Taxonomic rates can be quantified as number of genera / time span = average genera per million years , or one can take the reciprocal of this and ask: How long does a genus last? and thus quantify the duration of a taxon. By tabulating the period of first appearance of a taxon (genus, lets say) and the period of its last appearance in the fossil record one can obtain such taxonomic rates (see table below for example of data). In Pelecypod mollusks (bivalves) 13 genera appeared in the Ordovician and disappeared in the Ordovician; 4 genera appeared in the Ordovician and disappeared in the Triassic. Compiled over the entire data set the "average" genus of Pelecypod lasts 78 million years. Similar calculations for Carnivores show that in this group a genus lasts about 8 million years. See data below and compare with fig. 20.13, pg. 578. These data can be plotted on survivorship curves which tabulate the percent of taxa alive today that originated at the time indicated on the horizontal axis (broken line). For extinct taxa the graphs show the
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