Tusk shell

Tusk shells; Temporal range:
	Various Scaphopoda, from left to right: Fissidentalium, Gadilida, Gadila, and Gadilida.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Subphylum:	Conchifera
Class:	Scaphopoda; Bronn, 1862
Orders
	Dentaliida; Gadilida;

The tusk shells or tooth shells, technically the Scaphopoda /skæˈfɒpədə/ (the scaphopods /ˈskæfəpɒdz/, from Ancient Greek σκᾰ́φης skáphē "boat" and πούς poús "foot"), are members of a class of shelled marine mollusc with worldwide distribution, and are the only class of exclusively infaunal marine molluscs. Shells of species within this class range in length 0.5–18 cm (0.20–7.09 in) (with Fissidentalium metivieri as the longest).^[3] Members of the order Dentaliida tend to be larger than those of the order Gadilida.

These molluscs live in soft substrates offshore (usually not intertidally). Because of this subtidal habitat and the small size of most species, many beachcombers are unfamiliar with them; their shells are not as common or as easily visible in the beach drift as the shells of sea snails and clams.

Molecular data suggest that the scaphopods are a sister group to the cephalopods, although higher-level molluscan phylogeny remains unresolved.^[4]

^ Cite error: The named reference Steiner1992 was invoked but never defined (see the help page).
^ Reynolds, Patrick D. (2002). "The Scaphopoda". Molluscan Radiation - Lesser-known Branches. Advances in Marine Biology. Vol. 42. pp. 137–236. doi:10.1016/S0065-2881(02)42014-7. ISBN 9780120261420. PMID 12094723.
^ Vermeij, G. J. (2016). "Gigantism and Its Implications for the History of Life". PLOS ONE. 11 (1): e0146092. Bibcode:2016PLoSO..1146092V. doi:10.1371/journal.pone.0146092. PMC 4714876. PMID 26771527.
^ Giribet, G.; Okusu, A, A.; Lindgren, A.R., A. R.; Huff, S.W., S. W.; Schrödl, M, M.; Nishiguchi, M.K., M. K. (May 2006). "Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons". Proceedings of the National Academy of Sciences of the United States of America. 103 (20): 7723–7728. Bibcode:2006PNAS..103.7723G. doi:10.1073/pnas.0602578103. PMC 1472512. PMID 16675549.

[Steiner1992-1] Cite error: The named reference Steiner1992 was invoked but never defined (see the help page).

[2] Reynolds, Patrick D. (2002). "The Scaphopoda". Molluscan Radiation - Lesser-known Branches. Advances in Marine Biology. Vol. 42. pp. 137–236. doi:10.1016/S0065-2881(02)42014-7. ISBN 9780120261420. PMID 12094723.

[3] Vermeij, G. J. (2016). "Gigantism and Its Implications for the History of Life". PLOS ONE. 11 (1): e0146092. Bibcode:2016PLoSO..1146092V. doi:10.1371/journal.pone.0146092. PMC 4714876. PMID 26771527.

[4] Giribet, G.; Okusu, A, A.; Lindgren, A.R., A. R.; Huff, S.W., S. W.; Schrödl, M, M.; Nishiguchi, M.K., M. K. (May 2006). "Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons". Proceedings of the National Academy of Sciences of the United States of America. 103 (20): 7723–7728. Bibcode:2006PNAS..103.7723G. doi:10.1073/pnas.0602578103. PMC 1472512. PMID 16675549.

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