Fossil Record


Gastropod origins


The first small, marine gastropods appeared in the Upper Cambrian. Due to the high preservation potential of their hard shell (the only part normally fossilised), gastropods generally have an excellent fossil record. Although, very early specimens from Paleozoic rocks are unfortunately often poorly preserved. The first true, torted, gastropods probably evolved from bellerophont-like monoplacophorans, with bilaterally symmetrical shells coiled in a single plane. Fossil evidence suggests that Class Gastropoda is a monophyletic group, sharing an exclusive common ancestor. The first gastropods with asymmetrically coiled shells appeared in the Upper Cambrian (Pleurotomariacea and Macluritacea). This new shell form was produced as a result of an additional component of growth direction, parallel to the vertical axis through the apex. This is likely to have allowed more compact storage of the visceral mass.


  Diagram 1. Theoretical model of shell coiling in which differing growth parameters produce all shell shapes seen in the gastropods. W = the size ratio of sequential generating curves (the shape of the generating curve is equivalent to the shape of the aperture), D = the distance of the generating curve from the axis, and T = the height of one generating curve covered by folowing spirals. Developed by Raup, 1966 (cited by, Clarkson 1998). Based on an image from Clarkson, 1998.



Gastropod radiation


Near the end of the Cambrian the gastropods underwent a rapid diversification, which was accompanied by an increase in size in some groups. Gastropods were relatively unaffected by the end Cambrian extinction event (488 MYA) and by the Carboniferous the Class was highly diverse: having developed many of the shell forms found today.

 

 Photograph 1. Euophalus sp. Lower Carboniferous. Actual length 81mm.

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By this time, some groups had colonised fresh water and the first terrestrial groups had appeared, such as Maturipupa from the Coal Measures of Europe. Although, these early terrestrial gastropods are not directly related to the modern pulmonate land snails. The primitive archaeogastropods (now included in Subclasses Eogastropoda and Orthogastropoda) reproduced by external fertilisation: their eggs and sperm shed via the exhalent current into the sea for fertilisation. Reproduction on land and in fast flowing rivers required the evolution of internal fertilisation. In overcoming this barrier, members of Superorder Caenogastropoda developed two separate sexes with internal genital organs and ducts, while Order Opisthobranchia and Order Pulmonata evolved as exclusive hermaphrodites.


A number of groups dissapeared during the end-Permian mass extinction (252 MYA), but the majority survived into the Mesozic. The neogastropods (now included in Superorder Caenogastropoda with the mesogastropods) became highly diverse and successful during the late Mesozoic. The pulmonates can also be traced to the beginning of the Mesozoic and may have had a caenogastropod ancestor.

 

 Photograph 2. Aptyxiella sp. Jurassic, Portland. Actual length 35mm. The famous portland screw.

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At the beginning of the Cenozoic, there was a marked increase in gastropod diversity and many groups closely related to the modern forms appeared.

 

 Photograph 3. Actaeon sp. Tertiary (Paleogene or Oligocene), Isle of Wight. Actual length 30mm.

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Gastropod body fossils


Shelless gastropods such as many opisthobranchs and some pulmonates were very unlikely to be preserved in the fossil record. As a result little is known about their geological history, although an understanding of their relationships with other gastropod groups can be gained by studying living species.
Britain has a number of important gastropod fossil beds, deposited in freshwater and marine environments. The Jurassic "Purbeck marble" deposit in Dorset, and the Jurassic "Sussex marble" deposit, hold limestone beds densely packed with specimens. Carboniferous gastropod fossils can also be found along the Avon gorge.


   Photograph 4. Bellerophon sp. Carboniferous, Avon gorge. Actual length 37mm.



Gastropod trace fossils

 

Certain shallow water formations may preserve trails made by organisms that once moved along the substrate. Some trails are thought to have been made by the movement of the muscular foot as a gastropod crawled across the wet sand. Others are believed to be preserved borrows. However, it is often difficult to distinguish trails or burrows of gastropods from those made by other invertebrates. Distinctive bore holes, made by predatory gastropods such as Natica, in the shell of prey such as brachiopods have been found in Tertiary deposits. These give an insight into the lifestyles of prehistoric gastropods.

 

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Author: Jennifer Hoyal Cuthill
Last updated: 22.11.05
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