![]() ![]() This hand was clearly used in walking (hooves) and in defence or display (thumb spike). The most remarkable modifications are seen in the hand (Figure 8.13(c)), in which the carpals and metacarpal 1 are fused to form a single block in the wrist, digit 1 is reduced to a thumb spike, digits 2-4 form a bunch and digits 2 and 3 have small hooves. In the skeleton (Figure 8.13(b)), the prepubic process is ex panded, the postpubic process is very short and there is a complex lattice of ossified tendons over the neural spines of all vertebrae of the trunk and tail. Iguanodon from the Lower Cretaceous of Europe (Norman, 1980, 1986b) has a horse-like skull (Figure 8.13(a)). The 'iguanodontids' are a paraphyletic group representing stages of the acquisition of advanced hadrosaurian characters (Sereno, 1986). The foot touched the ground and the power stroke in which the body moves forward was achieved by the retractors and flexors, which pulled the femur and lower leg back respectively. As the leg swung forwards, the protractors pulled the femur forwards and upwards and the extensors extended the lower leg. (After Galton 1974.)ģ Extensors, muscles that extend the lower leg: ilio-tibialis, femorotibialis.Ĥ Flexors, muscles that pull the lower leg back: iliofibu-laris, flexor tibialis internus.ĭuring a single step all of these muscles came into play. 8.12 The ornithopod Hypsilophodon: (a) skeleton in running pose (b,c) skull in lateral and ventral views (d) foot in anterior view (e) restoration of the muscles of the pelvis and hindlimb, coded according to their functions. They fall into four groups that define their functions in walking.ġ Protractors, muscles that pull the femur forwards and up: iliofemoris, puboischiofemoralis internus (upper part).Ģ Retractors, muscles that pull the femur back: pubois-chiofemoralis internus (lower part), caudifemoralis longus and brevis, adductor femoralis. The muscle names record the bones to which they attach at each end. Galton (1974) made a detailed restoration of the muscles of the hindlimbs of Hypsilophodon (Figure 8.12(e)), based on muscle scars and processes on the bones and comparison with dissections of modern birds and alligators (see Box 6.2). Of a fast-moving gazelle, especially the very long shin and foot. (Modified from Norman and Weishampel, 1985.) Ornithopod jaw mechanics: the lower jaws of Heterodontosaurus (a) slide outwards as they close, hence producing a kind of 'chewing', whereas later ornithopods have a pleurokinetic hinge, which allows the cheek portion of the skull and the maxillary teeth, shown stippled in (b), to move outwards as the jaws close (c). Perhaps this ability explains the vast success of ornithopods in the Cretaceous after the heyday of the sauropods had passed. Ceratopsians could probably also slice their plant food effectively between powerful shearing jaws, but the other herbivorous dinosaurs lacked chewing adaptations. This pattern was taken to an extreme in hadrosaurids, which had a dental battery in which five or six rows of teeth below the jaw line could also be in use (see Figure 8.14(a-c)). As in all reptiles, teeth were replaced continuously as they became worn out and replacement teeth lined up below the currently functioning teeth on the inside of the jaws. Most ornithopods had single rows of teeth on the crests of their jaws. This specialized pleurokinetic hinge (illustration (b)) produces the same lateral shearing effect (illustration (c)) as did the rotating mandible of the heterodontosaurids. The hypsilophodontids, ' iguanodontids' and hadrosaurids have essentially fixed lower jaws that simply moved up and down without distortion during chewing, while the side of the skull (maxilla, lacrimal, jugal, quadratojugal, quadrate) as well as attached palatal elements (ectopterygoid, palatine, pterygoid) flap in and out. This mode of jaw rotation provided one solution to the problem of creating an efficient shearing scissor-like cutting movement between the cheek teeth (see illustration (a)).Īll later ornithopods adopted the other option, of rotating the maxilla, in order to achieve lateral shearing, and this adaptation is said to lie at the root of the great success of the ornithopods in the Cretaceous (Norman and Weishampel, 1985). The articular-quadrate joint at the back and a special ball and socket joint at the front between the dentary and predentary, allowed rotation as the jaws opened and closed. Heterodontosaurus had a rotating lower jaw. Ornithopods hit on two solutions to the problem, and both of these are different from the mammalian technique (see pp. We chew our food before swallowing in order to aid digestion: reptiles and birds just gulp their food down. ![]() Chewing, in which the back teeth move sideways and back and forwards, is normally thought to be a special feature of mammals. ![]() Ornithopod dinosaurs were unique among reptiles in that they could chew their food.
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