A Spiny Aptychus from the Cretaceous of Kansas

Abstract

Surficial characters and internal structures of a new species of ammonoid opercula, Spinaptychus sternbergi, discovered in the Cretaceous Niobrara formation of western Kansas, are described and illustrated. The Kansas specimens are better preserved and larger than any previously described species of the genus and represent the first record of the genus from North America. Orientation of growth lamellae leads to the conclusion that Spinaptychus was not a strictly external structure, because the outer margins of its exterior surface at least evidently were covered by mantle tissues.