Gastropods, the whirly molluscs

--- a commonly-found order of fossil

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Figure 1. On the left is a pair of fossil gastropods of the genus Pleurotomya (?), of Silurian age. Location unknown, but presumed to be the British Isles. On the right is a selection of modern gastropods from Koh Jum Island, Krabi province, southern Thailand. Note the diversity of shapes, colours and degree of ornamentation in these marine shells from the tropics.

"Rock of the Month #189, posted for March 2017" ---


are familiar to most people in many parts of the world, whether as freshwater snails or as marine species found by the shore (Fig. 1). They are widespread geographically and through time, from the Cambrian to the Recent, surviving to the present, in contrast to other formerly successful molluscan groups such as the ammonites. Whether gastropods evolved prior to the Cambrian is uncertain. Purported gastropod tracks were identified in the Chorhat sandstone of the Vindhyan Supergroup in central India, strata thought to be >1100 Ma, but the nature of the find remains controversial (Kathal, 1999).

Examples of gastropods are described in all general works on invertebrate fossils (e.g., BMNH, 1969; Chaumeton and Magnan, 2005). In rare cases, gastropods are so abundant that they become rock-forming components in their host rock (e.g., the Purbeck marble, a fossiliferous limestone from southern Dorset in England: Davies, 1956). Such rocks may form attractive building stones, as in the cases of the Purbeck marble in England and the Tyndall stone of Manitoba in Canada.

Gastropods are abundant in Pliocene to Recent beds, such as the deposits in northern Norfolk (Reid and Woodward, 1882). Gastropod shells in deposits of late Pleistocene and Holocene age can be used for radiocarbon dating (e.g., Campbell and Evans, 1990). The chemistry of the shells, reflected in the bulk composition, as well as in oxygen and carbon isotope signatures, can help with paleoenvironmental and paleoclimatic reconstructions (e.g., Ingram et al., 1998).


BMNH (1969) British Palaeozoic Fossils. British Museum (Natural History), London, 3rd edition, 208pp.

Campbell,IA and Evans,DJA (1990) Glaciotectonism and landsliding in Little Sandhill Creek, Alberta. Geomorphology 4, 19-36.

Chaumeton,H and Magnan,D (2005) Fossielen. Konemann, Tandem Verlag GmbH, 431pp. (in Dutch).

Davies,GM (1956) The Dorset Coast: A Geological Guide. Adam and Charles Black, London, 2nd edition, 128pp.

Ingram,BL, De Deckker,P, Chivas,AR, Conrad,ME and Byrne,AR (1998) Stable isotopes, Sr/Ca, and Mg/Ca in biogenic carbonates from Petaluma Marsh, northern California, USA. Geochim.Cosmochim.Acta 62, 3229-3237.

Kathal,PK (1999) Should the life's clock be backed by 400 million years? Current Science 76 no.6, 725, 25 March.

Reid,C and Woodward,HB (1882) The Geology of the Country Around Cromer, Explanation of Sheet 68E. Geol.Surv. G.B. Memoir, reprinted text, 56pp.

Graham Wilson, 15 March 2017

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