Geograpehic Variation and Episodic Evolution in an Ordovician Trilobite

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Science  22 Aug 1980:
Vol. 209, Issue 4459, pp. 925-927
DOI: 10.1126/science.209.4459.925


A single lineage of the trilobite Flexicalymene shows two evolutionary "punctuated equilibria" within a 2-million-year, 1000-square-kilometer stratigraphic interval. The 2 x 105-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.