PaleoBios

The fortuitous discovery of Cretaceous (late Maastrichtian) teleostean otoliths in boring samples (17–31 m below ground level) from the Arkadelphia Formation near Cabot, Arkansas, USA, has consequential and overarching ramifications. The otolith assemblage, which is relatively large with 2,109 specimens, represents the first Mesozoic otolith assemblage described from Arkansas and one of the largest Cretaceous assemblages from a single USA site. The diversity of the assemblage is fairly large with a richness of 19 species with three additional taxa in open nomenclature and one unknown lapillus, which more than doubles the known actinopterygians from the Arkadelphia Formation. The otolith assemblage is extremely uneven in its diversity with one species, a putative siluriform Vorhisia vulpes Frizzell (1965b), accounting for approximately 73% of the total. The most unique feature of the otolith assemblage is the presence of cool-water gadiforms,  which represent approximately 7.6% of the total assemblage. The presence of the gadiforms is related to the effects of the Western Interior Seaway and paleogeography during the Late Cretaceous in the western Gulf Coastal Plain. The gadiforms may represent relicts of a greater population and distribution in the early Maastrichtian. Percentage similarity measurements of the otolith assemblage indicate that the Arkadelphia Formation is more closely related to the Severn Formation in eastern Maryland (57.86%) and the  Kemp Clay Formation in northeast Texas (35.77%) than to the Ripley Formation in northeastern Mississippi (5.34%). The similarity measurements and other factors indicate that the Arkadelphia Formation otolith assemblage belongs to the Western Interior Seaway community (bioprovince). The Arkadelphia Formation otolith assemblage also contains several taxa that become extinct, such as the ubiquitous V. vulpes, at the K-Pg extinction event. The otoliths point to a very shallow marine environment (possibly inner shelf; less than 20 m in depth) with estuarine and freshwater input nearby and may be utilized for refinement of paleoshorelines for the southern reaches of the Western Interior Seaway during the Late Cretaceous.