PHYLOGENY OF THE UPPER CRETACEOUS AMMONITE BACULITES IN THE WESTERN INTERIOR OF NORTH AMERICA

Baculitid ammonites are among the most common fossils in the Upper Cretaceous strata of the Western Interior of North America and have been used in this region as the primary index fossil for the Campanian and Maastrichtian stages. Despite an extremely detailed understanding of the biostratigraphic distributions and paleoecology of these baculitids, relatively little is known about their evolution and phylogenetic relationships.

Here, we examine this group’s phylogenetic relationships using a combination of sixteen discrete and nine continuous characters. These characters include: shell aspect ratio, degree of shell taper, number of ventral ribs along a length equal to the shell height, number of lateral ribs per shell height, aspect ratios of suture lobes, size, shape of cross-section, presence of lateral and ventral ribs, shape of lateral ribs, shape of the venter, symmetry of both lobes and saddles, and presence of a keel. A phylogenetic matrix was put together for each species and the software program TNT (Tree search using New Technology) was used to recover the single most parsimonious tree.

The results of this study both support and alter interpretations of earlier, non-cladistic phylogenetic reconstructions as well as provides new data regarding the origin of Baculites in the Western Interior. Furthermore, this analysis suggests a more complex evolutionary and biogeographic history for these baculitid species than previously considered. The most distinctive result of this analysis, which also supports earlier non-cladistic phylogentic hypotheses, is the identification of two distinct lineages separating an endemic stock from a non-endemic lineage that inhabited both the Western Interior Seaway and the Gulf of Mexico. A comparison between the phylogeny and biostratigraphy of Baculites shows an excellent stratigraphic congruence for the endemic lineage and a poorer agreement in the non-endemic line, which indicates a greater number of emigration and extinction events in the Western Interior among baculitids than previously hypothesized.