New Anatomical Information on the Late Cretaceous Bony Fish, Micropycnodon kansasensis (Actinopterygii: Pycnodontiformes), from the Niobrara Chalk of Western Kansas, U.S.A.

Abstract

Micropycnodon kansasensis (Hibbard and Graffham 1941) is an extinct bony fish that lived in the Western Interior Seaway of North America during the Late Cretaceous. In this paper, we describe previously unreported anatomical features of the fossil fish based on a partial skeleton from the Smoky Hill Chalk Member of the Niobrara Chalk in Gove County, Kansas, U.S.A. The specimen consists of the anterior four-fifths of the fish, including a nearly complete skull with incisiform premaxillary and dentary teeth as well as a vomerine tooth plate and paired prearticular tooth plates. The body possessed imbricate squamation with platy bony scales posterior to the head that decreased in size from dorsal to ventral. Additionally, the lateral sides of the fish were covered with blunt tubercle-type denticles to sharp hook-shaped conical denticles. Furthermore, at least the posteroventral rim of the body was equipped with a series of laterally compressed, sagittally-arranged, triangular denticles. The collection of denticles and platy scales likely served as forms of protection against predators. Radiographic imaging of the specimen suggests that the vertebral column may consist of fused vertebrae. The fossil fish had a deep body with an estimated standard length of approximately 23 cm, indicating a slightly larger specimen than the holotype, and a slightly smaller specimen than the paratype.