LATE CAMBRIAN AND EARLY ORDOVICIAN STEM GROUP CHITONS (MOLLUSCA: POLYPLACOPHORA) FROM UTAH AND MISSOURI

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Abstract
Abundant silicified shell plates (valves) of some of the oldest-known chitons were recovered from the Late Cambrian Notch Peak Formation of Utah. The chiton fauna is dominated numerically by Matthevia wahwahensis new species, but also includes another mattheviid, Eukteanochiton milleri new genus and species, and a preacanthochitonid, Orthriochiton utahensis new genus and species. Robustum from the Early Ordovician Gasconade Formation of Missouri is herein reinterpreted as a septemchitonid chiton, and Hemithecella, from the Late Cambrian-Early Ordovician of the eastern United States, is once again considered a mattheviid chiton. Mattheviid valves are unique among chitons in that they are massive, elongate, and contain one or two tunnels; these characteristics have led some to exclude this family from the Polyplacophora. However, mattheviids and other chitons share many valve characters, including granules, an apical shelf, a thin anterior margin, bilateral symmetry, three valve types, and a shell ...

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