The Difference Between...Bivalves and Brachiopods

Diagram of Bivalve morphology. Image credit: keyword-suggestions.com

This is a group of fossils that I struggled to get to grips with initially, but hopefully this will make it easier to identify which one you have.

Both bivalves and brachiopods are very similar at first glance. They both have calcium carbonate, hinged shells that are opened and closed by muscles and ligaments in order to get oxygen and nutrients. They both have relatively sessile lifestyles

Diagram of Brachiopod morphology. Image credit 

and are extensive throughout the fossil record. However, these two organisms belong to different phyla. Brachiopods have their own phyla; the Brachiopoda, and bivalves belong to the phyla Mollusca.

There are many noticeable differences between the two, once highlighted it is clear that they belong in different phyla. I will go through the differences that will aid in identifying the fossil of either a bivalve or a brachiopod.

First, there is size. Bivalves can be much larger than brachiopods, growing anywhere between 1 millimetre and 1 metre, the largest being the Giant Clam. Brachiopods on the other hand only grow between 2 centimetres and 10 centimetres. I your shell is considerably larger than 10 centimetres, chances are you have a bivalve. There are more ways to increase your certainty.

Symmetry is one of the best ways to tell the difference between bivalves and brachiopods. Bivalves have a line of symmetry that runs along a plane between the two valves, meaning that they have symmetrical valves. BrachiopodsGryphaea arcuata which only has one valve, these are often called Devil's Toenails.

An example of a brachiopod from my own collection note
the larger pedicle valve overreaching the brachial valve.
Genus is Terebratula

are bilaterally symmetrical, from umbo to anterior edge of the pedicle valve. Thus if your specimen is complete and one valve is larger than the other you have a bivalve. An exeption to this is

If observing these animals in life it may be useful to know that the right and left valves are hinged by a ligament on the dorsal surface in bivalves. In brachiopods, the pedicle valve, the ventral valve, is larger and projects beyond the brachial valve, the dorsal valve. This is to allow the pedicle to emerge from the pedicle opening and anchor the brachiopod to a substrate at the posterior. The bivalves also have a foot that is released through the posterior by opening the valves.

I hope you find this useful when identifying bivalves and brachiopods, this works for extant species as well as extinct ones.