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Introduction

Our modern concept of ankylosaurs has been slow in development due to the mistaken belief that all ankylosaurs were characterized by spines projecting from the body and a large club on the end of the tail. Based on the work of Walter Coombs (1978a) and Carpenter (1982, 1984), nodosaurid ankylosaurs are now known to be characterized by outwardly projecting spines on the neck and shoulders (Fig. 1), while ankylosaurids have a tail club.

Figure 1: Skeleton of Sauropelta, the best known nodosaurid from the Early Cretaceous of North America. Characteristic features of the nodosaurids include the absence of a tail club, long spines on the neck and shoulder, bent ischium (pelvic bone seen protruding between legs and tail).

The earliest definitive nodosaurid is Sauropelta from the lower part of the Cloverly Formation, which is late Aptian in age (it is also present in the overlying Albian portions of the Cloverly Formation). Older specimens have been referred to the Nodosauridae (e.g., Sarcolestes leedsi from the Callovian of England), but these are either nomen dubia or have been referred to the family Polacanthidae (Carpenter, 2001). Sauropelta is one of the largest nodosaurids, with a length of 5.5 m (Fig. 1). Interestingly, such large nodosaurids are unknown from Cretaceous deposits of the European Archipelago, leading Pereda-Suberbiola (1992) to refer to the European nodosaurids as "dwarf insular forms." Nodosaurids appear in Antarctica by the Campanian (83-71 mya) (Gasparini et al., 1996), probably through South America because Campanian nodosaurids are also known from there (Coria and Salgado, 2001). Nodosaurids are not known from the continental deposits of China and Mongolia, although a possible specimen has been reported from the Cenomanian marine deposits of northern Japan (Hayakawa and Carpenter, 1996).

Galton (1980) has inferred a European origin for the Nodosauridae because of Middle Jurassic specimens referred to that family, and because, under the old bipartite classification of ankylosaurs, the Nodosauridae was the only family present in Europe. With the recognition of a second family in Europe, the Polacanthidae, and the dubious nature of the Middle Jurassic specimens, there is less certainty that nodosaurids originated in Europe.

The extinction of nodosaurids in North America and Europe appears to have occurred roughly at the same time in the late Maastrichtian (~66-67 mya), but well before the end of the Maastrichtian (65 mya) and the end of the dinosaurs. Carpenter and Breithaupt (1986) documented an absence of specimens above about the middle of the Lance and Hell Creek formations. Such an absence is believed to be real because of the thousands of person hours spent looking for fossils stratigraphically higher on both sides of the K-T boundary by various individuals. The extinction in both North America and Europe may have been due to a loss of coastal habitats coincidental with the regression of many shallow continental seas that occurred globally at that time (see coastal maps in Smith et al., 1994).

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