The oldest ankylosaur is Scelidosaurus from the Sinemurian (204-198 my) of England (Carpenter, in press). It bears many of the hallmarks of later ankylosaurs (see below) indicating an earlier origin of the group possibly in the Hettangian (208-204 my). Although Scelidosaurus is unequivocally an ankylosaur, it does not show any of the derived characteristics of the three ankylosaur families recognized here. The record of Middle Jurassic (Bajocian - Callovian, 183-163 my) ankylosaurs is very poor, with the named taxa considered nomen dubia because of their fragmentary nature (Carpenter, in press). More complete specimens are known from the Kimmeridgian (156-152 mya) of Colorado and Wyoming, and include two taxa, Gargoyleosaurus and Mymoorapelta (Carpenter et al., 1998; Kirkland and Carpenter, 1994) of the primitive ankylosaur family Polacanthidae (Kirkland, 1998; Carpenter, in press). This family persisted into the Early Cretaceous, and 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, in press).
Ankylosaurs underwent their greatest evolution during the Cretaceous (142-65 my), especially in the northern hemisphere. The Early Cretaceous was especially momentous, in that two ankylosaur families, the Nodosauridae and Ankylosauridae, first appear in the fossil record (Carpenter, in press). Their synapomorphies, however, indicate a much longer evolutionary history, with ghost lineages possibly extending back to the Middle Jurassic (Carpenter, in press). 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, in press, 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).
The skull of ankylosaurs has traditionally been thought to have armor fused to its surface (e.g., Maryanska, 1977; Sereno, 1986; Carpenter, 1997a, b). However, new evidence indicates that the "armor" is partially (Vickaryous, et al., in press) or entirely (Carpenter et al., in press) the result of remodeling of the bone surface. Hence, the term "ornamentation" is preferred to "armor" in referring to this surface texture. The ornamentation apparently corresponds to the former overlying scales (Vickaryous et al., in press). In nodosaurids, the ornamentation consists of a few, large scale patterns symmetrically arranged on the top and sides of the skull and lower jaw (Coombs, 1978a). In contrast, the ornamentation in polacanthids and ankylosaurids consisted of many, small, asymmetrically arranged scale patterns.
The overall shape of the skulls of the three families differs markedly, although there are more similarities between polacanthids and ankylosaurids (Kirkland, 1998). That of nodosaurids is significantly longer than wide, whereas it is nearly as wide or wider than long in polacanthids and ankylosaurids (Coombs, 1978a; Carpenter, in press). In addition, two of the five pairs of skull openings characteristic of most dinosaurs have fused shut. These two pairs of openings are the antorbital fenestrae located in front of the orbits and the supratemporal fenestrae located atop the skull roof. In addition, in ankylosaurids, a third pair of openings, the lateral temporal fenestrae located behind the orbits, are also closed.
Premaxillary teeth are present in primitive members of all families (Carpenter, in press). The cheek teeth of nodosaurids tend to be proportionally larger than in polacanthids and ankylosaurids. Except in polacanthids, the base of the crown often has a shelf or cingulum on both sides. In ankylosaurids, this cingulum may be replaced by a swelling of the crown base, but in polacanthids, the primitive ornithischian non-cingulated, non-swollen crown is retained. Wear patterns on the swollen bases and cingula indicate that chewing was more complex than simple orthal adduction (Rybczynski and Vickaryous, in press). The muzzle and beaks of nodosaurids are much narrower than those of ankylosaurids, suggesting that nodosaurids were browsers, selectively cropping plant parts or particular types of vegetation; in contrast, ankylosaurids were grazers, cropping all low vegetation (Carpenter, 1982, 1984). Polacanthids are variable, Gargoyleosaurus having a narrow muzzle and Gastonia a wider one; both share in having an unusual inverted U-shaped notch in the upper beak, the function of which is unknown.
The vertebral column in ankylosaurs consists of seven or eight cervicals, about 16 dorsals, three or four sacrals, and 20 caudals in ankylosaurids and 40 or more in nodosaurids (Carpenter, 1997a) and polacanthids. The neck region is typically short, whereas the trunk is very long. The facets for the ribs are often angled upwards so that the ribs produce a wide, barrel-like body. The pelvis is also wide, with the ilium rotated into a long, nearly horizontal structure that angles away from the midline. To provide this pelvis greater stability, the posterior five or more dorsals are fused to the sacrum producing a synsacral rod. These fused vertebrae prevent rotation of the pelvis about the vertebral column. The caudals are typically short near the pelvis, and the first one or two may actually fuse to the sacrum. The caudals rapidly become elongated, and in ankylosaurids the distal third of the tail may be fused to provide a rigid "handle" for the terminal tail club; in nodosaurids and polacanthids, the caudals remain long until near the end of the tail where the last three or so shorten rapidly.
The limbs of ankylosaurs are short relative to body length, especially in ankylosaurids. The scapula of polacanthids is a thin flange folded ventrolaterally towards the glenoid from the dorsal border of the scapula, whereas it is a long finger-like spur or prong near the glenoid in nodosaurids; in ankylosaurids, the acromion is typically a thickened area along the dorsal border of the scapula (in Pinacosaurus it is a polacanthid-like flange, although much smaller). The coracoid is usually longer than wide in nodosaurids, but about as long as wide in ankylosaurids (no complete coracoid is known for a polacanthid). The limb bones are rather stoutly built to carry the ponderous body. The humerus of ankylosaurs has a well developed deltopectoral crest for several muscles involved in locomotion (Coombs, 1978a). The humerus is proportionally more slender in nodosaurids than in ankylosaurids (Coombs, 1978b) and is intermediate in polacanthids (Carpenter, in press). The olecranon on the ulna in ankylosaurs is well developed and provides a tall lever for extensor muscles of the elbow. The pelvis is unusual among ornithischians because the ilium has rotated to a near horizontal position. In addition, the acetabulum, or hip socket, is not open but is cup-like. Most of the body weight was carried over the hind legs. Consequently, the femur is straight and pillar-like, and the tibia and fibula are short and stout; both the fore- and hind-feet are short and stout as well. The femur of nodosaurids and polacanthids is more slender in build than in ankylosaurids (Coombs, 1978a).
There have been several phyletic analyses of the ankylosaur families (see discussions for each family), but no comprehensive study of the group as a whole. The details of the analyses (compartmental method) are presented in Carpenter (in press), which includes several new taxonomic names not given here. Unlike most previous analyses of the families, which often relied only on cranial characters, the analyses in Carpenter (in press) used over 83 cranial and postcranial characters. Taxa represented with 50% or less missing data were excluded from the analyses in order to minimize the number of parsimonious trees. For details on the compartmental method, see Carpenter (in press) and Mishler (1994).
Carpenter, K. 1982. Skeletal and dermal armor reconstruction of Euoplocephalus tutus (Ornithischia: Ankylosauridae) from the Late Cretaceous Oldman Formation of Alberta. Canadian Journal of Earth Sciences, 19:689-697. Carpenter, K. 1984. Skeletal reconstruction and life restoration of Sauropelta (Ankylosauria: Nodosauridae) from the Cretaceous of North America. Canadian Journal of Earth Sciences, 21, p. 1491-1498. Carpenter, K. 1990. Ankylosaur systematics: example using Panoplosaurus and Edmontonia (Ankylosauria, Nodosauridae). Pp. 282-298 in K. Carpenter and P. Currie (eds.), Dinosaur Systematics: Perspectives and Approaches. Cambridge University Press, New York. Carpenter, K. 1997a. Ankylosauria. Pp. 16-20 in P. Currie, and K. Padian (eds.), The Encyclopedia of Dinosaurs. Academic Press, San Diego,. Carpenter, K. 1997b. Ankylosaurs. Pp. 308-316 in J. Farlow and M. Brett-Surman (eds.), The Complete Dinosaur. Indiana University Press, Bloomington, Indiana. Carpenter, K. in press. Phylogenetic Analysis of the Ankylosauria. In K. Carpenter (ed.). The Armored Dinosaurs. Indiana University Press. Carpenter, K., and B. Breithaupt, 1986. Latest Cretaceous occurrence of nodosaurid ankylosaurs (Dinosauria, Ornithischia) in western North America and the gradual extinction of the dinosaurs. Journal of Vertebrate Paleontology, 6:251-257. Carpenter, K. and J.I. Kirkland. 1998. Review of Lower and Middle Cretaceous ankylosaurs from North America. Pp. 249-270 in S.G. Lucas, J.I. Kirkland and J.W. Estep (eds.), Lower and Middle Cretaceous Terrestrial Ecosystems. New Mexico Museum of Natural History and Science Bulletin No. 14. Carpenter, K., J.I. Kirkland, D. Burge and J. Bird. 1999. Ankylosaurs (Dinosauria: Ornithischia) of the Cedar Mountain Formation, Utah, and their stratigraphic distribution. Pp. 244-251 in D. Gillette (ed). Vertebrate Paleontology in Utah. Utah Geological Survey Miscellaneous Publication 99-1. Carpenter, K., J.I. Kirkland, D. Burge and J. Bird. In press. Disarticulated skull of a new primitive ankylosaurid from the Lower Cretaceous of Eastern Utah. In K. Carpenter (ed.). The Armored Dinosaurs. Indiana University Press. Carpenter, K., C. Miles and K. Cloward. 1998. Skull of a Jurassic ankylosaur (Dinosauria). Nature 393: 782-783 Coombs, W. 1972. The bony eyelid of Euoplocephalus (Reptilia: Ornithischia). Journal of Paleontology 46:637-650. Coombs, W. 1978a. The families of the ornithischian dinosaur order Ankylosauria. Journal of Paleontology 21:143-170 Coombs, W. 1978b. Forelimb muscles of the Ankylosauria (Reptilia: Ornithischia). Journal of Paleontology 52:642-658. Gasparini, Z., X. Pereda-Suberbiola and R. Molnar. 1996. New data on the ankylosaurian dinosaurs from the Late Cretaceous of the Antarctic Peninsula. Memoirs of the Queensland Museum 39:583-594. Gaston, R.W., J. Schellenbach and J.I. Kirkland. In press. Mounted Skeleton of the Polacanthine Ankylosaur Gastonia burgei. In K. Carpenter (ed.). The Armored Dinosaurs. Indiana University Press. Fastovsky, D.E. and D.B. Weishampel. 1996. The Evolution and Extinction of the Dinosaurs. Cambridge University Press, New York. Hildebrand. A.R., G.T. Penfield, D.A. King, M. Pilkington, A.Z. Camaro, S.B. Jacobson, and W.B. Boynton. 1991. Chicxulub Crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico. Geology 19:867-871. Kirkland, J.I. 1998. A polacanthine ankylosaur (Ornithischia: Dinosauria) from the Early Cretaceous (Barremian) of Eastern Utah, Pp. 271-281 in S.G. Lucas, J.I. Kirkland and J.W. Estep (eds.), Lower and Middle Cretaceous Terrestrial Ecosystems. New Mexico Museum of Natural History and Science Bulletin No. 14. Kirkland, J. and K. Carpenter. 1994. North America's first pre-Cretaceous ankylosaur (Dinosauria) from the Upper Jurassic Morrison Formation of Western Colorado. Brigham Young University Geology Studies, 40:25-42 Maryanska, T. 1977. Ankylosauridae (Dinosauria) from Mongolia. Palaeontologia Polonica, 37:85-151. Maryanska, T. and H. Osmolska. 1985. On ornithischian phylogeny. Acta Palaeontologica Polonica, 30:137-150 Mischler, B.D. 1994. Cladistic analysis of molecular and morphological data. American Journal of Physical Anthropology 94: 143-156. Rybczynski, N. and M.K. Vickaryous. In press. Evidence of Complex Jaw Movement in the Late Cretaceous Ankylosaurid, Euoplocephalus tutus (Dinosauria: Thyreophora). In K. Carpenter (ed.). The Armored Dinosaurs. Indiana University Press. Sereno, P.C. 1986. Phylogeny of the bird-hipped dinosaurs (Order Ornithischia). National Geographic Research, 2:234-256 Vickaryous, M.K., A.P. Russell and P.J Currie. In press, Cranial Ornamentation of Ankylosaurs (Ornithischia: Thyreophora): Re-appraisal of Developmental Hypotheses. In K. Carpenter (ed.). The Armored Dinosaurs. Indiana University Press.