Abstract

Recent collecting provided fossil chitons (Mollusca, Polyplacophora) from upper Eocene to lower Oligocene deposits of Washington State, U.S.A. The study material consists of 140 valves from six localities in the Quimper, Makah, Lincoln Creek, Crescent and Gries Ranch formations. The material is mostly incomplete or very fragmented and/or worn so that a precise appreciation of diagnostic features has been in many cases difficult if not impossible. Fourteen species were identified, seven are described as new: Lepidopleurus propecajetanus n. sp., Leptochiton sp., Ischnochiton goederti n. sp., Ischnochiton? sp. A, Ischnochiton? sp. B, Stenoplax quimperensis n. sp., Stenoplax sp. A, Stenoplax sp. B, Lepidozona cowlitzensis n. sp., Lepidochitona lioplax (Berry, 1922), Lepidochitona washingtonensis n. sp., Lepidochitona squiresi n. sp., Lepidochitona sp. and Craspedochiton eernissei n. sp. The only species previously described from the study area is Lepidochitona lioplax and it is also the most common chiton in the study material (67% of the total). No species other than L. lioplax has been found in more than one locality. In four of these localities only a single species has been collected. The basal Lincoln Creek Formation at the Porter Creek site supplied the richest and most diverse chiton assemblage with seven species and 86% of the valves. The discovery and formal identification of such a diverse Paleogene fauna from the northeastern Pacific provides a comparative base for a better appreciation of Polyplacophora biogeography and evolution.

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