Ornithischia, Seeley, 1887

Norman, David B, 2021, Scelidosaurus harrisonii (Dinosauria: Ornithischia) from the Early Jurassic of Dorset, England: biology and phylogenetic relationships, Zoological Journal of the Linnean Society 191 (1), pp. 1-86 : 34

Ornithischia

The earliest known (Jurassic) ornithischians were small, bipedal cursors with long, muscular cantilever tails and a herbivorous diet ( Fig. 25A ). They processed food orally using a combination of orthal pulping and irregular occlusal shearing; they were also narrowsnouted (selective) feeders capable of utilizing more readily digested succulent plant material. Their enlarged plant-adapted gut could hang beneath the pelvis and between the legs because of the retroversion of the pubis; this arrangement did not compromize either their bipedal pose or locomotor ability. A large gut is a sine qua non for ornithischians, even if some were occasional omnivores (as argued by Barrett, 2000) because all living herbivorous-omnivorous lizards are characterized by their possession of comparatively large guts. Costal aspiration was likely to have been their primary respiratory mechanism, but might have been supplemented by some form of pelvic kinesis ( Carrier & Farmer, 2000a). The energetic losses incurred by actively raising and lowering the gut, if cuirassal aspiration had been an important component of their respiratory strategy, were thereby avoided.

The subsequent evolutionary history of the ornithischian clade demonstrates that opisthopuby was maintained, albeit with elaboration of the pelvic bones in particular subclades, and several subclades independently acquired a secondarily quadrupedal style of locomotion ( Barrett & Maidment, 2017; Fig. 25B, C ).

Figures

Figure 25. Ornithischian skeletal forms. A, Eocursor. B, Edmontonia. C, Stegosaurus. No gastralia are present in these or any other known ornithischians. Images kindly provided by Scott Hartman who retains the copyright of each. Scale bars in centimetres.