Palaeos Vertebrates: Ornithischia: Ankylosauria

The ankylosaurs were heavily armoured dinosaurs, 2 to 10 meters long, quadrupedal, slow moving, and fed on swampy soft vegetation. They are largely known from the Cretaceous. These creatures were equipped with bony plates, studs, spikes, and, in the case of one lineage, a bony tail-club. Despite their formidable defenses, they are never very common in the fossil record, although a large number of different species are known.

Traditionally, there were two families of ankylosaurs: the Nodosauridae and the Ankylosauridae. This picture has been somewhat obscured by the suggestion that Scelidosaurus is an ankylosaur, and the discovery of Minmi paravertebrata (shown at right), a small, long-legged running ankylosaur from the mid Cretaceous of south-east Gondwanaland. To avoid -- or perhaps create -- confusion, the term Ankylosauromorpha is used to describe all "ankylosaurs," i.e. everything closer to Ankylosaurus then to Stegosaurus. The term Ankylosauria is reserved for the traditional ankylosaurs: the last common ancestor of Ankylosaurus and Nodosaurus and all of its descendants.

The Nodosaurids appeared as small forms in the Jurassic of Europe, spreading to North America and Asia (Laurasia) during the early and middle Cretaceous. They had more extensive armoring then the Scelidosaurs, and often whole patches of external bone were fused into plates. The head was small, equipped with peg-like teeth. Nodosaurids differ from the contemporary ankylosaurids in the presence of side spines and absence of a tail club.

The ankylosaurids were the last of the armoured dinosaurs to evolve. They are distinguished by the heavy club-like tails, presumably used as a defensive weapon against predators. The primitive polacanthine group had small tail clubs, and was originally included under the Nodosauridae. (mostly MAK990512)

Descriptions

Characters: 2-3m; snout arches higher than the roof of the skull (as nodosaurs); vertebrae with bony projections along side the neural spines (paravertebrae), probably similar to ossified tendons in ornithopods; tail club probably absent; forelimbs about as long as hindlimbs; legs unusually long for ankylosaur; long post-acetabular region of the ilium; femur round in cross-section (plesiomorphic & unlike ankylosaurs); armor of large & small body scutes, large scutes on limbs, plates around neck, tear-drop shaped spiked plates behind hips, and triangular plates & large scutes along the tail; armor present as small scutes embedded in skin on ventral surface.

Notes: Widely speculated, on the basis of the long, unspecialized limbs and paravertebrae, that Minmi was adapted to running despite heavy armor. The only ankylosauromorph known from Gondwana.

Introduction: These squat, broad, heavily armoured creatures were equipped with bony plates, studs, and either spikes or a bone tail-club. Despite their amazing protection they were never very common in terms of individuals, although a large number of different species are known.

The Ankylosauria are conventionally divided into two families, the spiked Nodosauridae and the club-tailed Ankylosauridae, but there are also a few forms that don't fit in either category and would seem to constitute independent evolutionary lineages, outside the Ankylosauria, like the Australian Minmi.

The teeth are small and weak, indicating a diet of soft vegetation, and the massive limb bones are supported by particularly strong shoulder and pelvic girdles, presumably to carry the weight of the armour.

Ankylosaurian armored scutes are rectangular to oval bony plates organized in rows along the back and tail, giving them the appearance of giant dinosaurian armadillos. Smaller bony nodules fill the spaces between the large plates. The addition of spines and a solid bone tail club in some forms make these squat animals a prickly target for any carnivores. (MAK990512)

Characters: usually large (2.5-8m), heavy & broad [T00]; skull: skull flat and broad [CM90]; posterior skull wider than tall [CM90]; skull covered with armor [CM90]; cranial armor from remodeling of bone surface (= ornamentation, not fused osteoscutes); premaxilla broadly expanded on undersurface of snout [R56]; maxilla with deep, dorsally arched cheek emargination [CM90]; nares moderately large to very large, near terminal & facing laterally [R56]; sometimes with complex air passages [C97]; most fenestrae (groups differ) closed by dermal bone [T00] [C97]; orbit large and circular to elliptical [C97]; orbit walled in by supraorbitals, "postocular shelf" (from postorbital & jugal), ventral jugal extension of shelf, lacrimal shelf & "accessory bones of uncertain homology" [CM90]; extra supraorbitals (modified palpebrals) present [C97]; skull roof sutures obliterated in adults [CM90]; quadratojugal contacts postorbital [CM90]; quadrate slants forward ventrally below squamosal [CM90] [R56]; quadrate pterygoid process dorsoventrally short [CM90]; rectangular occiput [T00]; occipital condyle faces ventrally [R56]; palatal bones extensively fused [CM90]; premaxillae may have foramina for vomeronasal organ [CM90]; palate with strong central keel made up of vomers, pterygoids, premaxillae, nasals, and/or ethmoids [CM90]; keel extends to skull roof bones [CM90]; palate largely closed by pterygoids & palatines [CM90]; but secondary palate not primitive and derived separately in derived groups [CM90]; quadrate process of pterygoid laterally directed [R56]; lower jaw with co-ossified keeled plate along ventrolateral margin (like some pareiasaurs!) [CM90]; predentary small and symphysis narrow [CM90]; teeth primitive, small, laterally compressed, foliate, denticulate & non-interlocking [CM90] [C97]; primitively, premaxillary teeth present; dentary tooth row medial to maxillary teeth [CM90]; strong hyoids suggest large, mobile tongue [C97]; axial: vertebral column with 7-8 cervicals, ~16 dorsals (3-6 fused to presacral rod), 2-4 sacrals, and 20-40 caudals (1-3 fused to sacral rod) [C97] [CM90]; atlas with complete neural arch with elongate postzygapophyses ~40% of axis arch [CM90]; axis with massive odontoid and neural arch with short spine [CM90]; atlas and axis are fused in some forms [CM90]; short neck with shortened cervicals [C97]; posterior cervicals with anterior & posterior centra at different height [CM90] [T00]; cervical diapophyses horizontal or slightly ventral [CM90]; dorsal centra amphicoelous to amphiplatyan with similar length & diameter [CM90] [R56]; long & broad torso with elongated dorsal vertebrae [C97]; rib facets (diapophyses) angled upward 30-50° to give broad, barrel-shaped dorsal surface [C97]; posterior dorsal ribs tend to fuse with centra & transverse processes [CM90]; 3-6 dorsals fused with sacrum to form rigid synsacral rod, preventing rotation of the pelvis about the vertebral column [C97] [CM90]; diapophyses in sacrum + sacral rod are relatively flat [CM90]; presacral rod neural spines form continuous blade-like ridge [CM90]; sacral neural arches low & broad [CM90]; tail as long as body [CM90]; caudals are typically short near pelvis & 1-2 may fuse to sacrum; caudals rapidly elongate; neural & hemal spines of distal caudals elongated along axis of tail [CM90]; appendicular: limbs short [C97]; limb bones stout [CM90]; acromion (pseudoacromion?) typically a thickened area along dorsal border of scapula; scapula & coracoid may be co-ossified [T00]; forelimbs shorter (length 65-75% length of hind limbs) [CM90] [T00]; humerus broad, with strongly developed deltopectoral crest [C97]; ulna with very tall, long olecranon for extensor muscle (triceps?) attachment [C97]; manus short & broad, with 4-5 digits [C97]; pelvis wide, with ilium rotated into long, nearly horizontal structure that angles away from the midline, with attachment scars of retractor muscles [C97]; ilium with long anterior, short posterior processes [CM90]; ilium anterior process distally curves ventromedially [CM90]; ilium pubic peduncle absent [CM90]; ischium bent to project ventrally [C97]; pubes much reduced [C97]; anterior pubic process absent [CM90]; pubes posterior process short [CM90]; pubes almost excluded from acetabulum [CM90]; acetabulum cup-like & closed [CM90]; most of body weight carried over the hind legs; femur straight and pillar-like [CM90] [C97]; femoral neck absent [CM90]; femoral trochanters and cnemial crest poorly developed [R56]; lesser trochanter frequently absent [CM90]; 4th trochanter ridge-like [CM90]; tibia and fibula short and stout; proximal tarsals not fused to epipodials [R56]; metatarsals more slender & less spreading than metacarpals [CM90]; massive pes with 3-4 digits [C97]; other: $ armor covers dorsal surface; most of body armor plates of bone embedded as rows in the skin in species-specific pattern (so sexual selection?); plates oval to rectangular with longitudinal keel; plates commonly arranged in longitudinal rows [CM90]; gaps and limbs have small rounded osteoscutes; supplemented with spines; perhaps insectivorous as well as herbivorous [T00]; tracks show limbs close to midline, with elbow bent [CM90].

Note: [1] The majority opinion is probably now that Scelidosaurus is probably a basal ankylosaurian or ankylosaurid rather than a stem thyreophoran. [2] Very wide bodies combined with relatively weak dentition suggest highly-developed internal fermentation system. ankylosauria. [3] cranial bones poorly known because of extensive fusion and armor. [CM90].

Ankylosauridae: Ankylosaurus, Cedarpelta, Euoplocephalus, Gargoyleosaurus, Mymoorapelta.

Introduction: The ankylosaurids are the last of the armoured dinosaurs to evolve. They are distinguished by the heavy club-like tails, presumably used as a defensive weapon against predators, but lack the long spines of the nodosaurs. The primitive Polacanthine group were first classified as nodosaurids. They only had small tail clubs.

The ankylosaurid skull is heavy and re-enforced, and in comparison to the nodosaurids, wider and triangular with small horns at upper and lower corners and a mosaic of small armour plates over the skull. The beak is also wider, indicating a non-selective grazing or cropping of low vegetation, so it is clear that these animals had different feeding strategies than the contemporary nodosaurs.

In all ankylosaurids the armour is arranged in transverse bands of bony plates along the neck, back and tail.

Low, wide (75% of length or more), triangular skull, usually covered in bony plates, even eyelid (1spp); beak usually wide (non-selective cropper) but primitively longer than wide; nares usually forward; usually complex air passage includes loop; large postorbital "horns"; jugal large; lateral temporal fenestra closed by posterior expansion of postorbital; coronoid process of dentary low; small teeth with wide base (except Gargoyleosaurus), bulging base in derived species; neural spines low; 3-4 dorsals fused with sacrals & ilium = synsacrum; some other fusions; distal caudal vertebrae are stiffened with chevrons and zygapophyses; also stiffened with ossified tendons; typically distal 50% of caudal vertebrae fused; caudal bone club formed from fusion of 2 large & several small bone plates surrounding fused vertebrae; pectoral girdle massive; prominent knob at scapula- coracoid junction; deltopectoral crest not strongly projecting forward ??); "the distal condyles of the humerus are in nearly the same vertical plane as the deltopectoral crest" (Ankylosauridae); massive olecranon process on ulna (elbows flexed & close to body?); large preacetabular ilium (iliotibialis protractor?); ischium long, oriented dorsally; femur 4th trochanter distal to midpoint; distal tibia has large fibular process prevents movement between tibia & fibula); 3-4 functional toes (primitively 4); keeled bone plates (per Ankylosauridae "thin-walled scutes"); spines & spikes absent from shoulder & neck; neck has 2^nd layer of bone (?); skull surface ornamented scales); cranial sculpturing increases with age.

Nodosauridae: Amantarx, Anoplosaurus, Edmontonia, Niobrarasaurus, Nodosaurus, Panoplosaurus, Pawpawsaurus, Pricodon, Sauropelta, Silvisaurus, Stegopelta, Struthiosaurus, Texasetes.

Introduction: The nodosaurids appeared as small forms in the Jurassic of Europe, spreading to North America and Asia during the Early and mid Cretaceous. They had more extensive armoring then the scelidosaurs, and often whole patches of external bone were fused into plates. The head was small, equipped with peg-like teeth. Nodosaurids differ from the contemporary ankylosaurids in the presence of side spines and absence of a tail club.

In contrast to the broad triangular-skulled ankylosaurids, the elongated snout and relatively narrow beak seems designed for more selective cropping or browsing. A solid shield of fused keeled plates protected the pelvic area, and are supplemented by flank spines. The limbs are slimmer than those of the ankylosaurids, and these animals probably could get about faster, even able to use their flank spines to charge and attack predators. Their appearance suggests that could also draw in their front legs and crouch down like a tortoise when attacked, with the body close to the ground an hard to overturn. MAK

Similar to Ankylosaurs except: skull elongate with rounded edges (this may be the primitive condition); skull pear-shaped in dorsal view; skull length greater than width; nares face laterally; narrow rostrum & beak (selective browser?); single armor plate at center of skull; cranial plates (not bones) large and symmetrically arranged; jugal and postorbital "horns" small, if present, and usually rounded; anteriorly concave and anteroposteriorly flattened quadrate; hemispherical occipital condyle is composed of the basioccipital only; occipital condyle set off from braincase by short neck angled downwards ~50º; prominent W-shaped basioccipital tubera; basipterygoid processes usually consist of pair of rounded, rugose stubs; transversely continuous and straight posterior margin of the pterygoid aligned with the quadrate shaft; coronoid process is very tall; teeth may have cingulum; tooth row hour-glass shaped, narrowest at mid-maxilla; cutting edge of premaxilla connected to maxillary tooth row by low ridge; tall neck spines; outwardly projecting spines on the neck and shoulders; no bony tail club; spines, spikes along sides of body and on shoulders; scapula with distinct ridge-like acromion process above the glenoid for m. scapulohumeralis anterior; coracoid large and elongate; plates over pelvis may co-ossify; ischium is ventrally bent near mid-length; armor generally thick and solid.

Ornithischia: Ankylosauria

Taxa on This Page

Ankylosauria

Descriptions