Extinction (was Copes Law)

["Judith T. Lessin" <lessin@iceberg.org>]

In relation to Nick Longrich & Joe Daniel's discussion concerning a 
possible bias within terrestial extinctions towards large bodies 
animals, S.D. Webb (1984) found that during mammalian extinctions, the 
more genera that become extinct, the higher the proportion of 
large-bodies forms that contribute to the extinction count. J.E. 
Guilday (1984) noted that large mammals are generally characterized by 
small populations and small #'s of species, leaving them statistcally 
more prone to extinction. Such a bias also mirrors Bottom-Up community 
collapse.       At some point a critical value will be reached when 
large-bodied genera are largely absent from the "genera pool" and the 
proportion of small bodied genera begins to increase. A.D. Barnosky 
(1989) believed(s?) this study to be a possible padigram for 
terrestial extinctions as a whole, and urged testing of padigram. I, 
however, have not seen or heard of any such testing in relation to the 
K-T (anyone?).

        Note though, that Van Valkenburg (1994 Ecological Morphology) noted 
that mammalian diversification experienced a time lag of 150,000 yrs 
post K-T, presumably due to the amount of time needed to rebuild the  
community and provide niches for all ">those big things". What makes 
this really intriguing is that it implies that some of our extant 
communities may not be completely co-evolved because not enough time 
has passed since the last extinction peak. 

In conclusion though, this bias would not have been sensu stricto, a 
big contributing factor to the extinction of dinosaurs. It is only an 
indicator of widespread coevolutionary disequilibrium. I.E. stats and 
bias do not kill, they predict. The ultimate causation of dinosaur 
extinction was certainly more foreboding than a number.