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Fossil brachiopods

(in the Netherlands and adjacent parts of Belgium and Germany) 

Brachiopods are a separate phylum of the animal kingdom. Just like bivalves whose shells can be found in large numbers on the sandy beaches of the North Sea, they have a soft body that is protected by a shell that consists of two hinged shell parts. However brachiopods are very different animals from bivalves. The class of bivalves belongs to the phylum of molluscs (Mollusca), while the brachiopods form their own phylum, which in itself is subdivided into classes.

Image 1.
As many as 30,000 fossil species of brachiopods are known. Here is a small selection of the diversity of species and shapes.

The division of brachiopods and bivalves into two separate phyla obviously has to do with certain differences. The most striking difference can be found in the shells of the animals. For example, the ventral and dorsal shell of a brachiopod look different, while the shell halves of bivalves are identical. If we take a look at the individual shell halves, we see that the left and right sides are the same in brachiopods. In bivalves, the left and right halves are different; so they are not symmetrical (image 2).

brachiopoden versus bivalven
Image 2.
The most striking differences between brachiopods and bivalves are found in the animals' shells. For example, the ventral and dorsal shell of a brachiopod look different, while the shell halves of bivalves are identical (top left). If we look at the individual shell halves, we see that the left and right sides of the shells are the same in brachiopods. In bivalves, the left and right halves are different; so they are not symmetrical (top right). The brachiopod is on the left in both photos, the bivalve on the right.Below: The outside and inside of shell halves (of different animals) of St. James shells (see text).

schematic overview of the differences
brachiopods bivalves
ventral and dorsal shell are different ventral and dorsal shell identical
the left and right side of the shell halves are the same the left and right side of the shell halves are different

As is often the case, there are exceptions that prove the rule. For example, there are also bivalves whose two shell halves are not identical, while there are also brachiopods that have an almost identical ventral and dorsal shell. A good example of this exception in bivalves is the great scallop (Pecten maximus), which is also known as St. James shell and as an emblem of the oil company Shell. This bivalve has both a convex and a flat shell, while the left and right halves of the individual shells look the same (image 2).

In addition to the striking difference between the shells of brachiopods and bivalves, there are other differences such as the anatomy of the soft parts and the development of the animals. But of course you can’t see these differences when you find fossil specimens.

Image 3.
Brachiopods that are still in their matrix (also fossil seabed). The photos linked with an arrow show the top and side of brachiopods of the same species.

Brachiopods are sessile marine animals. This means that they are stuck on the surface and therefore they cannot move from one place to another. Many species are attached to that substrate with a fleshy, muscular stem. With special catching organs brachiopods swirl water into the shell where they then filter out the planktonic food particles. The water then leaves the shell again. Brachiopods have special opening and closing muscles for opening and closing the shell.

For the origins of brachiopods we have to go back very far in geological history. The first species appeared early in the Cambrian, more than five hundred million years ago. A number of species even had already a great development during the Cambrian period. They had the peak of their development during the early Devonian period, more than four hundred million years ago. They can occur in great numbers in rocks from that period. Not so far from the Dutch border, in the Eifel in Germany, we frequently encounter them in various places in the limestone from the Middle Devonian.

Image 4.
At the bottom of this rock bed (top right, see arrow; photo Els Weertz) in Steinbruch Auf Fuchsloch near Ahrhütte in the Eifel (Germany) a horizon with large brachiopod shells of the genera Stringocephalus and Bornhardtina can be seen (top left). Although tempting to view and photograph in this way, such an action is not entirely without risk. The photo below right shows that. The arrow here indicates where the piece of rock with the brachiopods was six months earlier. Due to frost in winter and other weather conditions, such heavy pieces of rock can become loose from the original rock and fall down. Take such possibilities into account when visiting (abandoned) quarries because they can contain all kinds of dangers. The rock bed in question was part of a coastal and beach environment of the tropical sea in the Middle Devonian at that time. The photo of the bivalves at the bottom left, taken on the beach (Plage des Ecardines) northeast of Calais in France, gives an impression of such a contemporary situation on the coast.

The dominant time of the brachiopods does not end until the Permian. So we could say that they are typical animals of the Paleozoic era. But even during the subsequent Mesozoic, the brachiopods continued to play an important, although less predominant role. During the first part of the Mesozoic, the Triassic, the number of species and genera decreases considerably, but during the Jurassic and Upper Cretaceous there is a certain revival. We again find many brachiopods in the limestone of the upper Cretaceous in the south of Belgian and Dutch Limburg.

Image 5.
To see brachiopods, we do not always have to visit quarries. We can also encounter them in villages and towns in the building blocks of older buildings. These building blocks are often hard limestones from the Devonian and sometimes also Carboniferous. The brachiopods in this image were photographed in Deventer (The Netherlands). The brachiopod at the top right can be seen in the Hofstraat, the brachiopod at the bottom right in the Keizerstraat.The drawing shows a brachiopod attached to the substrate by its stem.

Today, several hundred species of brachiopods exist. That is not so much if we consider that about 30,000 fossil species (see also image 1) are known. Almost all species that still exist, occur in the sea on the continental shelf, in the littoral zone adjacent to the mainland. But some species are found in the deep sea.

For those who want to know more about brachiopods, the following (German) books may be interesting. Especially these books were used to write (the original Dutch version of) this item about brachiopods.

Eifel-Brachiopoden by Hans J. Jungheim is published by Goldschneck-Verlag in 2000 (126 pages). The book not only provides good information about brachiopods in general, but also contains an extensive lexicon of terms and an extensive identification with images of brachiopods from the Eifel in Germany.

Die Eifel (Erdgeschichte, Fossilien, Lebensbilder) is also written by Hans J. Jungheim and published by Goldschneck-Verlag in 1996 (229 pages). This book contains an entire chapter with black and white photos and drawings of brachiopods. Here too, attention is paid to the identification of brachiopods from the Eifel.

Text and pictures: Jan Weertz
Drawing: Els Weertz

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