Geographic Variation and Episodic Evolution in an Ordovician Trilobite

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Abstract
A single lineage of the trilobite Flexicalymene shows two evolutionary "punctuated equilibria" within a 2-million-year, 1000-square-kilometer stratigraphic interval. The 2 × 10 5 -year-long "punctuation" may represent parapatric speciation. Commonness of depth-related clines before, during and after this event suggests that short-term adjustment to local conditions was important in long-term evolution.