THE ORDOVICIAN TRILOBITE CAROLINITES, A TEST CASE FOR MICROEVOLUTION IN A MACROFOSSIL LINEAGE

Abstract

We use geometric morphometrics to test a claim that the Ordovician trilobite Carolinites exhibits gradualistic evolution. We follow a previously proposed definition of gradualism, and define the criteria an ideal microevolutionary case study should satisfy. We consider the Lower-Middle Ordovician succession at Ibex, western Utah to meet these criteria. We discovered examples of: (1) morphometric characters which fluctuate with little or no net change; (2) characters which show abrupt 'step' change; (3) characters which show transitional change through intermediate states. Examples belonging to (2) and (3) exhibit reversals. The transitional characters were tested against a hypothesis of symmetrical random walk. The tests indicated that they were not changing under sustained directional selection. Two alternative interpretations are possible. (1) The characters are responding to random causes (genetic drift or rapidly fluctuating selection pressures) or to causes that interact in so complex a way that they appear random. This observation may be applicable to most claimed cases of gradualistic evolution in the literature. (2) Sampling was at too poor a resolution to allow meaningful testing against the random walk. If so, then this situation is likely to apply in most evolutionary case studies involving Palaeozoic macrofossils.