  Longevity of a taxonomic group is an important issue in understanding the
dynamics of evolution. In this respect a key observation is that genera,
families or orders can each be assigned a characteristic average lifetime [Van
Valen, L., (1973) Evolutionary Theory 1, 1-30]. Using the fossil marine animal
genera database [Sepkoski, J.J.Jr. (2002) A Compendium of Fossil Marine Animal
Genera, Bull. Am. Paleontol. 363, 563 pp.] we here examine key determinants for
robustness of a higher taxonomic group in terms of the characteristics of its
constituents. We find insignificant correlation between the size of an order
and its stability against extinction, whereas we observe amazingly large
correlation between the lifetime of an order and the lifetime of its
constituent genera.
