Chapter 3: Scaphitid Ammonites from the Upper Cretaceous (Coniacian-Santonian) Western Canada Foreland Basin

Abstract

The Upper Cretaceous (Coniacian-Santonian) of the Western Canada Foreland Basin, contains a rich record of scaphitid ammonites (scaphites). We describe four species: Scaphites (Scaphites) preventricosus Cobban, 1952, Scaphites (S.) ventricosus Meek and Hayden, 1862, Scaphites (S.) depressus Reeside, 1927, and Clioscaphites saxitonianus (McLearn, 1929). These are widespread index fossils that demarcate the upper lower-middle, middle, and upper Coniacian, and the lower Santonian, respectively. They occur in the lower part of the Wapiabi Formation, Alberta. The Coniacian part of the section has been divided into 24 informal allomembers based on the recognition of marine flooding surfaces, most of which can be traced through the >750 km extent of the study area. The most distinctive feature in the ontogenetic development of scaphites is the change in coiling during ontogeny. At the approach of maturity, the shell uncoils slightly, forming a shaft, which then recurves backward approaching the earlier secreted phragmocone. However, this sequence of scaphites shows an evolutionary trend toward recoiling, accompanied by an increase in size and degree of depression. These changes occurred against a background of changing environmental conditions resulting from the expansion of the Western Interior Seaway during the Niobrara transgression. This resulted in an increase in the area of offshore habitats, which may have promoted the appearance of larger species with more depressed whorl sections. Scaphites probably lived at depths of less than 100 m, and may have fed on small organisms in the water column.