EXPLORING THE PATTERN OF COORDINATED STASIS - SIMULATIONS AND EXTINCTION SCENARIOS

Originations and extinctions of lineages were simulated under three di fferent extinction scenarios, time-homogeneous, exponential and episod ic, 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 o f the patterns were evaluated: percentage holdovers - the proportion o f taxa from a given unit that also occur in the previous unit; percent age 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 th ose becoming extinct at the end of that unit); percentage originations - the proportion of taxa from a given unit originating within that un it (excluding those originating at the beginning of that unit). Coordi nated stasis of the magnitude reported for the Middle Devonian Hamilto n-Tully could not be generated under the time-homogeneous and exponent ial models but was produced in approximately 20% of simulations under the pulsed extinctions (episodic) model; when sampling is random, thes e results appear relatively insensitive to the sampling intensity. The se simulations suggest that coordinated stasis requires an evolutionar y/ecological mechanism consistent with long intervals of near-zero ext inction and origination punctuated by infrequent intervals of extremel y high extinction and origination.