Vertebrates of the Cannonball Formation (Paleocene) in North and South Dakota

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Abstract
We recognize at least 22 taxa of vertebrates from the Paleocene Cannonball Formation, which records the last marine incursion into central North America. These include 12 elasmobranchs, two chimaeroids, five actinopterygians, and three reptiles. All but three taxa are described and illustrated for the first time for the formation. One species is new, Dasyatis concavifoveus. Teeth of Carcharias taurus are most frequently found; other vertebrate fossils are relatively uncommon or rare. Cannonball vertebrates are known from 72 localities, most from southwest-central North Dakota. The marine fishes suggest a temperate Cannonball Sea of primarily shallow subtidal depths. Surprisingly, osteichthyan diversity is much lower than that of chondrichthyans, likely because of taphonomic factors. The marine fishes confirm a Paleocene, more specifically Thanetian, age for the Cannonball Formation. The Danian–Thanetian boundary, however, may occur within the Cannonball Formation because other fossil evidence implies a Danian age. Of the 15 marine Cannonball fishes identified to species, 13 are known from western Europe, seven from Maryland and Virginia, and five from western Greenland. These occurrences imply a connection of the Cannonball Sea with the Atlantic Ocean. These fishes, with the possible exception of Otodus obliquus, are only known from Paleocene or younger strata, supporting proposed K–T boundary marine fish extinctions and Paleocene fish renewals.