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.

A quick visit to Hastings

The mystery modern bone fragment

Hastings has always been a place I'd like to visit, both for the history and the geology. Hastings is the only coastal outcrop of Wessex Formation on the mainland of the UK, the most famous outcrops being the Isle of Wight. 

This location is definitely an interesting one, both from a palaeontological and a geological perspective. Palaeontologically, the range of fossils to find is rather wide, anything from Hybodus sharks teeth and fin spines to fossilised bone of Iguanodon dinosaurs can be found. Unfortunately, being the summer and scouring tides are few and far between I found neither, therefore I plan to revisit in the winter after some bad storms. This is the case with most locations, erosion in the summer is very little compared to the winter so fossil discoveries are scarce, although I did pick up a modern bone fragment. I keep these despite not being palaeontologically valuable as it gives me something to compare to the fossils of extinct fossils that I find. 

Straight ripple marks in a boulder at Hastings.
50 pence piece for scale.

But what I found interesting about this site is the geology. There were many geological wonders. The most fascinating that I haven't yet seen on the Isle of Wight are the straight ripple marks in a number of boulders. Straight basically means that they were formed under a unidirectional flow, for instance a river. Taking into account that the Wessex Formation was deposited in the Cretaceous period, at which time the South of England was a lagoon environment. These ripple marks are evidence for small rivers and streams flowing into said lagoon, on the Isle of Wight there are casts of smaller streams that have retained their characteristic channel shape, with shark and fish fossils in the bottom. I will do a separate post on the streams later on. Further evidence that these are indeed the unidirectional straight ripple marks is the cross bedding on many boulders. Straight ripple marks create cross laminae that dip in the same direction, these can be found in the cross section of many boulders. Cross bedding is formed by the migration of ripples downstream, This is caused by the build up of deposited sediment on the stoss (upstream) upstream side of the ripple, this continues until the crest is too heavy to hold more sediment and thus collapses down the lee (downstream) side of the ripple. These avalanches continue down river and create unidirectional cross bedding. This cross bedding is a good indicator for the palaeocurrent, the dip will always face downstream, this is also a good indicator, if the bedding is in situ, as to whether the rock has been rotated since deposition.

Unidirectional cross bedding. 50 pence piece for scale.

Also, a monospecific assemblage of bivalves was found in another boulder. These are most likely brackish water bivalves, tolerable to more stagnant water, as the Wessex Formation is a lagoon environment. The photograph I took shows numerous empty burrows, and some with the bivalves still inside. This could be an indicator to a storm that removed the bivalves from their shallow burrows and deposited them elsewhere, hence the empty burrows. Their preservation shows that they were filled in and fossilised at the time of the storm, a good palaeoenvironmental indicator. The climate would have had to have been much warmer than today to generate storms of such magnitude that they can reach down to the bottom of the lagoon and rework these bivalves and deposit sediment into their burrows. 

Burrows of brackish bivalves, some bivalves
still present. Hand for scale.

All in all, this was an interesting visit despite the lack of fossils, definitely worth a trip whether you are interested in geology or palaeontology as there is plenty there for both fields, although I recommend palaeontologists visit in the winter if they are hoping to find something.

If you have visited the Hastings Wessex Formation do share what you thought of it or what you have found there in the comments section.