LATEST CRETACEOUS FISH AND SHARKS FROM THE HELL CREEK FORMATION OF SOUTHWESTERN NORTH DAKOTA: EVIDENCE THAT THE CANTAPETA MARINE TONGUE (WESTERN INTERIOR SEAWAY) REACHED FURTHER WEST

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 most extensive records of vertebrate fossils 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. 9 of these screen-washed sites have produced a much larger number of small vertebrate fossils than from previous studies. This new collection includes, 3,312 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 approximately 65.6 million years old and is 8.4m below the K/P boundary. Locality 86002 is approximately 65.9 million years old and is 29.9m below the K/P boundary. 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 Western Interior Seaway was near this area, considerably further to the west than previously recognized.