The granddaddy of all extinctions occurred about 225 million years ago at the end of the Permian period.  (We don't know why, although the coalescence of all continents at about the same time must have set the basic stage.)  By wiping out many groups, permanently debilitating others, and allowing some to pass through relatively unscathed, this great dying set the fundamental pattern of life's diversity ever since.  But how profound was the Permian extinction?  An old and familiar figure states that half the families of marine organisms (52.0 percent to be exact) perished at that time.  But families are taxonomic abstractions.  What does 52 percent of familes mean for species, nature's real units? (Inconsistent taxonomic practices and an inadequate fossil record prevent the counting of species directly.  Families as bigger units,are harder to miss.) <P>

 We can be sure that a removal of half the families requires the death of a much greater percentage of species. A family is not gone until all its species die, and many families contain tens or hundreds of species.  The extinction of most individual species does not wipe out a family, just as, for example, the random excision of a single entry in a telephone directory rarely removes the name entirely--you'd have to bump off a lot of Smiths.  How many species must die before 50 percent of families are gone?  <P>

David  M. Raup of Chicago's Field Museum has recently considered this question (see bibliography).  The problem has no easy solution.  If all families contained about the same number of species, the a simple formula would suffice. But variation is enormous.  Many familes contain only a single species.  In this case, removal of the species also wipes out the family.  Phone books contain their Zzyzzymanskis as well as their Wongs.  Other families contain more than 100 species.  We must know the empirical distribution of species per family before we can make a proper estimate.  And we cannot construct an empirical distribution for Permain families because we cannot count the species directly. <P>

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 Raup therefore made tabulations for a group that we do know well, the echinoids, or sea urchins.  Echinoids include 894 species distributed into 222 genera and 40 familes.  How many species must, on average, be removed at random in order to eliminate 52 percent of the families?  Raup considered this question both empirically and theoretically and come up with the astounding figure of 96 percent.  If the rest of life maintains a distribution of species within families similar to the echinoids--and we have no evidence for major differences in this patterns--then the Permian debacle might have wiped out all but<A NAME="4 percent"></A> 4 percent of species.<A HREF="/biblio.html#SJG">Hen's Teeth and Horse's Toes</A>, p. 340.