Abstract from Vendrasco et al. (2004):

Articulated Palaeozoic fossil with 17 plates greatly expands disparity of early chitons

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, 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.

This specimen was collected from the the Crawford Shale (Carboniferous in age) near Bloomington, Indiana. It was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Troy Wood of the Department of Biology, Indiana University and Dr. Michael Vendrasco of the Department of Earth and Space Sciences, The University of California at Los Angeles. Funding for scanning was provided by a National Science Foundation IGERT grant to Dr. Wood.

The specimen was scanned by Matthew Colbert on 21 April 2003 along the coronal axis for a total of 540 1024x1024 pixel slices. Each slice is 0.057 mm thick, with an interslice spacing of 0.057 mm. and a field of reconstruction of 43 mm.

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Click on the thumbnail below to see an animation of the fossil chiton spinning as the matrix fades (1mb):

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