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

How the mollusc got its scales: convergent evolution of the molluscan scleritome
How the mollusc got its scales: convergent evolution of the molluscan scleritome
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.
Aculifera, Halkieria, Polyplacophora, scaly-foot gastropod, Wiwaxia
0024-4066
949-954
Chen, Chong
3faad3e1-b898-4f4b-b418-9c1736e53f95
Copley, Jonathan T.
5f30e2a6-76c1-4150-9a42-dcfb8f5788ef
Linse, Katrin
74d7ddc0-74a1-4777-ac1d-3f39ae1935ad
Rogers, Alex D.
fb474198-f059-48f7-b637-74617b5023f6
Sigwart, Julia
8bf86a20-94a9-4a0c-94cb-f1e61d19ccc6
Chen, Chong
3faad3e1-b898-4f4b-b418-9c1736e53f95
Copley, Jonathan T.
5f30e2a6-76c1-4150-9a42-dcfb8f5788ef
Linse, Katrin
74d7ddc0-74a1-4777-ac1d-3f39ae1935ad
Rogers, Alex D.
fb474198-f059-48f7-b637-74617b5023f6
Sigwart, Julia
8bf86a20-94a9-4a0c-94cb-f1e61d19ccc6

Chen, Chong, Copley, Jonathan T., Linse, Katrin, Rogers, Alex D. and 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. (doi:10.1111/bij.12462).

Record type: Article

Abstract

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.

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More information

Published date: April 2015
Keywords: Aculifera, Halkieria, Polyplacophora, scaly-foot gastropod, Wiwaxia
Organisations: Ocean and Earth Science

Identifiers

Local EPrints ID: 375970
URI: http://eprints.soton.ac.uk/id/eprint/375970
ISSN: 0024-4066
PURE UUID: f29096a6-6011-461b-a801-962a4bd9df12

Catalogue record

Date deposited: 10 Apr 2015 14:36
Last modified: 15 Jul 2019 21:24

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