Dinosaurs Extinction.
Faunal changes
During the 150 million years or so in which Dinosaur existed, there were repeated changes in the dinosaur communities. The stratigraphic record is too incomplete to establish whether these faunal turnovers were uniform, taking place at a steady rate, or episodic, but it seems to indicate the latter. The evidence shows a moderately rich Late Triassic fauna of plateosaurs and other prosauropods, primitive ornithopods, and theropods. Most of these kinds of Dinosaur are not represented in Early Jurassic strata, and by Late Jurassic time the fauna was very different, with sauropods, more advanced ornithopods, stegosaurs, and a variety of theropods predominating.
Early Cretaceous strata contain few sauropods (all new), a few stegosaurian holdovers, new kinds of theropods and ornithopods, and the first ankylosaurs. By Late Cretaceous time, sauropods apparently were rare, and advanced ornithopods (duckbills) had become the dominant browsers. A variety of new theropods of all sizes were widespread, stegosaurs no longer existed, and the ankylosaurs were represented by a collection of new kinds that were prominent in the North American and Asian faunas. A totally new group of dinosaurs, the horned ceratopsians, had appeared in Asia and had successfully colonized North America. The overall picture is quite clear: throughout Mesozoic time there was an ongoing turnover, or dying out and renewal, of dinosaurian life.
It is important to note that extinction is a normal, universal occurrence. On balance, it is as commonplace as is the appearance of new species. Old life-forms decline and diminish in numbers beyond the critical threshold below which the reproduction rate can no longer sustain the population. Ecological space and opportunities are created as a result of the void left by an extinguished species.
Sometimes new forms that originate by phylogenetic diversification are suitably adapted to make use of the vacated niche. That does not always happen, however, and the niche may remain empty or be parceled out among many occupants.
In a sense, the history of animal and plant life is replete with successions -early primitive kinds replaced by new and often more advanced kinds. In most instances, the stratigraphic record gives too little information to show whether the old forms were actually displaced by the new successors or the new kinds simply expanded into the declining population's ecological niches. Nor is the stratigraphic sequence adequate to document actual evolutionary lineages except in the most general way. For example, among dinosaurs, the sauropod group is generally thought to have originated from melanorosaurid prosauropods, but the sequence of ancestral to descendant species is not known specifically. Likewise, the hadrosaurs are widely believed to have derived from an Early Cretaceous iguanodont-like ornithopod (perhaps Probactrosaurus of Asia), but again the exact lineage is unknown.
Because of such stratigraphic gaps, it is not possible to say precisely how long Dinosaur species or genera actually existed. Moreover, because of the somewhat inconsistent, and thus inexact, anatomic definitions of the various Dinosaur taxa, the duration of any particular kind can be gauged only approximately‹usually by stratigraphic boundaries and presumed first and last occurrences.
The latter often coincide with geologic age boundaries; in fact, the absence of particular life-forms usually defines geologic boundaries. The Mesozoic ³moments² of apparently high extinction levels among Dinosaur were around the end of the Triassic (208 million years ago), the end of the Jurassic (144 million years ago), and of course the end of the Cretaceous (66.4 million years ago). Undoubtedly, there were lesser extinction peaks at other times in between, but these are poorly documented by fossil records.
The KT boundary event
It was not only the Dinosaur that disappeared at the end of the Mesozoic. Many other organisms became extinct
or were greatly reduced in abundance and diversity. Among these were the flying reptiles (pterosaurs), sea
reptiles (plesiosaurs and mosasaurs), and ichthyosaurs, the last disappearing slightly before the
CretaceousTertiary boundary known as the KT boundary. Strangely, turtles, crocodilians, lizards, and snakes
were not affected or were affected only slightly. Effects on amphibians and mammals were mild. But other
organisms, such as the molluscan ammonites, the belemnites and certain bivalves, the bryozoans, the crinoids,
and a number of planktonic life-forms like foraminifera, radiolarians, coccolithophores, and diatoms‹were
decimated.
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Original Source - http://www.crystalinks.com/
Alternative Theory On Dinosaurs
Paleontology
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