An actinopterygian-dominated fish fauna from the Upper Cretaceous Williams Fork Formation, northwestern Colorado, and evidence for provinciality across Laramidia at the Campanian/Maastrichtian boundary

The Williams Fork Formation (WFF) of northwestern Colorado preserves an understudied freshwater biota from the Campanian/Maastrichtian boundary. Here we describe a diverse actinopterygian-dominated fish assemblage from the ReBecca’s Hollow locality of Rio Blanco County. Chondrichthyans are rare from this site, but include a hybodontid, Lonchidion , and the hemiscyllid Chiloscyllium . A fragmentary element may represent a pycnodontiform. Chondrosteans are represented by a tentative acipenserid. Holosteans include the lepisosteids Atractosteus and an unnamed taxon, and the amiids Melvius , Cyclurus and Palaeolabrus . Teleosteomorphs at this site include Belonostomus , Paralbula casei , Coriops , Estesesox foxi , Acronichthys , hiodontids, and acanthomorphs. This locality yields at least 17 fish taxa, most of which have not been described previously from the WFF. Although the WFF is temporally correlative with the St. Mary River and Horseshoe Canyon Formations in Alberta and the Prince Creek Formation in Alaska, the ReBecca’s Hollow fish assemblage is markedly different from its northern contemporaries. Specifically, it contains several warm-climate taxa such as Lonchidion , Chiloscyllium , Melvius , Atractosteus , and Paralbula , and lacks higher latitude taxa such as Holostean A, documented from Alberta. The differences in these broadly contemporaneous fish assemblages supports the hypothesis that there was provincialism amongst freshwater fishes in Laramidia at the Campanian/Maastrichtian boundary. The fish fauna thus resembles that of Campanian localities in Utah, New Mexico, and Texas, and Maastrichtian (Lancian) localities in Montana, North Dakota, South Dakota, and Wyoming, suggesting that the boundary between these provincial zones remained north of the WFF locality throughout these periods of climate change. ● ReBecca’s Hollow (RH) (Williams Fork Formation) preserves at least 17 fish taxa ● RH’s fish fauna has key differences from coeval units in AK and AB ● RH’s fauna resembles that of assemblages from the Campanian of UT, NM, and TX ● RH’s fish assemblage also resembles that of the Maastrichtian of MT, ND, SD, and WY ● Laramidian fish were subject to provincialism at the Campanian–Maastrichtian boundary