An examination of freshwater euselachian fossils from the Maastrichtian lower and upper Hell Creek Formation, and the Bug Creek Anthills (Cretaceous-Paleogene boundary), and the early Paleocene (Puercan) Tullock Member of the Fort Union Formation of Montana, USA, revealed seven taxa: Lonchidion selachos , Protoginglymostoma estesi , Chiloscyllium , Restesia americana , Ischyrhiza avonicola , Myledaphus pustulosus n. sp., and Dasyatis (Dasyatidae). Squatirhina americana is redescribed to the new genus Restesia based on unique crown morphology. Dasyatis is only reported from the Paleocene. This is in contrast to the other collected taxa, which are only known from the Cretaceous. Ischyrhiza is not reported in our samples from the Hell Creek Formation despite earlier erroneous claims; however, the taxon is present in a sample from the Bug Creek Anthills. We suggest that this taxon infrequently moved into the freshwater rivers to forage. Lonchidion selachos occurs only in the upper Hell Creek Formation, and we hypothesize that the upper Hell Creek localities were deposited during a warm interval, as the paleodistribution of L. selachos shifted significantly to the north during warmer periods in the Santonian and Campanian. Of the taxa found in the Hell Creek Formation and the Bug Creek Anthills, only Myledaphus is found in Paleogene deposits; however, specimens are extremely rare. The occurrence of Myledaphus in the Paleogene is suggested to be due to the occurrence of reworked material.

Remains from vertebrate microfossil localities of the Hell Creek Formation provide a basis for evaluating the diversity and relative abundance of actinopterygians. Taxonomic composition of the basal actinopterygians of the Hell Creek Formation is increased by the recognition of a probable semionotiform designated Holostean A and a small-bodied amiid. A combined taxonomic/parataxonomic approach incorporating data from centra and tooth-bearing elements demonstrates that, at minimum, 17 kinds of teleosts are present in the Hell Creek assemblage. These include an ostariophysan with possible affinities to catfish and the first Cretaceous record of Priscacara . Latitudinal patterns are identified using both presence/absence and relative abundance data. Melvius is interpreted as having a more southern distribution, while the probable semionotiform referred to as Holostean A has a more northern distribution. A greater abundance of Holostean A in the lower Hell Creek localities than in the upper Hell Creek localities is interpreted as a result of shifts in distribution in response to changes in climate. A comparison of the relative abundances of fish in assemblages from the mid-Campanian Judith River Group and the Hell Creek Formation allows changes in the structure of aquatic paleocommunities to be identified. Amiids are of much greater abundance and the osteoglossomorph Coriops is of lower abundance in the Hell Creek Formation than they are in the mid-Campanian localities. In the Hell Creek Formation, teleosts are dominated by small-bodied taxa, but greater numbers of large taxa are present, suggesting that more teleosts occupied positions higher in the food web.