STEGOSAURS, ANKYLOSAURS

Ornithischians; herbivores without living descendants

Tetraradiate pelvis (not homologous to that of birds)

    -shades: palpebrals

    -nippers: predentary

    -chewers: cheek fossa, straight quadrate

    -stiff back: ossified tendons

    -runners: (forward) long preacetabular process on ilium

                   (backward) pendant 4th trochanter on femur

All of the specialized ornithischian groups (stegosaurs, ankylosaurs, ornithopods, pachycephalosaurs, ceratopsians) appeared after the end of the Triassic; radiation after that of saurischians.

Lesothosaurus model for basal ornithischians

THYREOPHORA

Shield bearers; plated and armored dinosaurs. These are archaic ornithischians and the first to become specialized.

• jugal with transversely broad postorbital process
• parasaggital rows of keeled scutes (osteoderms)

armor

broadening of pelvis

Scutellosaurus TEXT, fig. 20.2 (Sereno 1997, Ann. Rev. Earth Planet. Sci. 25: 435-489)

• 2 m, immature animal
• 6 pmx teeth (as in Lesothosaurus, cf. prosauropods)
• long tail, bipedal, long legs (resembles Lesothosaurus where tail is shorter and the legs are longer)
• hundreds of isolated scutes
• triangular palpebral sutured to orbital margin
• armor consists of scutes (cf. ankylosaurs), short plates (cf. stegosaurs)
• metacarpals subequal, hoof-shaped unguals
• reversion to quadrupedality

• palpebral, additional osteoderm incorporated into orbital rim
• rugose areas for osteoderms on skull roof, jaws
• three rows of plates on each side of body
• four rows on each side of the tail
• broadening of pelvis, reversion to quadrupedalism
• occurrence in shallow marine sediments (cf. nodosaurs)

Completely known taxa:

Huayangosaurus

Stegosaurus, TEXT: fig. 21.1, 4 - 9 m long (partial list of synapomorphies from Sereno and Dong 1991)

Stegosaurus 2-2-2-2-?1 manus

Huayangosaurus 0-2-3-3-0 pes

• supraorbital osteoderms fused to skull
• neural canal, neural arches of anterior dorsals vertically elongate
• mid-dorsal transv. processes >45 deg above horizontal
• 5-6 sacrals, including 1-2 sd, and 1 cs
• no ossified tendons along back (no need)
• massive prox carpals block-like, fused
• pubis with oval acetabular surface (cf. Ankylosauria)
• pedal digit I absent, reduction of phalanges
• parasaggital plates grading into spines posteriorly, elongated, paired terminal caudal spines, forward projecting parascapular spine with expanded base

Huayangosaurus (Sereno and Dong 1992, JVP 12: 318-343), archaic stegosaur from poorly correlated strata of middle Jurassic age, China; small (4.3 m), like Scelidosaurus

• 7 premaxillary teeth (more than any other ornithischian)
• 25-30 mx teeth (very high for thyreophorans)
• antorbital fenenestra present
• 3 supraorbitals (cf. Stegosaurus, Pinacosaurus)
• sacral ribs not expanded or co-ossified, as in later stegosaurs (open in ankylosauria)
• parascapular spine present
• intercostal flange (ankylosaurid Saichania also possesses dermal plates in this position, as well as Thescelosaurus "Willo")
• at least one lateral row of body scutes (ankylosaurs have an armor coat)
• carpal block of 4 bones (rad, int, uln, pisiform) instead of two (rad, pisiform separate) as in later stegosaurs
• hindlimb relatively shorter than in later stegosaurs

Stegosaurus, about 8 m long

First stegosaur to be restored, the largest, most derived and best-known stegosaur

(S. stenops: 2 pair spines, 3.7 metric tonnes)

(S. ungulatus: 4 pair spines, 6.4 metric tonnes)

• first "fin-backed" dinosaurs
• secondary expansion of anterior spines (cf. Huayangosaurus) into thermal plates
• 17 plates also largest number among stegosaurs

• double row of plates in five genera of stegosaurs, no stegosaur with single median row known
• plates histologically highly vascularized, were held vertically in place (not raised or lowered)
• plates offset, probably in double row, heat dissipaters through both radiation amd convection
• more thermally stressed than other stegosaurs, where there are more spines than plates
• never been scaled to body size in different genera

• hind limbs long with respect to body length, rotated easily by front limbs, distal tail spikes obviously for defense (laterally oriented cf. Carpenter)
• flexible backs and porcupine-like defense posture

• sacral enlargment in sacrals 2-3 (cf. Giffen)
• 20 times size of brain cavity, comparable in volume to encephalization of primitive living mammals
• most likely glycogen body, as in birds - function unknown

African Kentrosaurus first evidence of herding

Chinese radiation, early Cretaceous survivors in China, UK, S Afr (see Dong 1993, Stegosaurs of Asia, Carpenter and Currie (eds.) Dinosaur Systematics, Cambridge U. Press)

Regnosaurus, jaw fragments from the Wealden (Valanginian, early Cretaceous) of England (Barrett and Upchurch 1995 (Geol. Mag. 132: 213-222). Original specimen reported in 1838, one of the oldest dinosaur names. The jaw contains numerous small teeth, a lateral ridge and deep depression below, as in Huayangosaurus
 
 
 

ARMORED DINOSAURS (ANKYLOSAURS)

• Massive, armored horned toads 5-8 m long (rump not elevated), neck and head covered with armor plates
• Short legs, divergent elbows, straight hind limbs
• Flattened skulls, eyes usually faced ventrally
• Small brains (half of reptilian average)
• Isotopically non-homogenous heat distribution

• more than one palpebral, fused to skull roof (Sereno 1997)
• median septum extends to rear end of palate (formed from pmx + na above, vo + pt below)
• mandibular symphasis slender
• fourth trochanter of femur small or absent
• preacetabular region of ilium long, divergent, postacetabular region short
• ischium unfooted
• short, spreading metapodials

• stegosaur carpal block
• stegosaur reduction of manus
• stegosaur loss of digit I, pes

ANATOMY (see TEXT)

Skull:

• osteoderms fused to top of skull
• "cheeks" shielded by large, oval scute
• antorbital and supratemporal fenestrae closed
• small, simple, primitive teeth

• anterior dorsals with vertically expanded rib articulations
• middle dorsals restricted by zygapophyses to move in a vertical plane (opposite of stegosaurs)

• (armor plates short to facilitate flexibility)
• posterior dorsals fused ("ankylosed") to ribs
• presacral "rod" of 3-6 co-ossified vertebrae
• proximal caudals with elongated transverse processes
• anterior blade of ilium horizontal, acetabulum closed

• rudimentary pubis
• femur head caps femur

NODOSAURS: TEXT, p. 473-479

There are no completely known nodosaur taxa:

Nodosaurs are the more archaic of the two groups, and appear with the earliest stegosaurs in middle Jurassic time (Sarcolestes, known from a jaw). They are known from more than a dozen genera but no good skeletons. Late Jurassic nodosaurs are known from Portugal (Dracopelta, rib cage and armor) and North America (Mymoorapelta, postcranial elements and osteoderms, Kirkland and Carpenter 1994, BYU Studies in Geology 40: 25-42). The North American and European record extends to the end of the Cretaceous; North American taxa were 5-7 m long.

Skull:

• long-headed

• "scoop" beak anterior to mx tooth rows (presence of pmx teeth primitive, cf. stegosaurs)
• nostrils face laterally
• secondary palate formed from vomers and maxillae (parallel to palate in small aviform theropods)
• quadrate exposed on side of skull posteriorly

• armor plates not excavated from below
• shoulder and neck spines present (site for carnivore attack)
• long tail with "saw" edges (tail clubs unknown)

• knob on scapula for articulation with the furcula/clavicle has migrated ventrally onto the lateral surface of the scapula

2-2-2-2-?1 - for Stegosaurus

- pes: 2-3-4-4/5-0 — for nodosaurs

2-3-4-5-0 - for Scelidosaurus, (primitive thyreophoran)

0-2-3-3-0 - for primitive stegosaur (Huayangosaurus)

The animals defended themselves with shoulder thrusts and by whipping its tail. They were adapted to feeding at about the same level as stegosaurs, but the beaks are broader and indicative of a less selective diet. There is a noticeable tendancy for skeletons to be turned upside down (bloated, floating carcasses). They seem to have frequented nearshore but terrestrial environments.
 

As many as 7 genera (Nodosaurus, Pawpawsaurus, Priconodon, Sauropelta, Silvisaurus, Stegopelta, Texasetes) of nodosaurs may be present in the early and early late Cretaceous of North America (Carpenter and Kirkland 1998 New Mexico Mus. Nat. Hist. Sci., Bull. 14: 249-270). North American Late Cretaceous nodosaurs include Panoplosaurus and Edmontonia (see Carpenter 1993, in Carpenter and Currie (eds.) Dinosaur Systematics, Cambridge U. Press).

Only one genus (Anoplosaurus) can be identified in the middle Cretaceous of Europe (England, Pereda-Suberbiola and Barrett 1999. Special Papers in Palaeontology 60: 177-208).

A dwarf form 2-3 m long (Struthiosaurus) occurred on European islands extending from Spain to Romania during terminal Cretaceous time. It possessed premaxillary teeth and unfused atlas-axis vertebrae, both of which are primitive characters (Pereda-Suberbiola 1992 Terra Research 4: 641-648; Pereda-Suberbiola et al. 1995 Bull Soc. géol. France 166: 207-211).

Cretaceous Southern Hemisphere "nodosaur" records include generically indeterminate remains from Patagonia, New Zealand, and India (Chatterjee and Kudra 1996). There are suggestions that they differ from Northern Hemisphere forms.

An unusual form 2.5-3.5 m long is known from excellent material from the middle Cretaceous of Australia (Minmi: Molnar 1996, Mem. Queensland Mus. 39: 653-668). It is peculiar:

• elongate bony rods formed from an ossified aponeurosis between the vertebral spine and the tips of the transverse processes — termed a paravertebra
• coronoid process more powerful than in any other thyreophoran
• temporal fenestrae closed
• well developed pubis (pubis small or a nubbin in nodosaurs and ankylosaurids)
• tail flexible, no indication of tail club

• possibly juvenile animal 3-4 m long
• presacral rod
• ilium horizontal plane
• mosaic armor
• teeth different from Minmi

Polocanthines (including the late Jurassic Mymoorapelta):

• long, posteriorly grooved shoulder spines
• large, solid-based, erect presacral spines
• sacral shield of fused armor
• triangular, hollow-based caudal plates along the tail

• nodosaur-like scapula, ischia sharply decurved at midlength
• ankylosaur-like skull with triangular posterolateral scute at each corner posteriorly

Referring to the genus Gastonia to be described by Kirkland (1988), Carpenter et al. 1986 JVP 16(3): 25A note:

Nodosaurid features:

• lateral temporal fenestra present
• occipital condyle without neck
• laterally projecting head, body and tail spines
• prominent scapular spine (acromion)

• asymmetrical cranial armor
• posterolaterally projecting jugal and cranial horns
• no ridge between pmx beak and mx tooth row
• broad postocular shelf - eye socket closed posteriorly

Completely known taxa:

Pinacosaurus

Euoplocephalus

More specialized in almost every feature, 9 genera

Middle Cretaceous (Aptian) to end Cretaceous (Maestrichtian),

• short headed
• "squared" beak lateral to maxillary tooth row (resembling the beak of diplodocids)
• no premaxillary teeth (unlike in diplodocids)
• nostrils face to the front
• narial passages sigmoid above secondary palate, air sinuses lateral to narial passage
• bony eyelid, postocular shelf
• pair of accessory plates form temporal "horns"
• quadratojugal lateral to quadrate

• plates commonly excavated below ("hollow")
• tail clubs (sauropod parallel)

• distal articulations of radius + ulna flat

Stegosaurus 2-2-2-2-?1

Pes: 2-3-4-4-0

Huayangosaurus 0-2-3-3-0

Ankylosaurids are known from arid environments in Central Asia and Cordilleran North America, of late Cretaceous age. Young Pinacosaurus specimens were buried together in Cretaceous dune deposits in the Mongolia; they were only 1.5 m long, and bore armor only on their heads, and two small neck rings. The remainder of their bodies were covered with little nodules of bones, which would become shield-like osteodersm. Five individuals were found associated with Velociraptor teeth; 7 more individuals were found lying parallel to each other 20 m away. The occurrence was in Inner Mongolia.

Mahiro Watabe sent me a letter from Outer Mongolia in 1996, noting that 20 immature Pinacosaurus skeletons had been found in outer Mongolia, plus one adult.

David Eberth has just documented a relatively abundance occurrence of Ankylosaurus specimens from the terminal Cretaceous Scollard Formation in Alberta — the area was evidently in a rain shadow cast by the nearby Rockies.
 

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During early Jurassic time, ornithischians were seemingly pursuing the armored route:

• heading for evolutionary stasis/dead end

• selective, Panzer nippers - food intake not high - good candidates for very low metabolic rates (distribution of body heat also supports low metabolic rates)
• a great ornithischian rennaissance to parallel that of the "bird dinosaurs" occurred in late Cretaceous

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