Mark Wilson April 21st, 2013
Another brachiopod this week. This simple fossil is an internal mold of the brachiopod Pentamerus oblongus (J. de C. Sowerby, 1839). It was a very common and widespread taxon throughout North America and Europe in the Early Silurian. This particular specimen was found in a dolomite of the Clinton Group of New York State. This species has been an important fossil for reconstructing Early Silurian paleocommunities, and it is useful in biostratigraphy as well.
I chose this specimen because it has the preservation I have seen in almost every pentamerid brachiopod I have collected: it is an internal mold formed when sediment filled the calcitic shell, was cemented, and then the shell dissolved. We are looking at an impression of a sort of the interior surface of the brachiopod. The posterior (hinge region) of the brachiopod is at the top of this view. You can see a straight slit that represents the ventral muscle field complex (spondylium) that was part of the ventral valve. This was a kind of shelly septum on the floor of the brachiopod interior. we would not see this feature (or rather what is left of it) if the exterior shell had not been removed.
The above is a drawing of Pentamerus oblongus as it looked with its original shell. In this view, unlike our specimen, we are looking at the dorsal valve with the ventral valve visible beneath it.
The genus Pentamerus was named in 1813 by James Sowerby (1757-1822), a prolific scientist we met earlier with our specimen of the Cretaceous bivalve Inoceramus. The species Pentamerus oblongus was fittingly named by his eldest son, James de Carle Sowerby (1787-1871), in 1839. J. de C. Sowerby is shown above in his latter years. The younger Sowerby was an unusual combination of a paleontologist, botanist and mineralogist. He was a friend of the extraordinary scientist Michael Faraday (1791-1867), so he would have had encouragement to be an accomplished polymath. He is said to have conceived one of the first classification of minerals by their chemical compositions. In 1838, J. de C. Sowerby and his cousin Philip Barnes founded the Royal Botanic Society and Gardens (now part of Regent’s Park, London). On top of all this, he was a spectacular scientific illustrator. How many such diverse scientists do we have today?
Johnson. M.E. 1977. Succession and replacement in the development of Silurian brachiopod populations. Lethaia 10: 83-93.
Johnson, M.E. and Colville, V.R. 1982. Regional integration of evidence for evolution in the Silurian Pentamerus-Pentameroides lineage. Lethaia 15: 41-54.
Ziegler, A.M., Cocks, L.R.M. and Bambach, R.K. 1968. The composition and structure of Lower Silurian marine communities. Lethaia 1: 1-27.