The species extinctions might have been from continent collision, rather than meteor strikes.
pbs.org
Q and A
sci.monash.edu.au
en.wikipedia.org
These and a selection of other extinction events are outlined below. The articles about individual mass extinctions describe their effects in more detail and discuss theories about their causes.
1. 488 million years ago — a series of mass extinctions at the Cambrian-Ordovician transition (the Cambrian-Ordovician extinction events) eliminated many brachiopods and conodonts and severely reduced the number of trilobite species.
2. 444 million years ago — at the Ordovician-Silurian transition two Ordovician-Silurian extinction events occurred, and together these are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct.
3. 360 million years ago — near the Devonian-Carboniferous transition (the Late Devonian extinction) a prolonged series of extinctions led to the elimination of about 70% of all species. This was not a sudden event — the period of decline lasted perhaps as long as 20 million years, and there is evidence for a series of extinction pulses within this period.
4. 251 million years ago — at the Permian-Triassic transition Earth's largest extinction (the P/Tr or Permian-Triassic extinction event) killed 53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species (including plants, insects, and vertebrate animals). The "Great Dying" had enormous evolutionary significance: on land it ended the dominance of the mammal-like reptiles and created the opportunity for archosaurs and then dinosaurs to become the dominant land vertebrates; in the seas the percentage of animals that were sessile dropped from 67% to 50%.
The whole of the late Permian was a difficult time for at least marine life - even before the "Great Dying", the diagram shows a late-Permian level of extinction large enough to qualify for inclusion in the "Big Five".
5. 200 million years ago — at the Triassic-Jurassic transition (the Triassic-Jurassic extinction event) about 20% of all marine families as well as most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians were eliminated.
6. 65 million years ago — at the Cretaceous-Paleogene transition (the K/T or Cretaceous-Tertiary extinction event) about 50% of all species became extinct. It has great significance for humans because it ended the reign of the dinosaurs and opened the way for mammals to become the dominant land vertebrates; and in the seas it reduced the percentage of sessile animals again, to about 33%. The K/T extinction was rather uneven — some groups of organisms became extinct, some suffered heavy losses and some appear to have got off relatively lightly.
7. Present day — the Holocene extinction event. A 1998 survey by the American Museum of Natural History found that 70% of biologists view the present era as part of a mass extinction event, possibly one of the fastest ever. Some, such as E. O. Wilson of Harvard University, predict that man's destruction of the biosphere could cause the extinction of one-half of all species in the next 100 years. Research and conservation efforts, such as the IUCN's annual "Red List" of threatened species, all point to an ongoing period of enhanced extinction, though some offer much lower rates and hence longer time scales before the onset of catastrophic damage. The extinction of many megafauna near the end of the most recent ice age is also sometimes considered a part of the Holocene extinction event.[4] Some paleontologists, however, question whether the available data support a comparison with mass extinctions in the past.[5]
EC<:-} |
