Tetrapod Zoology

Welcome to day 2 of the 'ankylosaur week' series - for the background on this go see day 1 on Hungarosaurus. Before talking about today's ankylosaur, here's a quick 'everything you wanted to know about ankylosaurs but were afraid to ask'. Ankylosaurs were ornithischian dinosaurs from the Jurassic and Cretaceous, they have a highly modified skull where the sutures are closed, the external openings are mostly or entirely closed, the dorsal surface is covered with thick bony layers and bony bosses, and the small teeth are inset from the jaw margins. Horns often project from the cheek and rear margin of the skull. Within the group, snout shape varies from long and narrow to short and very broad. All ankylosaurs were quadrupeds (obligate bipedality has been suggested for one taxon, but this is almost certainly wrong); their limbs are typically short and strongly muscled. An average ankylosaur was probably 4-5 m long, but there were giants twice as size, as well as dwarfs of only 2-3 m long or less.

Their hips are broad, sometimes incredibly so, and some of them would have been extraordinarily broad, flat-backed animals. All of these features indicate that ankylosaurs were herbivorous (insect-eating has been suggested in the past, as has a marine seaweed-eating lifestyle), and gut contents from an Australian taxon confirm this. Tooth and jaw morphology indicates that ankylosaurs mostly fed by orthal pulping (simple vertical mashing of plant material), but complex jaw movements may have been present in some taxa (Rybczynski & Vickaryous 2001). Various details indicate that cheeks were probably present. Complex air passages show that ankylosaurid ankylosaurs in particular were doing something odd with inspired air, but we aren't quite sure what this was, and thick skulls and a mobile joint in the braincase suggest to some workers that head-butting was practised by at least some species.

Rows of armour lined the ankylosaur neck, body and tail: articulated armour is rare, but often seems to have consisted of plate-like scutes that were arranged in half-circles around the dorsal surface of the neck, and a covering of keeled plates and/or spikes across the back, hips and tail. A large fused plate covered the hip region in some taxa, one poorly known ankylosaur even had a 'belly plate', and scutes protected the limbs in at least some forms. The armour appears to have been absent in juveniles. Ankylosaurid ankylosaurs possessed a tail club formed from large osteoderms connected to a handle-like structure formed by fused caudal vertebrae. The old idea that ankylosaurs were 'passive defenders' which just sat like tortoises and waited for predators to go away seems extraordinarily unlikely given the evolution of big shoulder spikes, laterally projecting tail plates, and tail clubs. However, armour may have been important in intraspecific combat and display as much as in predator avoidance. Some time I'll elaborate on the crazy idea that tail-clubbed ankylosaurs were mimics of iguanodontian ornithopods.

Ankylosaurs are thyreophorans, and share a list of characters with stegosaurs, the plated dinosaurs. A recent claim that Scelidosaurus [shown in adjacent image] from Lower Jurassic England is closer to ankylosaurs than stegosaurs are, and that the Ankylosauria + Scelidosaurus clade be named Ankylosauromorpha (Carpenter 2001), has proved less well supported than the idea that stegosaurs and ankylosaurs - grouped together as Eurypoda - form a clade (Butler et al. 2008). Since the 1970s, ankylosaurs have been classified in two groups: Nodosauridae and Ankylosauridae (Coombs 1978). Some recent work indicates that a group of taxa that share pelvic shields and long shoulder spines that bear posterior grooves deserve to be separated as Polacanthidae (this isn't a new name: Polacanthidae was first named in 1911).

Huh - that went on for much longer than I'd planned. To business...

Reaching about 8 m in length, Tarchia gigantea from the Maastrichtian Nemegt Formation of Mongolia is the largest of Asian ankylosaurid ankylosaurs, and also the last. The Tarchia image used at the top of the article was produced by Mineo Shiraishi and is used with permission (© M. Shiraishi, all rights reserved). Mineo's gallery of excellent dinosaur reconstructions can be found here. A club-tailed, broad-skulled form, Tarchia is probably a basal member of the ankylosaurid clade Ankylosaurinae. It has been suggested that the Asian ankylosaurids Tarchia and Saichania are close relatives of the North American Nodocephalosaurus (Sullivan 1999), as all three share bulbous, polygonal, symmetrically arranged bony lumps on their nasals and frontals [Tarchia skull shown here borrowed from wikipedia].

The 40-cm-long skull was not only covered in bony bumps, but also decorated with large, flattened triangular spines which projected sideways from its cheeks. Ankylosaurids could have used these in fights with members of their own species, perhaps butting one another on the side of the head. Tarchia also has backwardly curved horns on the posterior corners of its skull. Originally named in 1956 as a new species of the North American genus Dyoplosaurus (itself now regarded as synonymous with Euoplocephalus), Tarchia was named as a distinct genus in 1977 when Teresa Maryanska recognised a second species (T. kielanae). Though Maryanska's idea about the generic distinctness of Tarchia is regarded as correct, T. kielanae is mostly regarded as a synonym of T. gigantea.

More ankylosaurian action tomorrow!

Refs - -

Butler, R. J., Upchurch, P. & Norman D. B. 2008. The phylogeny of the ornithischian dinosaurs. Journal of Systematic Palaeontology 6, 1-40.

Carpenter, K. 2001. Phylogenetic analysis of the Ankylosauria. In Carpenter, K. (ed) The Armored Dinosaurs. Indiana University Press (Bloomington and Indianapolis), pp. 455-483.

Coombs, W. P. 1978. The families of the ornithischian dinosaur order Ankylosauria. Palaeontology 21, 143-170.

Rybczynski, N. & Vickaryous, M. K. 2001. Evidence of complex jaw movement in the Late Cretaceous ankylosaurid Euoplocephalus tutus (Dinosauria: Thyreophora). In Carpenter, K. (ed) The Armored Dinosaurs. Indiana University Press (Bloomington and Indianapolis), pp. 299-317.

Sullivan, R. M. 1999. Nodocephalosaurus kirtlandensis, gen. et sp. nov., a new ankylosaurid dinosaur (Ornithischia: Ankylosauria) from the Upper Cretaceous Kirtland Formation (Upper Campanian), San Juan Basin, New Mexico. Journal of Vertebrate Paleontology 19, 126-139.