Ornithopoda

Ornithopods (ornitho - bird; pod - foot) were the cows, deer, bison, wild horses, antelope, and sheep of the Mesozoic (Figure 7.1, see p. 133). Magnificent herbivores all, they were one of the most numerous, diverse and longest-lived groups in all Dinosauria. From the Jurassic, when they first appeared, until the end of the Cretaceous, when they all went extinct, ornithopods evolved nearly 100 species at present count.

Ornithopods spread all over the globe. They ranged from near the then-equator to such high latitudes as the north slope of Alaska, the Yukon, and Spitsbergen in the Northern Hemisphere, and Seymour Island, Antarctica, and the southeast coast of Australia in the Southern Hemisphere (Figures 7.2 and 7.3). Local conditions in these regions varied widely, so ornithopods lived in quite diverse habitats and in a wide range of climates.

Figure 7.2. Global distribution of Heterodontosauridae and basal Euornithopoda.
Figure 7.3. Global distribution of Iguanodontia.

They also evolved a range of sizes: early in their history, ornithopods were generally small (ranging from 1 to 2 m in length); however, later some members of the group attained quite large body sizes (upward of 12 m; Figure 7.4).

Ornithopods Dinosaurs

We know as much about ornithopods as about almost any other group of dinosaurs: Iguanodon was a charter member of Sir Richard Owen's original 1842 Dinosauria (see Chapter 14). Hadrosaurids ("duckbills") are known from single bones to huge bonebeds. Their remains include skin impressions and ossified tendons, as well as delicate skull bones such as sclerotic rings (that support the eyeball), stapes (the thin rod of bone that transmits sound from the eardrum to the brain), and hyoid bones (delicate bones that support the tongue). Paleontologists have also found hadrosaurid eggs and all growth stages represented, from hatchling, to "teenager," to adult. Ornithopod footprints and trackways abound in many parts of the world.

Who were the ornithopods?

As we have seen, ornithopods are genasaurian cerapodans (Figure 7.5). As ornithopod phy-logeny is currently understood, there is a basic split between some primitive ornithopods, including Agilisaurus and Hexinlusaurus, and the remaining ornithopods, Euornithopoda (eu - true). Within euornithopods, iguanodontians, and hadrosaurids are two important monophyletic groups.

Dinosaurs Cladogram Derived Character

Ornithischia

Figure 7.5. Cladogram of Genasauria, monophyly of Ornithopoda. Derived characters include: at 1, pronounced ventral offset of the premaxillary tooth row relative to the maxillary tooth row, crescentic paroccipital processes, strong depression of the mandibular condyle beneath the level of the upper and lower tooth rows, elongation of the lateral process ofthe premaxilla to contact the lacrimal and/or prefrontal; at 2, scarf-like suture between postorbital and jugal, inflated edge on the orbital margin of the postorbital, deep postacetabular blade on the ilium, well-developed brevis shelf, laterally swollen ischial peduncle, elongate and narrow prepubic process.

Ornithischia

Ornithopod lives and lifestyles

Gettin' around. Ornithopods functioned as bipeds and quadrupeds, with both locomotor modes commonly occurring in the same beast (Box 7.1). The smallest were predominantly bipedal, with gracile bones that suggest agility and speed. When eating or standing still, however, they also may have adopted a quadrupedal stance. Some of the larger ornithopods, however, such as Iguanodon, may have functioned more as full-time quadrupeds, only going bipedal when in a hurry. A number of larger ornithopods have a sturdy wrist and a hand with thickened hoof-like nails on the central digits, a hand that clearly was capable of considerable weight support. Interestingly, juveniles may have been more bipedal than their fully grown, adult counterparts.

In all cases, the tail was long, muscular, strengthened by ossified tendons, and held at or near horizontal, making an excellent counterbalance for the front of the animal. In general, the powerful hindlimbs tend to be at least as long as, and in some cases more than twice the length of, the forelimbs.

How fast could these dinosaurs have traveled? Larger iguanodontians such as hadro-saurids may have been able to reach 15 to 20 km/h during a sustained run, but upward of 50 km/h on short sprints. Quadrupedal galloping appears unlikely given the rigidity of the vertebral column and the limited movement of the shoulder against the ribcage and sternum. For smaller ornithopods, running speeds were higher. Maximum speeds were probably on the order of 60 km/h (see Box 12.3).

Arms and hands. The primitive ornithopod Heterodontosaurus may have used powerful fore-limbs and clawed hands to grab at vegetation or to dig up roots and tubers. On the other hand, many euornithopods appear to have had less powerful forelimbs, and likely used their hands to grasp at leaves and branches, bringing foliage closer to the mouth so that it could be nipped off by the toothed beak.

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