OCCURRENCE OF ARTICULATE BRACHIOPODS (RHYNCHONELLIDA: ORBIRHYNCHIA? PETTITT, 1954) FROM THE UPPER BEARPAW SHALE (CRETACEOUS: LATE CAMPANIAN) OF MONTANA

Brachiopods are poorly represented and, therefore, a minor component of the marine environments of the Cretaceous Western Interior Seaway. Although inarticulate brachiopods, such as Lingula , are not uncommon in the Redbird Member of the Bearpaw Shale, the occurrence of any articulate brachiopod is considered novel. Recently four specimens of an articulate brachiopod were recovered from fine-grained siltstone concretions within the Exiteloceras jennyi zone in the Bearpaw Shale of Carter County, Montana. This zone is considered to be early late Campanian (~73-74 mya). The specimens are housed at the Wyoming Dinosaur Center, accessioned as WDC-BpS-001.

The shells are strongly ribbed and wedge-shaped with a zig-zag commisure and impunctate microstructure. These characters are all found in rhynchonellid brachiopods, which are common taxa in the Mesozoic. The shallow dorsal fold, an anterior commissure with low arcuate uniplication, numerous radiating ribs or costae, and rounded to slightly angular accordion-like folds on the sulcus indicate placement in the genus Orbirhyncha. Serial sectioning of the specimens would be needed to determine if the specimens represent a new species.

The genus Orbirhynchia has a stratigraphic range from the Late Albian to the Middle Campanian. If correctly identified, these specimens extend the range into the Late Campanian.

Because of the various factors that control the distribution of sessile benthic epifaunal suspension feeders, these specimens provide an opportunity to learn more about the paleoenvironments prior to the cessation of deposition during the final transgressive phase of the Western Interior Sea.