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I had to make a trip to the "big" city of Rochester, Mn today. As I drove by a new building site that exposed some Decorah Shale, I had to make a stop. The Decorah Shale is an impervious layer that keeps pollution from seeping into deeper rock layers and contaminating our ground water. Obviously, building permits are being obtained without adherence to the zoning which prevents interuption of this great geologic feature! I will drink my own water but collect fossils from these ill conceived sites. While visiting such a site, I discovered this tiny gastropod that I can not identify.
Rhynchonellids are hard to identify by exterior morphology as they often need to have their internal structures visible to be sure of an id. However if you know the faunal lists from a specific area, you can reduce the candidates considerably. The specimen here has 22 costae with 4 on the fold and thus, at this size must be one of two species, Rostricellula minnesotensis or Rhynchotrema wisconsinensis. The only completely safe way to differentiate between the two is the presence or absence of a cardinal process in the brachial valve but this is not possible here. However, Rostricellula usually, though not always, devoid of shell ornamentation, such as ridges or the presence of growth lines, and Rhynchotrema wisconsinensis usually, though not always, shows these, though they can also be seemingly absent through wear. But, R. wisconsinensis never shows a length to width ratio of 1.00, only from 0.80 to 0.95 and this specimen has a ratio of 1.00 which does occur in Rostricellula. Furthermore, the fold of Rostricellula is wider and less sharply developed Finally, good specimens of Rostricellula are far more common than R wisconsinense at the locality as the species most commonly found here is Rhynchotrema ainsiei which is describe elsewhere and not to be confused with the other two due to it's larger number of costae Thus, I am fairly confident with my id
As with the adult this has more costae than any other brachiopod found in this formation. In this case 32. and 5 of them on the fold. The fold and sulcus are not yet very noticeable, as this species only develops a noticeable fold as it matures.
Brachiopods, perhaps rhynchonellids most of all, are notoriously hard to identify without their internal features exposed. However, if you know the formation and rough location and have faunal lists it can be possible. Rhynchonella ainsliei, for example, has 26-34 costae with 5-7 of these appearing on the fold. This specimen has 30 and 5 respectively and is the only species that has so many found in this formation. It also has the correct shape and size to support the match.