FIRST REPORT AND BIOGEOGRAPHIC SIGNIFICANCE OF AN EXTREMELY LARGE GLADIUS OF TUSOTEUTHIS LONGA LOGAN (COLEOIDEA, TEUTHIDA) FROM THE PEMBINA MEMBER OF THE PIERRE FORMATION (CAMPANIAN) IN NORTH DAKOTA

The Pembina Member of the Pierre Formation (Campanian) is exposed at the Pembina Gorge site in a Pembina Escarpment road cut west of Walhalla, Cavalier County, northeastern North Dakota. At this site the Pembina Member, the basal member of the Pierre Formation, rests on the Niobrara Formation. Numerous bentonite layers occur at the base of the Pembina Member which is otherwise a black carbonaceous shale. These bentonites have been determined to be equivalent to the Ardmore bentonite succession and thus provide an age of about 80 Ma for the member. This member is equivalent to the Sharon Springs Member of the Pierre Formation in South Dakota and the Pembina Member of the Pierre Shale in Manitoba, Canada. Gypsum encrusted and impregnated skeletal parts of several vertebrate taxa including mosasaurs (Plioplatecarpus, mosasaur indet.), fish (Enchodus, Xiphactinus, Stratodus), plesiosaurs?, and birds (Hesperornis regalis) are common in the bottom, bentonitic, part of the Pembina Member at this site. Invertebrate fossils are sparse in the Pembina Member; however, an extremely large gladius of the coleoid cephalopod, Tusoteuthis longa Logan, 1898 has been recovered. Although first described from the Niobrara Formation of Kansas, this species has also been reported from the Pembina Member of the Pierre Shale in Manitoba. This is the first report of Tusoteuthis longa from North Dakota and confirms its presence in the basal part of the Pierre Formation in the United States. The North Dakota specimen is extremely well preserved although the distal tip of the rhachis is missing. It is about 30% larger (length of the gladius is 1700 mm and greatest width of the posterior expansion of the gladius is 460 mm) than the largest Manitoba specimen. This specimen provides additional information about the extreme size that the giant Pierre Sea squid could attain. The North Dakota fossil exhibits no signs of predation as noted on other Tusoteuthis specimens.