Three important groups in the Phylum Mollusca (both fossil and extant organisms) are bivalves, gastropods and cephalopods.
Bivalves evolved around 500 million years ago during the mid-Cambrian Period and species survive today. They were most common in the Mesozoic and Cenozoic Eras i.e. from 251 million years ago until today. Many fossilised bivalves bear a close resemblance to living species which enables us to understand the lifestyle of those species that lived in the past.
Bivalves are entirely aquatic. Most marine species living in shallow seas. Some species burrow into sediments, some cement themselves onto or bore into objects such as rocks and some are attached by threads. A few species can swim.
The majority of bivalves have a shell comprised of two valves of equal size and shape (bilaterally symmetrical) with each valve the mirror image of the other. The hard shells enclose the soft body. A few species such as oysters do not, however, have symmetrical shells. The shells can be made of calcite or aragonite and have growth lines recording the history of growth. Usually the valves are closed by 2 main muscles and scars are often left on the inside of the shells after death showing where these muscles were attached. The fossil shells would have afforded protection against predation and helped to prevent the mollusc from dehydrating if it lived in the intertidal zone.
Most bivalves are filter feeders.
The presence of fossil bivalves in a rock will indicate that the rock was formed in either a marine, brackish or freshwater environment.
Gryphaea - an extinct species of oyster
Gryphaea, also known as "Devil's Toenail" lived from the Jurassic to Cretaceous Period (200 - 65 million years ago). The shell was composed of calcite and the animal would have lived on muddy sea-floors cemented to a rock. The shell shaped like a bowl was an adaptation to living in soft, fine-grained sediment.
It was believed in Scotland during the 17th and 18th centuries that carrying one of these fossils would help prevent arthritis and rheumatism.
Aquatic gastropods first evolved early in the Cambrian around 542 million years ago and during the last 65 million years have become the most common mollusc group as they are able to live in so many different habitats. There are around 105,000 living gastropod species and 15,000 fossil species have been found. In the Carboniferous Period (354-290 million years ago) gastropods began to inhabit freshwater environments and terrestrial snails may have evolved by late Carboniferous times from these freshwater species. In order to live on dry land snails evolved lungs to allow them to breathe out of water and aestivated(became dormant during hot, dry periods waking up when humidity was high and conditions were wetter).
Although gastropods suffered some species loss during the mass extinction at the end of the Permian when 90% of marine organisms became extinct, they were not as badly affected as many other groups.
Gastropods possess a muscular, flattened foot used for movement, eyes,tentacles and a radula composed of minute teeth for feeding. Most have a coiled or conical shells composed of calcite and/or aragonite but some species, slugs for example, have no shells.
The majority of gastropods are marine living in shallow seas but many also live in freshwater environments such as rivers, lakes and ponds, and some species live on dry land.
Helix astra - from the Pleistocene
I should have added a coin for scale for this photo - the shell on the right is 4/5 inches long and the one on the left around 3 inches. I haven't been able to confirm the exact species of either fossil but I believe that the one on the right may be Bourgetia which lived in shallow water.
Turitella imbricata. From the Eocene (55 - 36 million years ago.
A sea snail species
Cephalopods, such as ammonites and belemnites, were covered in an earlier posting.