Bivalve, Trigonia costata

Both Bivalves and brachiopods are a type of shell with a living organism within the valves. Although they may look similar at a first glance, there are a few differences between them which become obvious with a close up look at them.

Firstly bivalves are from the phylum known as Mollusca (molluscs), whereas brachiopods have their own individual phylum Brachiopoda.

The physical characteristics also define whether a shell is a brachiopod or a bivalve. For example brachiopods are made up of two valves which are different sizes and possibly shape, whereas the two valves on a bivalve are the same (symmetrical). Brachiopods consist of a top and bottom valve whereas on bivalves they are referred to left and right valves.

Brachiopod, Atrypa sp. from the Wrens Nest

The largest valve on a brachiopod is the pedicle valve and contains a hole (pedicle opening) where the pedicle (stalk) would have been attached. This then would have been attached to the sea floor or a rock. Bivalves have a fleshy foot or siphon which come out the sides of the shell to feed.

Today there are approximately 300 species of brachiopod living in cold, often deep waters after the majority of the group became extinct during the end-Permian mass extinction. Bivalves are very common today and throughout the fossil record. They became an increasingly successful group of fossils after the brachiopod numbers rapidly declined.