Part Ii

Ornithischia: armored, horned, and duck-billed dinosaurs

Clades Dinosaurs

Ornithischia, one of the two major clades of dinosaurs, was first recognized by Harry Govier Seeley of Cambridge University, England, in 1887, but little could he have guessed at that time that ornithischians were such a diverse and anatomically wide-ranging group of closely related dinosaurs. Since then, we have learned an immense amount not only about the existence of new ornithischian taxa (e.g., Pachycephalosauria, Heterodontosauridae) but also about the detailed anatomy and evolutionary diversity of both earlier-known and newly discovered groups. Nonetheless, all the diversity and anatomical details do not cloud the issue of ornithischian monophyly: Ornithischia is monophyletic.

Diagnostic features for the entire clade abound. As we have already learned and as is clear from the name Ornithischia, the pelvis is reminiscent of that found in birds.1 That is, at least a part of the pubis has rotated backward to lie close to and parallel with the ischium; this is called the opisthopubic condition (Figure II.1). The other landmark condition of ornithischians is the presence of a bone called the predentary, an unpaired, commonly scoop-shaped bone that caps the front of the lower jaws (Figure II.2) and is found nowhere else.

Although these are the sine qua non of ornithischians, there are numerous other derived features shared by these dinosaurs, including a toothless and roughened front tip of the snout, a narrow bone

Ornithiscia Pubic Bone
Figure II. I. Left lateral view of the ornithischian pelvis as exemplified by Stegosaurus. Note that the pubic bone is rotated backwards to lie under the ischium in what is known as the opisthopubic condition. (Photograph courtesy of the Royal Ontario Museum.)

I Ornithischian dinosaurs are confusingly called "bird-hipped"; but birds themselves belong to the "lizard-hipped" clade (Saurischia) of dinosaurs (see Chapter 13).

Dinosaurs Lizard Hip Bone Name
Figure 11.2. Left lateral view of the skull of the lambeosaurine hadrosaurid Corythosaurus. Note the predentary bone capping the front of the lower jaw. (Photograph courtesy of D. B.Weishampel.)

(the palpebral) that crossed the outside of the eye socket, a jaw joint set below the level of the upper tooth row, cheek teeth with low crowns somewhat triangular in shape, at least five sacral vertebrae, and ossified tendons above the sacral region (and probably further along the vertebral column as well) for stiffening the backbone at the pelvis, among a host of others.

The basal split of Ornithischia is into the lone form Lesothosaurus and a clade termed Genasauria (gena - cheek) by P. C. Sereno (Figure II.3). The small, long-limbed herbivore Lesothosaurus (named for Lesotho, South Africa, where this dinosaur was discovered) was first christened by P. M. Galton in 1978 (Figure II.4). This Early Jurassic form had earlier been grouped with Ornithopoda, principally on the basis of primitive characters. With more recent cladistic analyses, however, it now appears to be fully ensconced as the most basal of known ornithischians.

In contrast, all remaining ornithischians - Genasauria - share the derived characters of muscular cheeks (as indicated by the deep-set position of the tooth rows, away from the sides of the face), a spout-shaped front to the mandibles, and reduction in the size of the opening on the outside of the lower jaw (the external mandibular foramen), among others.

Genasaurs subsequently split into Thyreophora and Cerapoda. Talcing each in turn, Thyreophora (thyreo - shield; phora - bearer; a reference to the fact that these animals have dermal armor) - a name originally proposed by F. Nopcsa in 1915 - consist of those genasaurs in which the j ugal (one of the cheek bones) has a transversely broad process behind the eye and there are parallel rows of keeled dermal armor scutes

Cladogram Thyreophora

Figure 11.3. Cladogram of Dinosauria, emphasizing relationships within Ornithischia; in particular those of Lesothosaurus, cerapodans, and basal thyreophorans. Derived characters include: at I opisthopubic pelvis, predentary bone, toothless and roughened tip of snout, reduced antorbital opening, palpebral bone, jaw joint set below level of the upper tooth row, cheek teeth with low subtriangular crowns, at least five sacral vertebrae, ossified tendons above the sacral region, small prepubic process along the pubis, long and thin preacetabular process on the ilium; at 2 emarginated dentition (indicating large cheek cavities), and reduction in the size of the opening on the outside of the lower jaw (the external mandibular foramen); at 3 gap between the teeth of the premaxilla and maxilla, five or fewer premaxillary teeth, finger-like anterior trochanter; at 4 transversely broad postorbital process of the jugal, parallel rows of keeled scutes on the back surface of the body.

Figure 11.3. Cladogram of Dinosauria, emphasizing relationships within Ornithischia; in particular those of Lesothosaurus, cerapodans, and basal thyreophorans. Derived characters include: at I opisthopubic pelvis, predentary bone, toothless and roughened tip of snout, reduced antorbital opening, palpebral bone, jaw joint set below level of the upper tooth row, cheek teeth with low subtriangular crowns, at least five sacral vertebrae, ossified tendons above the sacral region, small prepubic process along the pubis, long and thin preacetabular process on the ilium; at 2 emarginated dentition (indicating large cheek cavities), and reduction in the size of the opening on the outside of the lower jaw (the external mandibular foramen); at 3 gap between the teeth of the premaxilla and maxilla, five or fewer premaxillary teeth, finger-like anterior trochanter; at 4 transversely broad postorbital process of the jugal, parallel rows of keeled scutes on the back surface of the body.

on the back surface of the body. The most familiar thyreophorans are stegosaurs and ankylosaurs, but there are some more primitive, yet no less important, thyreophorans. At the base of the cladogram is Scutellosaurus (scutellum - small shield), which was described from Lower Jurassic rocks of Arizona by E. H. Colbert in 1981. Scutellosaurus was a

Lesothosaurus Skeleton
Figure 11.4. Left lateral view of the skull (a) and skeleton (b) of the basal ornithischian Lesothosaurus.
Scelidosaurus
Figure 11.5. Left lateral view of the skull and skeleton of Scelidosaurus.

small, gracile bipedal herbivore with a back covered by small, oval-shaped plates of dermal armor. Slightly higher up within Thyreophora, we come to one of the newest of these armor-bearers to be discovered, Emausaurus (EMAU is the abbreviation for Ernst-Moritz-Arndt-Universitat), also from Lower Jurassic strata (but from Germany) by H. Haubold in 1991. Finally, we have another Early Jurassic thyreophoran, Scelidosaurus (skelis - limb). This primitive ornithischian comes from the southern coast of England. Scelidosaurus is a moderate-sized, heavily built herbivore whose limb morphology suggests that it may have been at times a quadruped and at others a biped (Figure II.5). First described by R. Owen in 1860, Scelidosaurus is the closest relative to the crowning thyreophoran clade, a taxon called Eurypoda (eury - wide; pod - foot).

Eurypodans consist of both stegosaurs (Chapter 6) and ankylosaurs (Chapter 7) and share as many as 20 important features not found in any of the more basal thyreophorans. These characters include short and stocky metacarpal and metatarsal bones, reduction in the large process on the shaft of the femur called the fourth trochanter, shortened post-acetabular process of the ilium (the part of the ilium behind the hip socket), and loss of a phalanx in digit IV of the foot.

As earlier mentioned, Thyreophora has as its sister-taxon Cerapoda (cera - horn), which are those genasaurs that share a diastema (or gap) between the teeth of the premaxilla and maxilla, five or fewer pre-maxillary teeth, and finger-like lesser trochanter (a process at the top of the femur), among other derived characters. Within this large group, we encounter Ornithopoda, who we will meet in Chapter 10, and Marginocephalia. Marginocephalians (margin - margin; cephal -head) - a group united by having a narrow shelf formed from both the parietal and squamosal bones that extends over the back of the skull, a reduced contribution of the premaxillary bone to the palate (this bone primitively forms an important part of the front of the palate), and a relatively short pubis - is formed of two well-known ornithischian taxa, pachycephalosaurs (Chapter 8) and ceratop-sians (Chapter 9).

Important readings Haubold,H. 1991.EinneuerDinosaurier(Ornithischia,Thyreophora)aus dem unteren Jura des nordlichen Mitteleuropa. Revue de Paleobiologie. 9,149-177.

Norman, D. B„ Witmer, L. M. and Weishampel, D. B. 2004. Basal Ornithischia. In Weishampel, D. B., Dodson, P. and Osmolska, H. (eds.), The Dinosauria, 2nd edn. University of California Press, Berkeley, pp. 325-334.

Norman, D. B., Witmer, L. M. and Weishampel, D. B. 2004. Basal Thyreophora. In Weishampel, D. B., Dodson, P. (eds.), The Dinosauria, 2nd edn. University of California Press, Berkeley, pp. 335-342.

Sereno, P. C. 1986. Phylogeny of the bird-hipped dinosaurs (Order Ornithischia). National Geographic Research, 2, 234-256.

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