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The Ankylosauridae, or ankylosaurids, are the stereotypical armor dinosaurs having a large club of bone on the end of the tail. The body is encased in armor plates like the nodosaurids', but long spines on the neck and shoulder are not present. In some species, however, there are tall, thin-walled cones located on the back and shoulder that superficially resemble spines. Ankylosaurus, the name best known by non-paleontologists, is actually very poorly known scientifically. Only three specimens have been found and only one of these, the holotype, has been described in any detail (Brown, 1908). Perhaps the best known scientifically is Euoplocephalus because it is known from several specimens, including at least two that have armor preserved in life position (Nopcsa, 1928). This has allowed accurate life-reconstructions (e.g., Carpenter, 1982, 1997).
Figure 1. Skeleton of Euoplocephalus, the best known ankylosaurid from the Late Cretaceous of North America. Characteristic features of the ankylosaurids include the tail club and two rows of neck armor.
The earliest ankylosaurid is Cedarpelta from the Lower Cretaceous (Albian) of Utah (Carpenter et al., 2001). The taxon is represented by two partial skulls and skeletons having an estimated skull length of 60 cm. One skull is mostly disarticulated, allowing the individual bones of an ankylosaur to be described in multiple view for the first time. The skull shows some similarities with Shamosaurus from the Lower Cretaceous of Mongolia, especially in its boxy appearance.
Ankylosaurids diversified during the Campanian (83.5-71.5 mya), but only in Asia and North America. What prevented their migration to South America is puzzling considering that nodosaurids were not hindered. Apparently the ecological requirements of nodosaurids and ankylosaurids were different enough to form a barrier to ankylosaurids but not to nodosaurids. Such an interpretation is complicated by the apparent extinction of the ankylosaurids in North America at about the same time as the extinction of the nodosaurids (i.e., pre-asteroid impact). If true, this would imply that ankylosaurids were also affected by the loss of habitat during the regression of the Cretaceous Seaway. As for the Asian ankylosaurids, the end of the Cretaceous extinction of all dinosaurs is too poorly documented to know when ankylosaurid extinction occurred.