INTEGRATIVE APPROACHES TO LATE CRETACEOUS MARINE BIOSTRATIGRAPHY AND BIOGEOGRAPHY

Stratigraphically Correlated Assemblages (SCAs) are integrative biochronostratigraphic units that function as a series of successive temporal windows through which marine biodiversity can be quantitatively evaluated. Designed to provide a broad framework for correlations among Late Cretaceous marine shelf and epicontental sea deposits, SCAs synthesize Cobban’s ammonite zonations with calcareous nannofossils, benthic foraminifera, eustatic sea level changes, and radioisotope-derived ages. In so doing, SCAs offer a convenient and robust method for compiling and analyzingz marine fossils from far-ranging localities while retaining high levels of biostratigraphic resolution.

SCAs have been employed to test genus-level richness trends and morphological diversification among mosasaurs throughout the Late Cretaceous (Ross 2009). Here, I apply SCAs to address the hypothesis that marine vertebrate diversity is reduced along poleward-trending latitudinal gradients within the Western Interior Seaway. Nicholls and Russell (1990) noted a decrease in biodiversity poleward among selected WIS deposits from Alabama, Kansas, South Dakota, Saskatchewan, and the Northwest Territories, but conceded that several of the lithologic units studied might not be contemporaneous, and therefore they could not rule out temporal diversity fluxes, as opposed to biogeographic and environmental factors.

Reanalysis of these units using SCAs demonstrates that there were two distinct temporal windows in Nicholls and Russell’s analysis. To test their postulate of decreasing poleward diversity, a data set of 756 mosasaur fossils from these units was analyzed using rarefaction. This anaylsis confirms astatistically supported decrease in poleward diversity among these taxa. Further analysis of this kind using a suite of marine taxa (plesiosaurs, turtles, osteichthyans, etc.), could result in a far more resolved and comprehensive picture of Late Cretaceous biodiversity than has been heretofore available.