MACROEVOLUTIONARY TREND MECHANISMS IN CAMERATE CRINOID CUP ORGANIZATION

The morphological evolution of the camerate cup is characterized by a reduction in the number of plates and a general simplification of organization. This simplification occurs in most camerate sub-groups and occurs over the whole duration of the clade. Two distinct macroevolutionary processes are identified as mechanisms involved in generating this trend. 1) Genus level origination and extinction rates are negativly correlated with cup simplicity. If this pattern is contstant over time a trend toward more complexly organized cups would occur, in contrast to the actual pattern observed. 2) Indirect (non-phylogenetic) evidence suggests that there is a bias in the transition in cup organization between ancestoral and descendent genera. Descendent genera tend to be simpler than their ancestors. The evidence for this is provided by a subclade test on camerate families. The biased direction of production pattern will generate the observed trend toward simpler cups. This is a case of antagonistic trend mechanisms. A simulation study suggests that directed speciation does not always swamp out the effects of differential taxonomic rates across morphological types.