Bivalves

BGS fossils and geological time

Living bivalves — Xenostrobus pulex (little black mussel) New Zealand.
Information icon

Living bivalves — Xenostrobus pulex (little black mussel) New Zealand. By Avenue [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)

Expand icon

Bivalves have inhabited the Earth for over 500 million years. They first appeared in the Mid Cambrian, about 300 million years before the dinosaurs. They flourished in the Mesozoic and Cenozoic eras, and they abound in modern seas and oceans; their shells litter beaches across the globe. Some also occur in lakes and rivers. Fossil bivalves were formed when the sediment in which they were buried hardened into rock. Many closely resemble living forms which helps us to understand how they must have lived.

The animal

Bivalve shells may be elongate, round or highly irregular in shape. The valves are often mirror images, e.g. Laevitrigonia, but each can have a different shape, e.g. Volviceramus. Patterns of concentric and/or radial ribbing add strength and provide anchorage in sediment.

Nodose (knobbly) ornaments may prevent shells being dislodged by water currents, while spines defend against predators or offer support on soft sediments. All forms have fine concentric lines marking shell growth.

The valves consist of layers of crystals of the mineral calcite or aragonite. Particular strengths conferred by different crystal layer arrangements help bivalves adapt to a variety of environments. A hinge, formed by the interlocking of rows of projecting nodes (teeth) and notches (sockets) along one of the inside edges of each valve, guide the opening and closing of the valves.

The most important functions of fossil bivalve shells were to protect against predators and prevent dehydration in intertidal environments. The inside surface of a bivalve shell is marked by the attachment areas of the muscles and ligament responsible for opening and closing the valves. These features, with the teeth and sockets of the hinge, are important for classification.

The most important functions of fossil bivalve shells were to protect against predators and prevent dehydration in intertidal environments. The inside surface of a bivalve shell is marked by the attachment areas of the muscles and ligament responsible for opening and closing the valves. These features, with the teeth and sockets of the hinge, are important for classification.

Bivalves, which belong to the Phylum Mollusca and class Bivalvia, have two hard, usually bowl-shaped, shells (called valves) enclosing the soft body. The valves are the parts usually found as fossils, but decay of the elastic hinge tissue that joins them means that they are rarely preserved together.

Information icon

Bivalve environments and enemies: an artist’s impression of a seascape showing the different modes of life of modern and fossil marine bivalves. Mussels (1) and oysters (2) attach themselves to rocky surfaces, while burrowing (3 & 4) and rock-boring (5) bivalves hide beneath the sea bed. Spiny shells (6) can deter some predators, while scallops (7) can escape by rapid flapping of their valves (8). The bowl-shaped shell of the Jurassic oyster Gryphaea (9) supported it on soft, muddy sea beds. ‘Enemies’ shown are fish, lobster, starfish and gastropod. BGS ©UKRI. All rights reserved. Based on a painting by Richard Bell.

Shell shapes

Feeding habits

Most bivalves lived by filtering water-borne food particles, although some extracted nutrient directly from the sediment.

In the Mesozoic Era, the evolution of extendable tubes of soft tissue (siphons) enabled bivalves to burrow more deeply whilst keeping their food supply accessible. A special embayment of the inner shell margin (pallial sinus) allowed storage of the siphons when danger threatened.

Reconstructed life position of Gryphaea arcuata (Lamarck, 1801) Jurassic. BGS ©UKRI. All rights reserved.
Information icon

Reconstructed life position of Gryphaea arcuata (Lamarck, 1801) Jurassic. BGS ©UKRI. All rights reserved.

Modes of life

Different modes of life are reflected by the shape of the bivalve shell. Streamlined burrowing forms contrast with the irregular form of oysters, e.g. Deltoideum that reflect the irregular surfaces they encrust. The bowl-shaped Gryphaea is adapted to resting on soft, fine-grained sediment, while Teredo and Penitella used their abrasive shell ornaments to bore respectively into wood and soft rocks. The rapid flapping of the fan-like shells of fossil scallops, e.g. Pecten, propelled them through the water to escape predators or to find new food.

The geologists’ tool

Bivalves can be used to show if the rocks in which they occur were formed in a marine, brackish or freshwater environment. Abundant fossil oysters might suggest deposition in shallow water, or proximity to ancient shorelines.

Sometimes bivalves are a useful guide to the age of the rocks in which they occur. This is the case in the coal-bearing rocks of the Late Carboniferous in which Carbonicola has been used to identify and correlate individual coal seams. The strata of the Late Cretaceous Chalk Group, forming the famous White Cliffs of Dover, are rich in the remains of inoceramid bivalves. They have been used to refine the established Chalk biostratigraphy, and so aid correlation.

The strong influence of environmental factors on bivalve distribution coupled with rather slow rates of evolution limits their widespread use for biostratigraphy. Even in the rather uniform environment of the Chalk, some groups display faunal provincialism.

Myths and legends

In the old Scunthorpe coat of arms, images of this oyster signify its occurrence in the formerly commercially important Jurassic ironstone deposits of the area.
Information icon

In the old Scunthorpe coat of arms, images of this oyster signify its occurrence in the formerly commercially important Jurassic ironstone deposits of the area. Courtesy of the North Lincolnshire Council.

The strongly recurved form of some Gryphaea is popularly known as the Devil’s toenail. Some 17th and 18th century Scottish accounts show that its possession was believed to cure arthritis.

Four-footed beasts

Fossil bivalves can also resemble parts of ‘four-footed beasts’. Robert Plot (1640–1696) illustrated internal moulds of Jurassic specimens of Myophorella and Protocardia as horse’s heads and bull’s hearts respectively.

3D fossil models

Cyprina sedgwickii (SM B 19578 – Holotype). Albian Age (Cretaceous Period) – Aptian Age (Cretaceous Period) (100.5 – 126.3 Ma B.P.) See 3D fossils online.
Information icon

Cyprina sedgwickii (SM B 19578 – Holotype). Albian Age (Cretaceous Period) – Aptian Age (Cretaceous Period) (100.5 – 126.3 Ma B.P.) See 3D fossils online. BGS ©UKRI. All rights Reserved.

Expand icon

Many of the fossils in the BGS palaeontology collections are available to view and download as 3D models. To view this fossil, or others like it, in 3D visit GB3D Type Fossils.

Reference

Woods, M A. 1999. Bivalves: fossil focus. Nottingham, British Geological Survey.

Need more information?

Please contact the BGS enquiries team

You may also be interested in:

Ammonite p521007

Fossils

What is a fossil and why do we study fossils? This section explains the different methods of fossil preservation and links to a set of detailed pages that describe 14 of the most common fossil types, including ammonites, belemnites, bivalves and trilobites.

Show more

Was this page helpful?

  • How can we make this section better?*

  • Please select a reason*

  • How can we make this section better?*