the possibility of competitive extinction of the polacanthids by the end of the Aptian (Carpenter 2001, fig. 21.11). Nodosaurid taxa dominate the ankylosaur fossil record in the late Early Cretaceous (Aptian-Albian, 121-98.5 my) and middle Cretaceous (Cenomanian-Santonian, 98.5-83.5). Many of the nodosaurid occurrences are from marine deposits. These specimens represent carcasses of individuals that were carried to sea from their near-shore coastal plain habitat. The absence of ankylosaurids from marine deposits suggests a preference for more inland or upland habitats. Since nonmarine strata are rare for the middle Cretaceous, the observed dominance of nodosaurids may be more apparent than real.

Nevertheless, this dominance changed during the latest Cretaceous (Campanian-Maastrichtian, 83.5-65 my) when ankylosaurids increased in diversity and relative abundance. They are especially diverse in Asia, where nodosaurids are as yet unknown. In marked contrast, ankylosaurids are unknown in Europe. The Asian and North American ankylosaurids are known from both arid (eolian) and lowland coastal environments.

The extinction of the ankylosaurs at the end of the Cretaceous does not appear to have been abrupt. Both their relative abundance and diversity shows a decline during the last three million years of the Cretaceous in North America (Carpenter and Breithaupt, 1986), suggesting that, in North America at least, they became extinct before the asteroid impact (Hildebrand et al., 1991).

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