Longevity of orders is related to the longevity of their constituent   genera rather than genus richness
  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. 
Stefan Bornholdt (U Bremen), Kim Sneppen (Niels Bohr Inst), Hildegard   Westphal (U Bremen)