RE-EVALUATION OF A MYSTERIOUS MISSISSIPPIAN BIVALVE: PINNA MARSHALLENSIS WINCHELL 1865

Pinna marshallensis was collected, named, and described by Alexander Winchell in 1865 from the Napoleon Member of the Marshall Sandstone (Osagean, Mississippian) in Jackson County, Michigan, USA. The specimen was unillustrated in its initial description and not included in any subsequent work. The oldest accepted pinnid reported in literature is late Osagean or early Meramecian in age and is known in North America and Europe. As such, if Pinna marshallensis is a pinnid, it may represent the oldest member of the group. Recent systematic work on late Paleozoic pinnid bivalves prompted a reexamination of this singular specimen housed in the University of Michigan Museum of Paleontology (UMMP 26759). The holotype is preserved as an external mold with several other moldic individuals and fragments on the slab. There are three individuals on the slab that show a swollen area along one margin, the presence of a radial lineation, and the outline shape of the shell. These characteristics are not seen on each individual. Growth lines are not readily apparent but short, faint oblique growth lines are visible on some areas of the specimens. The general shell shape is unusual, with increasing width for approximately the first half of the shell length and then changing near to ear parallel margins of the second half of growth. This is uncharacteristic of other bivalves. Herein, we redescribe the holotype and other individuals using high resolution imaging,including reflectance transformation imaging and X-ray microCT,to reveal specific anatomical details that may provide insight into the taxonomic affinities of this specimen. The specimen was compared to other regional Carboniferous mollusks and unusual specimens.