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Department
Natural Science
Course
NATS 1540
Professor
Robert Levine
Semester
Fall

Description
DINOSAUR EXTINCTION THEORIES Michael E. Williams Dinosaur Extinction Alvarezs proposed test: Dinosaur remains in latest Cretaceous rocks were randomly distributed throughout most of the Hell Creek Formation, with the exception of a three- meter gap at the top. The gap indicated the extinction of the dinosaurs before the iridium was deposited, and that there was no indication that the population was declining. He believed dinosaurs were reproducing at a constant rate until they were suddenly wiped out, and believed that the gap between the highest known occurrence of dinosaur bone and the iridium layer lacked statistical significance. Taphonomic Filter: a variety of factors, such as scavengers, decomposers, weathering, destruction by soil acids, burial and subsequent erosion, may intervene to prevent an animals incorporation into the fossil record. Two prime factors influence the number of remains passing through the filter and into the fossil record: 1) the relative coarseness of the filter 2) the number of remains being sent into it Evaluating the Two Theories: 1) Alvarez believed a catastrophic mass extinction wiped out the dinosaurs. Alvarezs proposed test was based upon an implicit assumption of stable populations continuing through time until they were demolished at the time the iridium was deposited. The fossils sought to fill the gap before the iridium layer proved that this theory failed. However, a random distribution through time suggests constant mortality rates, and this means that there was no catastrophic mass extinction. The only distribution that would provide evidence for Alvarezs case in an increase in mortality rates, and that means an increase in the number of remains. 2) Based on the differing taphonomic predictions of extinctions, Archibalds theory suggests that the distribution of remains is compatible with a gradual extinction. Not only is there no increase in the number of remains encountered as we approach the iridium layer, but the remains become rarer until not a single unequivocal occurrence is known at the time the iridium was deposited. Archibald pointed out that the height of the last known occurrence of dinosaur bone varies from section to section. Falsification: Alvarezs was no falsified because there are a variety of factors that might account for the absence of bone where the model predicts it ought to be: 1) The dinosaurs may have migrated out of the Western Interior only to meet their fate at an unknown time and at an unknown tempo at one or more locations where the transitional rocks are not preserved. 2) The taphonomic filter may have effectively shut down even as the skeletal input was maximized. If we ever had a catastrophic mass extinction with no preserved evidence, we would by definition have no evidence of a catastrophe. We cannot currently discount migration and could never discount an extinction that destroys all the remains and leaves no trace of its coming. Finding unequivocal Paleocene dinosaurs would potentially falsify Alvarezs argument, but only if the theory that dinosaurs died before the iridium is false; if it is true, there are no post-iridium dinosaurs and the theory cannot be falsified. The decline in both numbers and kinds of dinosaurs suggested by the evidence of the channels and the sparseness of the last few meters of the Hell Creek Formation are consistent with a gradual decline or possibly an accelerating decline, but not a catastrophic one. This distribution is not consistent with either Alvarezs own test or the more discriminating predictions based on taphonomic considerations. There is simply no evidence for a catastrophic mass extinction. Richard Cowen the K-T Extinction These organisms suddenly became extinct about 65 MA, at the end of the Cretaceous Period: - Almost all the large vertebrates on Earth, on land, at sea, and in the air (all dinosaurs, plesiosaurs, mosasaurs, and pterosaurs) - Most plankton and many tropical invertebrates, especially reef-dwellers - Many land plants were severely affected However, most groups of organisms survived: insects, mammals, birds, and flowering plants on land, and fishes, corals and mollusks in the ocean went on to diversify tremendously soon after the end of the Cretaceous. The K-T extinctions were worldwide, affecting all the major continents and oceans; geographic change, oceanographic change, climatic change, or an extraterrestrial event are globally effective agents that could explain the extinction. Two hypotheses suggest a violent end to the Cretaceous: An asteroid or cometary impact - a meteorite big enough to be called a small asteroid hit Earth at the time of the K-T Extinction - Evidence of this impact: Alvarez discovered extraordinary amounts of iridium in the rocks at the K-T Boundary - Iridium is very rare on Earth; its much rarer than gold, but the K-T boundary clay iridium is usually twice as abundant as gold, and the same high ratio is found in meteorites - A layer of iridium is in the ocean floor sediments in the Pacific Ocean and the Caribbean; it is found in continental shelf deposits of Europe; from Canada to New Mexico, it occurs in coal-bearing rock sequences laid down on floodplains and deltas iridium layer has been identified in more than 100 places around the Earth - Iridium is only present in the boundary rocks. Therefore it was deposited in a single large spike (a very short event) the iridium spike at the K-T boundary is very large- Alvarez suggested that iridium was scattered worldwide from a cloud of debris that formed as an asteroid struck somewhere on Earth - The asteroid may have been about 10 km across and blasted a crater in the crust about 100 km across - Shocked quartz, tektites (unusually-shaped large glass beads) and spherules (tiny glass spheres) are found in meteorite impact structures - All over North America, the K-T boundary clay contains glass spherules and just above the clay is a thinner layer that contains iridium and shocked quartz; the zone of shocked quartz extends west onto the Pacific Ocean floor, but it is rare in K-T boundary rocks elsewhere - Based on this evidence, the K-T impact occurred on or near North America with the iridium coming from the vaporized asteroid and the shocked quartz coming from the continental rocks it hit. Haiti was 800 km from Chicxulub at the end of the Cretaceous; the K-T boundary at Beloc and other Haiti sites is marked by a normal but thick clay boundary layer than consists mainly of o spherules and black tektites (formed at about 1300 C from two different kinds of rocks) and yellow tektites (from evaporate sediments with a high content of sulfate and carbonate) - The K-T crater is an egg-shaped structure, 180 km across, called Chicxulub, found buried under the sediments of the Yucatan peninsula of Mexico. The igneous rock under the Chicxulub contains high levels of iridium and its age is 65 Ma - Some places contain fossils
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