Ankylosauromorpha

Introduction

Ankylosauromorphs are more popularly known as the armored-plated dinosaurs. They first appeared in the Early Jurassic (~208-204 my) and became extinct at the end of the Cretaceous (65 my) (Carpenter 2001). Their fossils are known from every continent, including Antarctica (which was not ice covered during the Mesozoic) (Gasparini et al., 1996). Ankylosauromorphs are taxonomically most diverse in the last half of the Early Cretaceous (~130-112 my) and again in the later half of the Late Cretaceous (~83.5-65 my) (Carpenter and Kirkland 1998; Carpenter et al, 1999).

The oldest ankylosauromorph is Scelidosaurus from the Sinemurian (204-198 my) of England (Carpenter 2001). It bears many of the hallmarks of later ankylosaurs (see below) indicating an earlier origin for the ankylosauromorphs, possibly in the Hettangian (208-204 my). Although Scelidosaurus is unequivocally an ankylosaur, its primitiveness prevents it from being assigned to any of the three ankylosaur families, grouped as the Ankylosauria (Carpenter, 2001); it is the closest sister taxon of the Ankylosauria.

Characteristics

Ankylosauromorphs are quadrupedal ornithischians with a proportionally wider body than seen in most other ornithischians. This wide body accommodates a large gut, suggesting that hindgut fermentation (analogous to that seen in horses) was important to break down their vegetation diet. The most distinctive feature of ankylosauromorphs is the armor that encases the entire body (Carpenter 2001). The armor was undoubtedly embedded within the skin, much like the armor of crocodiles today. The armor consisted of outwardly projecting spines and larger, narrower spikes, thin walled cone-like plates, keeled nearly flat plates, solid, keeled scutes, and small rounded ossicles that filled the gaps between the larger armor, as well as covered the belly. The armor is arranged along the top and sides of the neck as bands or half-rings, in rows or alternating rows along the back, sides and tail. As in crocodiles, this armor was arranged in distinct patterns in the different genera, especially in the neck region (Carpenter, 1982, 1984, 1990). In Scelidosaurus, the armor is predominately thin-walled, laterally compressed cones, although some smaller solid scutes also occur. The armor on the neck consists of two rows, and that on the body is arranged in rows extending parallel to the body.

The skull surface of ankylosauromorphs is extensively modified either by reworking of the bone surface by the overlying scales, or by fusion of armor (Vickaryous et al., 2001; Carpenter, 2001; Carpenter et al. 2001). The rough textured surface is called "ornamentation", although at one time it was thought be to exclusively fused armor (see Ankylosauria). In Scelidosaurus, remodeling of the bone is confined to the postorbital, maxilla, jugal, and mid-section of the lower jaw.

Scelidosaurus retains the primitive ornithischian shaped skull, which is taller than wide, unlike the low, wide skull of the Ankylosauria. Furthermore, Scelidosaurus still retains the five pairs of skull openings characteristic of primitive ornithischians. These openings are the nares at the front of the snout, the antorbital fenestrae located in front of the orbits, the orbits, the supratemporal fenestrae located atop the skull roof, and the lateral temporal fenestra located behind the orbit. In the Ankylosauria, the antorbital and supratemporal fenestra have fused shut, and the lateral temporal fenestra is covered over in one family (Ankylosauridae).

Premaxillary teeth are present in Scelidosaurus, as well as primitively in the Ankylosauria (Carpenter 2001). The characteristic cropping beak of the Ankylosauria is not developed in Scelidosaurus, suggesting that it was a selective feeder. The cheek region in Scelidosaurus is not inset to the great extent seen in the Ankylosauria The cheek teeth are leaf-shaped, much like those of the primitive ornithischian, Lesothosaurus. Wear facets on the teeth indicate a puncture-crushing method of food processing between the teeth (Barrett, 2001). In many, but not all, ankylosaurs, the teeth are modified and are less leaf-shaped, especially by the development of a cingulum, or expansion of the crown just above the root.

The vertebral column in ankylosauromorphs consists of seven or eight cervicals, about 16 dorsals, three or four sacrals, and around 40 or more caudals. The neck region is typically short, whereas the trunk is very long. The pelvis retains the three pairs of bones seen in other dinosaurs, the large upper ilium, the lower, forward positioned pubis, and the lower, rearward projecting ischium. However, ankylosauromorphs have modified these bones from the standard ornithischian pattern. The ilium is expanded horizontally and angled away from the midline, thus producing a wide hip to accommodate a wide gut. The width of the pelvis is even greater in the Ankylosauria than in Scelidosaurus indicating the increased importance in hindgut fermentation. The pubis retains the primitive overall shape seen in Lesothosaurus, but the pubic body is expanded and rotated to partially close the back wall of the acetabulum. This trend is continued in the Ankylosauria until it is completely closed off so that the acetabulum is a cup. The caudals are typically short near the pelvis and become elongated near the middle of the tail. The limbs of ankylosauromorphs are rather stoutly built to carry the ponderous body, even more so in the wider-bodied Ankylosauria.

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Figure 1. Pelvis of Scelidosaurus in top and side view (A), compared with that of a more "primitive", bipedal ornithischian Thescelosaurus (B), Stegosaurus (C) and the ankylosaur Euoplocephalus (D). Note that in side view, the pelvis of Scelidosaurus retains the primitive ornithischian appearance (most similar to that of Thescelosaurus in side view). However in top view, the pelvis is more like that of the ankylosaur Euoplocephalus. The horizontal expansion of the entire ilium (upper pelvic bone) is a uniquely ankylosaur character. In Stegosaurus, only a part of the ilium is expanded horizontally. Modified from Carpenter 2001.

Discussion of Phylogenetic Relationships

Traditionally, Scelidosaurus was considered the closest sister taxon to the Stegosauria and Ankylosauria (Sereno 1986; Maryanska and Osmolska, 1985; Fastovsky and Weishampel, 1996). Together, these three taxa, plus several other armored forms, constituted the Thyreophora (Sereno 1986). But a restudy of Scelidosaurus shows that it has several of the apomorphies of the Ankylosauria in the pelvis and armor, thus cannot be a close sister taxon to the Stegosauria (see above; also Carpenter 2001). On the other hand, these apomorphies occur in the Ankylosauria, so are plesiomorphic for the group. To show this relationship, the Ankylosauromorpha consists of Scelidosaurus + Ankylosauria.

Other Names for Ankylosauromorpha

• armor-plated dinosaurs
• Armored-plated dinosaurs

References

Barrett, P.M. 2001. Tooth wear and possible jaw action of Scelidosaurus harrisoni Owen and a reviw of feeding mechanisms in other thyreophoran dinosaurs. Pp. 25-52, in K. Carpenter (ed.). The Armored Dinosaurs. Indiana University Press, Bloomington.

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. 2001. Phylogenetic Analysis of the Ankylosauria. Pp. 455-483 in K. Carpenter (ed.). The Armored Dinosaurs. Indiana University Press, Bloomington.

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. 2001. Disarticulated skull of a new primitive ankylosaurid from the Lower Cretaceous of Eastern Utah. Pp. 211-238 in K. Carpenter (ed.). The Armored Dinosaurs. Indiana University Press, Bloomington.

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.

Fastovsky, D.E., and D.B. Weishampel. 1996. The Evolution and Extinction of the Dinosaurs. Cambridge University Press, New York.

Maryanska, T. and H. Osmolska. 1985. On ornithischian phylogeny. Acta Palaeontologica Polonica, 30:137-150.

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. 2001. Cranial ornamentation of ankylosaurs (Ornithischia: Thyreophora): re-appraisal of developmental hypotheses. Pp. 318-340 in K. Carpenter (ed.). The Armored Dinosaurs. Indiana University Press, Bloomington.