BRACHIOPOD

Taxonomy

Brachiopod

Kingdom: Animalia
Phylum: Brachiopoda
Class: Rhynchonellata
Order: Rhynchonellida
Family: Trigonirhynchiidae
Genus: Rostricellula
Species: R. minnesotensis
Author Citation Sardeson 1892

Geological Time Scale

Eon: Phanerozoic
Era: Paleozoic
Period: Ordovician
Epoch: Middle

Stratigraphy

Cummingsville Formation

Provenance

Acquired by: Purchase/Trade

Dimensions

Length: 1.2 cm
Width: 1.2 cm
Height: 0.9 cm

Location

Rochester
Minnesota
United States

• brachiopod
• caradoc
• cummingsville formation
• middle ordovician
• minnesota
• ordovician
• rochester
• rostricellula
• rostricellula minnesotensis
• ryhnchonellid
• trigonirhynchidae
• usa

Comments

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