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How the mollusc got its scales: convergent evolution of the molluscan scleritome

Chen, Chong; Copley, Jonathan T.; Linse, Katrin; Rogers, Alex D.; Sigwart, Julia. 2015 How the mollusc got its scales: convergent evolution of the molluscan scleritome. Biological Journal of the Linnean Society, 114 (4). 949-954. 10.1111/bij.12462

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Abstract/Summary

Radiation of dramatically disparate forms among the phylum Mollusca remains a key question in metazoan evolution, and requires careful evaluation of homology of hard parts throughout the deep fossil record. Enigmatic early Cambrian taxa such as Halkieria and Wiwaxia (in the clade Halwaxiida) have been proposed to represent stem-group aculiferan molluscs (Caudofoveata + Solenogastres + Polyplacophora), as complex scleritomes were considered to be unique to aculiferans among extant molluscs. The ‘scaly-foot gastropod’ (Neomphalina: Peltospiridae) from hydrothermal vents of the Indian Ocean, however, also carries dermal sclerites and thus challenges this inferred homology. Despite superficial similarities to various mollusc sclerites, the scaly-foot gastropod sclerites are secreted in layers covering outpockets of epithelium and are largely proteinaceous, while chiton (Polyplacophora: Chitonida) sclerites are secreted to fill an invaginated cuticular chamber and are largely calcareous. Marked differences in the underlying epithelium of the scaly-foot gastropod sclerites and operculum suggest that the sclerites do not originate from multiplication of the operculum. This convergence in different classes highlights the ability of molluscs to adapt mineralized dermal structures, as supported by the extensive early fossil record of molluscs with scleritomes. Sclerites of halwaxiids are morphologically variable, undermining the assumed affinity of specific taxa with chitons, or the larger putative clade Aculifera. Comparisons with independently derived similar structures in living molluscs are essential for determining homology among fossils and their position with respect to the enigmatic evolution of molluscan shell forms in deep time.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1111/bij.12462
Programmes: BAS Programmes > Polar Science for Planet Earth (2009 - ) > Environmental Change and Evolution
ISSN: 0024-4066
Additional Keywords: Aculifera, Halkieria, Polyplacophora, scaly-foot gastropod, Wiwaxia
Date made live: 02 Feb 2015 15:16 +0 (UTC)
URI: http://nora.nerc.ac.uk/id/eprint/507429

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