Exploring the pattern of coordinated stasis: Simulations and extinction scenarios

Originations and extinctions of lineages were simulated under three different extinction scenarios, time-homogeneous, exponential and episodic, in order to determine whether the patterns of stratigraphic ranges produced are consistent with those described as coordinated stasis by Brett and Baird (1992, 1995) and Brett et al. (1996). Four features of the patterns were evaluated: percentage holdovers - the proportion of taxa from a given unit that also occur in the previous unit; percentage carryovers - the proportion of taxa from a given unit that persist into the subsequent unit; percentage extinctions - the proportion of taxa from a given unit becoming extinct within that unit (excluding those becoming extinct at the end of that unit); percentage originations - the proportion of taxa from a given unit originating within that unit (excluding those originating at the beginning of that unit). Coordinated stasis of the magnitude reported for the Middle Devonian Hamilton-Tully could not be generated under the time-homogeneous and exponential models but was produced in approximately 20% of simulations under the pulsed extinctions (episodic) model; when sampling is random, these results appear relatively insensitive to the sampling intensity. These simulations suggest that coordinated stasis requires an evolutionary/ecological mechanism consistent with long intervals of near-zero extinction and origination punctuated by infrequent intervals of extremely high extinction and origination.