Dinosaurs Ornithopods.
The postcranial anatomy of the ornithopods was fairly uniform. All members had hind legs that were much longer and sturdier than their forelegs. The thighbone (femur) was always shorter than the shinbones (tibia and fibula) and usually bore a prominent process, called the fourth trochanter, just above mid-length for the attachment of the retractor, or walking, muscles. The pelvis was expanded, usually with an elongated and broad prepubic process for the attachment of the protractor, or recovery, leg muscles. The tail was long and sometimes quite deep and flat-sided (commonly equated with tails that afford a sculling action in swimming animals). The vertebral spines of the tail and thoracic region were reinforced by a rhomboidal latticework of ossified tendons. These features, taken together, have led to the conclusion that all ornithopods were at least facultatively bipedal but that most kinds could assume a quadrupedal stance and gait. Some may have been obligatory bipeds‹for example, Fabrosaurus, whose forelimbs were only one-third the length of the hind limbs and whose hands seem better constructed for grasping than for walking.
Ornithopod feet were modified from the primitive five-toed pattern. The three middle toes served as the functional foot; the inside toe was shortened and often held off the ground; and the outside toe was greatly reduced or absent altogether. The toes terminated in broad, almost hooflike bones, especially in the duckbills. The hand reflected the primitive five-part design, although the first, or inside, finger may have been missing in some. The fingers usually ended in broad, blunt bones rather than in claws. In the duckbills the fingers apparently were encased in a mittenlike device sometimes thought to have been a paddle to aid in swimming; more probably it simply broadened the hand to better support the animal's weight on soft ground.
Members of the infraorder differ from one another in the structure of their skulls. In particular, their jaws and teeth show considerable variation among the different families. In the fabrosaurids the teeth were simple, leaf-shaped, laterally compressed elements arranged in a single front-to-back row in each jaw. They were not set in from the outer cheek surface as in most ornithopods. Small incisor-like teeth existed on the premaxillary bones above, but no teeth were present on the predentary below. The lower jaw had no coronoid process for large muscle attachment, and the upper temporal fenestra (the jaw muscle site) was relatively small. Upper and lower teeth alternated in position when the jaw was closed; they did not occlude directly.
In heterodontosaurs the cheek teeth were crowded together into long rows and set in slightly from the outer cheek surface. They occluded directly to form distinct chisellike cutting edges with a self-sharpening mechanism maintained by hard enamel on the outer side of the upper teeth and the inner side of the lower. There were prominent upper and lower tusklike teeth at the front of the mouth, the upper tusks in the premaxillaries, the lower tusks on the dentary bones of the jaw and not on the predentary. The upper temporal fenestra, larger than that of the fabrosaurids, and a prominent coronoid process beneath it indicate the existence of much larger jaw muscles than in the fabrosaurids.
The hypsilophodonts had cheek teeth arranged in tightly packed rows set well in from the outer cheek surfaces. The teeth occluded directly, and the opposing rows formed a long shearing edge similar to that of the heterodontosaurs. There was, however, no canine tusk either above or below. The premaxillaries had small, simple, incisor-like teeth above the beak-covered, toothless predentary. Strong coronoid processes extended up from the lower jaws toward the moderate-size upper temporal fenestrae.
In the iguanodonts the coronoid processes and temporal openings provided for still larger jaw muscles, but the cheek teeth were less regular and compacted than in the primitive ornithopods and consequently did not occlude as uniformly. Both the premaxillaries and the predentary were toothless but probably were sheathed in horny beaks.
Specialization of the teeth and jaws reached a pinnacle in the hadrosaurs, or duck-billed ornithopods. Here a very prominent, robust coronoid process jutted out and up at the back of the stout lower jaw. A large adductor muscle chamber was present above this process and beneath the lateral and upper temporal fenestrae‹clear evidence of powerful jaw muscles. The dentition consisted of numerous tightly compacted teeth crowded into large grinding batteries. The battery in each jaw was composed of as many as 200 functional and replacement teeth with distinct, well-defined wear or grinding surfaces that resulted from very exact occlusion. Both the predentary and premaxillaries were toothless but were enclosed in broad horny beaks, or bills. These bills apparently had edges sharp enough to close, clamlike, for shredding and stripping leaves or needles from low shrubs and branches. Pine needles have been identified in duck-billed Dinosaur remains and presumably represent stomach contents.
Some varieties of hadrosaurs are also noted for the peculiar crests and processes on
the top of the head. These structures were expansions of the skull composed almost
entirely of the nasal bones. In genera like Corythosaurus , Lambeosaurus ,
Parasaurolophus, and a few others, the crests were hollow, containing a series of
median and lateral chambers that formed a convoluted passage from the nostrils to the
trachea.
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Alternative Theory On Dinosaurs
Paleontology
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