EXAMINING EVOLUTIONARY AND ENVIRONMENTAL CHANGES OF AMMONITES IN THE LATE CRETACEOUS (MAASTRICHTIAN) WESTERN INTERIOR SEAWAY, USA

As ‘classic’ biostratigraphic index fossils, ammonites have contributed heavily to theories about the patterns and processes of evolution in the fossil record. However, quantitative analyses of phyletic evolution based on samples of populations from the same lineage traced through a single stratigraphic section are rare. One of the best records of ammonite evolution is contained within the upper Cretaceous sediments of the Western Interior Seaway, USA. Among the most abundant and well preserved ammonites in these deposits are those belonging to the Family Scaphitidae (scaphites), a group which has an increasingly stable taxonomy. In this study we focused on three species from the Maastrichtian portion of the Pierre Shale and overlying Fox Hills formations of South Dakota (Hoploscaphites birkelundae-Hoploscaphites nebrascensis range zones): Hoploscaphites melloi, H. nicolletii, and H. comprimus, which are considered to constitute a single endemic evolutionary lineage. The stratigraphic distribution of these taxa is limited to two intervals in the Pierre Shale (Mobridge and Elk Butte members), and 5 assemblage zones in the Fox Hills Formation (Trail City and Timber Lake members) which contain thousands of specimens. The total amount of time represented by these horizons is probably <1 million years. Using a large collection of more than 1500 specimens we took a series of morphometric measurements to capture changes in the size and shape of the shell through time. We also analyzed the oxygen and carbon isotopic composition of well-preserved shell material to document co-occurring changes to the local paleoenvironment. Results indicate no directional change in size or shape within species over time followed by apparently rapid transitions between species. An unbiased random walk is the best model to explain evolutionary changes to ammonite taxa through the stratigraphic section. Isotopic data are suggestive of a stable environment with average water temperatures recorded by scaphites between 22 and 25°C. This stability occurs despite local changes in grain size and lithology which probably reflect progradation and shallowing of the seaway during the late Maastrichtian.