The discovery of fossils gives us the only hard facts about the distribution of dinosaurs across the Mesozoic Earth. As such, an understanding of dinosaur distribution is strongly dependent on the availability of fossils, which is subject, in turn, to the availability of fossil deposits of Mesozoic age. The time of the dinosaurs spanned an enormous gulf in time—164 million years—composing the better part of three long geologic periods: the Triassic, the Jurassic, and the Cretaceous.
There is a natural and unavoidable bias in the fossil record that affects the availability of fossils. Paleontologists speak of bias in the fossil record to explain why some forms of prehistoric life are more likely to become fossilized than others. The likelihood that an organism will become a fossil depends on a combination of factors, including the living conditions and anatomical form of the organism as well as the way in which it died. When compared with other organisms, vertebrates have a preservational advantage because their anatomy includes an abundance of hard, bony parts. The large size of many dinosaur bones also makes their discovery more likely. The best of skeletons may never be discovered, however, if they are not deposited in a location that is easily accessible to paleontologists.
The surface of the Earth was subject to great geologic change during the Mesozoic and the following Cenozoic. As a consequence, not all of the terrestrial environments that were once occupied by dinosaurs have been preserved at the surface of today's world. Pick any continent, and you will find numerous gaps in the strati-graphic record of dinosaurs. In eastern North America, there are no dinosaur-bearing deposits available that span a 55 million-year-gap from the Early Jurassic to the Early Cretaceous epochs. In South America, evidence of dinosaurs from the Early Jurassic and Late Jurassic Epochs is virtually unknown. Evidence of dinosaurs in China for the entire Late Triassic and Jurassic Epochs is mostly nonexistent. Dinosaurs of Australia are known only from the Middle Jurassic and Early Cretaceous Epochs, while the few Antarctic dinosaurs that have been found date only to the Early Jurassic and Late Cretaceous. Late Cretaceous dinosaurs from Africa are very poorly known, while Indian and Malagasy dinosaurs are almost totally unknown from the Late Jurassic to the mid-Cretaceous. Even in western North America, probably the best-sampled region for dinosaurs in the world, dinosaurs from the Middle Jurassic are essentially unknown.
Add to these geologic gaps in the record of dinosaurs the factor of nonrecovery of fossils that might actually be there, and it would appear that the full extent of dinosaur diversity and populations can never be known for certain. Paleontologists currently have validated about 500 dinosaur genera, almost half of which have been described since 1990. Considering that this number represents only the kinds of dinosaurs that have been accessible to science because of various natural biases affecting preservation, the number of actual dinosaur genera that may have existed, including those that have yet to be discovered, is estimated to be between 1,500 and 2,000.
The known geographic distribution of sauropods is naturally affected by the biases just described. Western North America is famous for its wealth of sauropod specimens from the Late Jurassic, but because the fossil record of Early Cretaceous dinosaurs is so poor on much of the continent, little is known of North American sauropods that transitioned into the Early Cretaceous. This is not to say that sauropods disappeared from North America during the Early Cretaceous, but that paleontologists merely have less evidence for them because of the scarcity of fossil deposits of the appropriate age.
Even given a bias in the fossil record toward the discovery of sauropods outside of North America, the Cretaceous Period represents a definite shift in the anatomical adaptations and evolutionary success of some sauropod groups over others. Gone were the diplodocids, camarasaurs, and most of the brachiosaurs—the bulky giants that towered over all other life-forms. In their place was a proliferation of stockier and often smaller sauropods, including the titanosaurs. The transition from the Jurassic to the Cretaceous seems to have been a defining time for the future of sauropods. Their numbers gradually diminished in the Northern Hemisphere until, by the Late Cretaceous, more than 80 percent of known sauropods hailed from regions in the Southern Hemisphere that included South America, Africa, Madagascar, and India (then in a transitional stage, moving from the Southern to the Northern Hemisphere). One thing can be said for certain: The number and diversity of sauropods gradually diminished during the Cretaceous Period, even as the success of the herbivorous ornithopods and ceratopsians and feathered dinosaurs, large theropods, and birds raged.
The reason for this change in the fortune of Northern Hemisphere sauropods is not fully understood. American paleontologist Spencer Lucas suggests that the fate of sauropods might be tied to two related developments in the world of dinosaurs: The rise of flowering plants and the evolution and diversification of ornithopod dinosaurs adapted to eat such flowering plants. The fact that sau-ropods changed little in their eating adaptations might mean that they were poorly adapted to eat and digest flowering plants even as the range of tall gymnosperm trees began to slacken, outcompeted by angiosperms. Sauropods also may have had difficulty competing for what plant material was left for them because of the growing numbers of ornithopods that could eat practically anything, tough-skinned evergreens and flowering plants alike. Lucas notes that the relative scarcity of ornithopod fossils in the Southern Hemisphere corresponds with a stronger presence of sauropods in continents below the Equator during the Late Cretaceous Epoch. A lack of success of ornithopods in the southern continents might help explain the abundance of the sauropods in those locations.
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