Dinosaurs Theropoda.
Theropoda. This group includes all the other known carnivorous dinosaurs. No herbivores are recognized in the group. Theropods ranged in size from the smallest known adult dinosaur, Compsognathus , the size of an ordinary chicken and probably weighing 1 or 2 kilograms (2 to 4 pounds), up to the great Tyrannosaurus , which was 15 or more metres (50 feet) long and over 5 metres (16 to 18 feet) tall and which weighed 6 tons or more. In all theropods the hind leg bones were hollow to varying degrees‹extremely hollow and lightly built in small to medium-size animals like Compsognathus, Coelurus, and Ornitholestes and more solid in the larger forms like Allosaurus, Daspletosaurus, and Tarbosaurus. Theropods have been recovered from rocks of the Late Triassic through the latest part of the Cretaceous and from all continents except Antarctica.
In stance and gait, theropods were obligatory bipeds. Their bodies conformed to a common shape in which the hind legs were dominant and designed for support and locomotion. The forelimbs, on the other hand, had been modified from the primitive design and entirely divested of the functions of locomotion and body support. Hind limbs were either very robust and of graviportal (weight-bearing) proportions, as in Allosaurus, Megalosaurus, and the tyrannosaurids, or very slender, elongated and of cursorial (adapted for running) proportions, as in Coelurus, Coelophysis, Ornitholestes, and the ornithomimids.
Theropod feet, despite the group's name, which means ³beast (i.e., mammal) foot,² usually were designed like those of birds. Three main toes were directed forward and splayed in a V-arrangement; an additional inside toe was directed medially or backward; and the whole foot was functionally digitigrade, with the ³heel² elevated well above the ground. Toes usually bore sharp, somewhat curved claws.
The forelimbs varied widely from the slender, elongated ones of Struthiomimus, for example, to shorter, more massively constructed grasping appendages like those of Allosaurus, to the greatly abbreviated arms and hands of Tyrannosaurus. The hands typically featured long, flexible fingers with pronounced, often strongly curved claws, which must have borne sharp piercing talons. Primitive theropods like Coelophysis had four fingers, but the majority were three-fingered. Tyrannosaurids (Albertosaurus, Daspletosaurus, Tarbosaurus, and Tyrannosaurus) and apparently the diminutive Compsognathus were notable for their two-fingered hands on unusually short arms. This separation of function between fore and hind limbs set theropods apart from all other dinosaurs.
As obligatory bipeds, the theropod body plan was reorganized so that the animal, large or small, balanced at the pelvic pedestal by using its heavy tail behind and thrusting the thighs and knees forward to positions directly below the centre of gravity. To shift the centre of gravity back toward the supporting hind legs, the head and neck were arched upward, especially so in those kinds with very large heads. In addition, the bipedal stance required reinforcing the backbone by enlargement of the interspinous ligaments (between the back vertebrae) and the ligaments of the neck. The tail probably served not only as a counterweight to the large body and head but also as a dynamic inertial stabilizer, moving up and down and from side to side to counteract changes in the animal's movement and direction such as lunging and turning during an attack.
Bipedality has sometimes been explained as an adaptation for fast running or for energy conservation. The latter seems unlikely in view of several experiments showing that it requires no less energy to run or walk on two legs than it does on four. Speed does not seem to have been the primary factor either, although some investigators have claimed that a seven-ton Tyrannosaurus could achieve an unlikely velocity of 70 kilometres (45 miles) per hour - faster than a greyhound or a racehorse. Rather, because of their great weight, tyrannosaurids probably could have barely kept ahead of a charging elephant (20 kilometres per hour), whereas the more cursorial-limbed ornithomimids might have been able to keep up with a modern ostrich (70 kilometres per hour). Rather than as a specialization for running, bipedality may have come into being and perfection more as an enhancement of viewing range. Theropods all had large eyes and a wide field of vision.
Theropods that featured large heads, like Allosaurus and Tyrannosaurus, had long, strong lower jaws that undoubtedly were powered by massive jaw muscles. The skull was a highly fenestrated strut work, both for lightness and for strength, providing ample attachment areas for muscles. The jaws are noted for their complement of sharp, bladelike teeth. In nearly all theropods these laterally compressed blades had steak-knife-like serrations along the rear edge and often along the front edge as well. Among the predatory adaptations displayed by most kinds of theropods, the characteristic teeth were the most conspicuous. The diversity of the suborder Theropoda, with its various modes of predation and carnivory, is suggested by the following summary of the group's two infraorders.
Ceratosauria
These are the basal or primitive theropods of medium size like Ceratosaurus, Dilophosaurus, and perhaps
Coelophysis from the Late Triassic and Late Jurassic. They may include ancestral stock of most later theropods.
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Alternative Theory On Dinosaurs
Paleontology
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