1. Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA
2. Institute of Geophysics and Planetary Physics and Molecular Biology Institute, University of California, Los Angeles, California 90095, USA
3. Department of Biology, JH 142, 1001 E. Third Street, Indiana University, Bloomington, Indiana 47405, USA
4. Present address: NASA Astrobiology Institute, MS 240-1, Ames Research Center, Moffett Field, California 94035, USA

Correspondence to: Michael J. Vendrasco¹ Email: mikev@ess.ucla.edu

Abstract

Modern chitons (Mollusca: Polyplacophora) possess a highly conserved skeleton of eight shell plates (valves) surrounded by spicules or scales, and fossil evidence suggests that the chiton skeleton has changed little since the first appearance of the class in the Late Cambrian period (about 500 million years before present, Myr bp). However, the Palaeozoic problematic taxon Multiplacophora^{1, 2, 3, 4, 5}, in spite of having a more complex skeleton, shares several derived characters with chitons. The enigmatic status of the Multiplacophora is due in part to the fact that its members had an exoskeleton of numerous calcium carbonate valves that usually separated after death. A new articulated specimen from the Carboniferous period (about 335 Myr bp) of Indiana reveals that multiplacophorans had a dorsal protective surface composed of head and tail valves, left and right columns of overlapping valves (five on each side), and a central zone of five smaller valves, all surrounded by an annulus of large spines. Here we describe and name the articulated specimen and present evidence that multiplacophorans were chitons. Thus the highly conserved body plan of living chitons belies the broad disparity of this clade during the Palaeozoic era.

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