Dinosaur Extinction
Notes

  1. Evolutionary biologists argue that the group 'Dinosauria' never really became extinct, because birds are descended from dinosaurs and hence a part of the 'Dinosauria' lineage. This may be perfectly true, but it only begs the original question. To satisfy their objection, we could rephrase the question, "Why did non-avian dinosaurs become extinct?" The problem still hasn't gone away.

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  1. There is always some uncertainty as to the exact timing of any extinction, and the extinction of the dinosaurs is certainly no exception. Workers in Montana have discovered dinosaur remains, mostly teeth, in the sediments of a stream channel that is clearly Tertiary in age. In places, this stream actually eroded parts of the terminal Cretaceous sediments in the area, and also contains fossil remains of Early Tertiary mammals. It has been argued that this proves nothing, as the dinosaur remains could have been washed out of earlier (i.e. Cretaceous) sediments, and redeposited in the stream channel. The discoverers argue, however, that the remains are unabraded, the teeth are still sharp, and show none of the wear associated with stream redeposition. The discovery of dinosaur remains above the terminal Cretaceous deposits not in a stream channel would undoubtedly resolve the issue, but so far such remains have not been discovered.

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  1. Percentage of extinction figures are for comparison only, and are based on the number of fossil species known. In no way does this actually reflect the actual number of species that might have been extant before or after any extinction event. The vast majority of the 2 million or so species known on Earth today are insects, and over half of those known species are beetles. Insects have been found at least as far back as the Carboniferous Period, yet very few fossil insects are known for any geological period. It can only be assumed that insects were vastly more common than the fossil record would indicate. As insects appear to have been rather resilient to mass extinction events, one must surmise that the actual percentage of species exterminated was greatly lower than the given figures indicate.

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  1. Many researchers have argued that the number of different species of dinosaurs diminished considerably during the last 2 million years or so of the Late Cretaceous Period. Whether this represents simply a randomness in the sampling process or a real trend remains hotly debated. This "randomness of sampling" argument is a serious problem in identifying extinctions, because the mere fact that no fossils of a species have been discovered certainly doesn't prove that it was extinct, particularly with creatures that were as rarely preserved as dinosaurs.

    A more convincing statistical argument is based on the relative percentages of each species found within a particular formation. A drastic increase in the abundance of one species relative to other species in the same population is strongly suggestive that the other species are becoming less competitive in their environment, and may be dying out. If those other species are nowhere to be found in successive strata, a strong argument for early extinction can be made. This is precisely the case that is presented for latest Cretaceous dinosaurs in western North America.

    A similar reduction in species diversity is reported among calcium-secreting foraminiferans in the uppermost Cretaceous chalk deposits. The number of different species present in the very latest chalk layers, immediately below the Fish Clay, is significantly less than is found in lower, or older, layers. Here, the "randomness of sampling" objection is not applicable, because such vast number of individuals are preserved.

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  1. At the time of the Alvarezes' discovery, plate tectonics and sea floor spreading had long been documented. Periodic reversals of the Earth's magnetic field are recorded in the sea floor, as bands of magnetized basaltic material. As the Atlantic Ocean widens, new sea floor is deposited by lava flows from fissures along the Mid-Atlantic Ridge. As the new lava cools and hardens, it 'records' the orientation of the Earth's magnetic field. As the rate at which new sea floor is being created is known, the duration of each field reversal can be determined.

    The Fish Clay, and some of the chalk deposits on either side of it, were all deposited during what is known as Chron C-29-R, a brief period when the magnetic field was opposite of what it is now. C-29-R was known, from other measurements, to have lasted less than 500,000 years. Thus, the interval during which the clay layer was deposited could not have lasted long enough for cosmic dust to have contributed all of the iridium concentration.

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  1. There are many paleoclimate computer models in use today, but so far as I know, all of them lack predictive power. The most recent information that I have suggests that they are basically at the stage of "adjust the parameters of the model until it agrees with the observed data."

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  1. Evidently, at some time earlier in the Late Cretaceous, Mongolia and Western North America were in contact, because the early ceratopsians were able to migrate from Mongolia, where the earliest and most primitive ceratopsians are found, to North America, where they had ample time to diversify into over a dozen different genera.

    There is no evidence, however, that this migration, at a time when sea levels were near their highest, was over the Bering land bridge. Whatever migration route the ceratopsians followed seems to have been one way only, as the entire ceratopsian lineage appears to have died out in Mongolia, and all known later forms are found only in western North America.

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