Dinosaurs Psittacosaurus.
Except for Psittacosaurus, all ceratopsians were obligate quadrupeds with a heavy, ponderous build. The leg bones were stout and the legs themselves muscular, the feet were semiplantigrade for graviportal stance and progression, and all the toes ended in ³hooves² rather than claws. As in most other four-legged animals, the rear legs were significantly longer than the front legs (again suggesting to some experts that such animals had a bipedal ancestor). The hind legs were positioned directly beneath the hip sockets and held almost straight and vertical. The front legs, on the other hand, projected out to each side from the shoulder sockets in a ³push-up² position. Consequently, the head was carried low and close to the ground. This mixed posture was probably related to the large horned head and its role in combat, the bent forelegs providing a wide stance and stable base for directing the horns at an opponent and resisting attack.
The first four neck vertebrae of ceratopsians were coossified, presumably for strength and impact resistance during hostile engagements. The first joint of the neck was unusual in that the condyle of the skull formed a nearly perfect sphere that fitted into a cuplike socket of the fused neck vertebrae. Such an arrangement would seem to have provided solid connections along with maximum freedom of the head to pivot in any direction without having to turn the body. There can be no doubt that ceratopsians used their head horns in an aggressive manner, but whether they used them as defense against possible predators or in rutting combat with other male ceratopsians, or both, is not so clear. Evidence of puncture wounds in some specimens suggests rutting encounters, but the fact that both sexes apparently had horns seems to indicate defense as their primary use.
Ornithischia
Thyreophora
The suborder Thyreophora is divided into two infraorders: Stegosauria, the plated dinosaurs, and Ankylosauria, the armoured dinosaurs.
Stegosauria With their unique bony back plates, the stegosaurs were one of the most distinctive Dinosaur varieties. Relatively few specimens have been found, chiefly from North America, Africa, Europe, and Asia. Stegosaurian remains have appeared in Early Jurassic to Early Cretaceous strata. Their immediate ancestry is uncertain, and no descendants are known. The most familiar genus is Stegosaurus , found in the Morrison Formation (Late Jurassic) of western North America. Stegosaurus was 3.7 metres in height and 9 metres in length, probably weighed two tons, and had a broad, deep body. Not all varieties of the infraorder were this large; for example, Kentrosaurus , from eastern Africa, was less than 2 metres high and 3.5 metres long.
All stegosaurs were graviportal and undoubted quadrupeds, although the massive legs were of greatly disparate lengths - the hind legs more than twice as long as the forelegs. Some authorities have interpreted the short front legs as evidence that stegosaurs were secondarily quadrupedal and had a bipedal ancestry, probably some ornithopod. This theory of ancestry may be correct, but such a precursor need not have been bipedal. In fact, among virtually all quadrupedal animals, it is the rule rather than the exception that the front legs are shorter than the hind - often much shorter. (Notable exceptions are the giraffe and, among dinosaurs, the giant sauropod Brachiosaurus, whose front legs were much longer than its rear legs.)
Whatever walking and running skills were possessed by the stegosaurs, their limb proportions must have made them move extremely slowly. The humerus of the upper arm was longer than the bones of the forearm, the femur was much longer than the shinbones, and the metapodial bones of the feet were all very short, which means that the stride must have been short. The feet were graviportal in design, with no cursorial adaptations. The stegosaurian skull was notably small, long, low, and narrow with weakly developed dentition of small, laterally compressed, leaf-shaped teeth in short, straight rows. There was no significant coronoid process on the lower jaw, nor was there much space for sizable jaw muscles. This combination of features seems entirely incompatible with the large, bulky body, especially in view of the apparent absence of Jurassic vegetation of suitable nature. The weak dentition would indicate that the food eaten must have required little preparation by the teeth and yet have provided adequate nourishment.
The best explanation is that the digestive tract may have contained a flora
of fermenting bacteria capable of breaking down the cellulose-rich Jurassic plant tissues. Perhaps that process
was assisted by a crop or gizzard full of pulverizing stomach stones (gastroliths), although none have so far been
discovered in stegosaurian specimens. Even so, such features would still not explain how these animals, with such
small mouths and dentition, could feed themselves adequately to sustain their great bulk. (The same problem has
been encountered in speculations about the feeding habits of sauropods.)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Original Source - http://www.crystalinks.com/
Alternative Theory On Dinosaurs
Paleontology
Print Version - Print This Article
