LATEST CRETACEOUS FISH AND SHARKS FROM THE HELL CREEK FORMATION OF SOUTHWESTERN NORTH DAKOTA: EVIDENCE FOR MORE EXTENSIVE MARINE TRANSGRESSION OF THE WESTERN INTERIOR SEAWAY (CANTAPETA MARINE TONGUE)

The Late Cretaceous Hell Creek Formation is exposed in the Great Plains of North America and represents the last 1.4 million years of the Cretaceous. It was deposited in a coastal floodplain adjacent to the Western Interior Seaway. The Hell Creek Formation has one of the world’s most extensive fossil records of vertebrates preceding the Cretaceous/Paleogene (K/P) mass extinction. 11 microvertebrate sites were collected from the Hell Creek near Marmarth, North Dakota, all measured to the K/P boundary. Nine of these screen-washed sites have produced a much larger collection of small vertebrate fossils than from previous studies. This new collection includes, 4,122 fish and shark specimens (teeth and scales). Within the sharks, there are 538 Lonchidion selachos, 286 Myledaphus (guitarfish), 165 Restesia americana (carpet shark), 124 Cantioscyllium (nurse shark), 60 Orectolobidae unid., 7 Ischyrhiza (sawfish), and 37 unidentified specimens. Within the fish, there are 304 Amiidae unid., 150 Melvius (bowfin), 7 Belonostomus (needlefish), 1348 Lepisosteidae (Gar), 7 sturgeon, 2 Teleost unid., 2 Paddlefish, 219 Cyclurus (bullhead catfish), and 866 unidentified specimens. There are two localities, 89003 and 86002, that have particularly high shark and ray concentrations. Locality 89003 is 8.4 meters below the K/P boundary and is approximately 65.6 million years old. Locality 86002 is 29.9 meters below the K/P boundary and is approximately 65.9 million years old. Both of these localities are similar in stratigraphic position to the Cantapeta Marine Tongue (an incursion of the Western Interior Seaway) which has been documented near Fort Rice, North Dakota, ~245km to the east. Based on 1) the high abundance of sharks and rays in our samples from the upper Hell Creek Formation near Marmarth, North Dakota and 2) the similar stratigraphic position of the Cantapeta Marine Tongue, we argue that the transgression of the Western Interior Seaway reached this area, considerably further to the west than previously recognized.