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

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 Multiplacophora1,2,3,4,5, 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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Figure 1: Articulated multiplacophorans and a reconstruction of the skeleton.
Figure 2: Fine structure of multiplacophoran and chiton valves.
Figure 3: Cladogram showing placement of the Multiplacophora (Polysacos) and Echinochiton within the Polyplacophora.

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Acknowledgements

We are grateful to M. Graham and the staff of the US Army Corps of Engineers, Middle Wabash Area, for their cooperation in the collection and study of this fossil. We thank R. Mapes, R. Hoare, B. Haissini, E. Landing, D. Blake and C. Mah for lending specimens. K. McKeegan, G. Jarzebinski and R. Turner helped with scanning electron microscopy work and G. Gastony with the Greek. The computerized tomography scanning and image processing was carried out by M. Colbert at the X-ray CT Laboratory, University of Texas at Austin, with funding from a US National Science Foundation IGERT Fellowship to T.E.W. C. Fernandez drew the reconstruction and made the epoxy casts of modern chiton aesthete canals. G. Gastony, D. Eernisse and L. Rieseberg provided comments on the manuscript. R. Raff provided advice throughout the project.Authors' contributions All three authors made significant contributions to this article.

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Correspondence to Michael J. Vendrasco.

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Supplementary information

Supplementary Movie 1

Movie of X-ray computerized tomography (CT) scan of holotype obtained at the CT Laboratory, University of Texas at Austin. (MP4 197 kb)

Supplementary Table 1

Data matrix for cladistic analysis plus explanatory notes. (XLS 25 kb)

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Vendrasco, M., Wood, T. & Runnegar, B. Articulated Palaeozoic fossil with 17 plates greatly expands disparity of early chitons. Nature 429, 288–291 (2004). https://doi.org/10.1038/nature02548

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