A new marine vertebrate assemblage from the Upper Cretaceous Dakota Formation in Nebraska, USA, and its paleoecology and taphonomy

Abstract

The upper part of the Dakota Formation (lower part of middle Cenomanian) is a marine sedimentary rock unit formed at the dawn of the major transgression of the Late Cretaceous Western Interior Seaway of North America. In this study, remains of marine vertebrates from a fossiliferous bed in the stratigraphic horizon in southeastern Nebraska were examined, which resulted in a collection of over 7292 taxonomically identifiable specimens, consisting minimally of 51 distinct vertebrate taxa. They include at least 26 chondrichthyans, 20 osteichthyan fishes, and five tetrapod taxa, many of which mark the first occurrence in the Cretaceous of Nebraska. The vertebrate assemblage is represented primarily by many small piscivorous pelagic fishes (e.g., cf. Stratodus, Enchodus, and Pachyrhizodus) and durophagous benthic fishes (e.g., Rhinobatos, Pseudohypolophus, Ischyrhiza, cf. Hadrodus, cf. Palaeobalistum, and Albulidae indet.) as well as a small squamate (Coniasaurus) and a diving bird (hesperornithiform). It also includes multiple medium–large predatory pelagic fishes (e.g., Haimirchia, Protolamna, Cretodus, Cardabiodon, Cretoxyrhina, Archaeolamna, Cretalamna, and Protosphyraena) as well as plesiosaurs, that are thought to have occupied the highest trophic positions in their ecosystem. This study is significant because it represents the first formal account of the vertebrate assemblage from the Dakota Formation. Although the fossil assemblage may be time-averaged for possibly as much as 50,000 years timespan, it offers a glimpse into the paleoecology of a nearshore, shallow marine environment during the early part of the middle Cenomanian representing the initial phase of a major transgression of the Western Interior Seaway.