mirrored file at http://SaturnianCosmology.Org/ For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== Extinctions [ Up ] [ Origin_of_Life ] [ Genetics ] [ Evolution ] [ Extinctions ] horizontal and up links [ Earth Impact Craters ] links to lower pages * Exhibit 3.8 The Discovery of Extinctions * As geologists and paleontologists looked at the fossils in the geologic record, they discovered that time and again, multiple abundant species died out fairly suddenly. Every species dies out in the long run; the average species life span is four million years, but there is a lot of variation. For a species to die, every single individual of that species must die. Numbers of individuals per species are highly variable, ranging from a few (pandas, for example) through billions (people) to almost uncountable (some bacteria). Now, all fossils are fairly rare; only a tiny fraction of organisms ever get fossilized, so even a abrupt die-off may not leave fossils right up to the last day, which may cause the die-off to not look so sudden. This is especially the case for organisms that were always relatively rare, such as the large top-predator dinosaurs like Tyrannosaurus Rex. *Figure 3.8.1 Marine family numbers* Red dot at each major extinction. Data from Sepkoski. Graphs after Sole & Newman. The late University of Chicago paleontologist Jack Sepkoski patiently assembled records of thousands of species? life histories. When many species die out at the same time, we refer to this as a "mass extinction". In general, at any given time, about 5% of all species are going extinct; this is the "background extinction rate". If there are usually 200 species that are abundant, widely distributed, and likely to leave fossils (this is a very small subset of all living things) then 5% would be 10 species extinctions per year. This is about what is seen in the fossil record, over the past 500 million years. This of course leads to two conclusions: first that that new species must be coming into being at least at the same rate or there would be nothing left, and second that there may have been 5 billion fossilizable species in the last half billion years. And this is only a small subset of all the species that have existed. Fossils are rare; there are only 250,000 known species in the fossil record, mostly since the Cambrian. Plotting Sepkoski?s data on marine family numbers versus stratigraphic levels since the Cambrian 550 million years ago, we first notice that their number has been in nearly constant increase, even taking into account that recent fossils are easier to find. Next we note the existence of five dips, corresponding to the five major peaks of extinctions in the second graph, which also shows the spike of experimentation with body forms in the Cambrian. Many extinction events are known (see the lesser peaks), and there is no qualitative difference between the Big Five and the rest of the 28 or so extinctions known in the fossil record. The Great Extinctions of marine organisms are tabulated below, with figures for land taxa, where available (based on data from Jablonski, 1991). Note that it is far easier to track the smaller numbers of higher taxa such as genera and families and use these results to generate estimates of species extinctions. *Figure 3.8.2 Marine family extinctions* Arrows mark Great Extinctions Left-most is Cambrian explosion/experimentation Species are grouped into genera: there are currently some 1000 genera containing about 4000 species of mammals. Most of these genera have but a single species; when the species of a genus all go extinct, so does the genus. And so on up through the higher taxa: families, orders, classes and phyla. It does get progressively harder to knock off entire groups; in the KT event 85% of species were lost, while only 47% of genera, 16% of families, 10% of orders and 1 out of 82 classes went extinct. No phyla were lost * * *Table 3.8-1 Great Marine Extinction Percentages* *Name Ma Families Genera All Species Land Species* *Cretaceous -Tertiary (KT)* 65 16 47 85 18% of vertebrate families *Triassic - Jurassic * 214 22 53 83 unclear *Permian -Triassic * 251 53 82 95 70% of land species *Late Devonian * 364 22 57 83 little known *Ordovician ? Silurian* 439 25 60 85 nonexistent Note that Genera and All Species % are observed, while Land Species % is estimated. Related information can be found on the following two pages: [ Earth Impact Craters ] Materials in this site are copyright University of California, unless otherwise indicated. Images are generally from NASA sites. Mail5332.gif (4196 bytes)For problems or questions regarding this web contact [ProjectEmail] . Last updated: July 19, 2001. Hit Counter