THE PHYLOGENY AND EVOLUTION OF THE HETEROMORPHIC AMMONITE BACULITES IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

The heteromorphic ammonite Baculites is among the most distinctive, yet common Upper Cretaceous marine mollusk, which are especially prominent in Campanian and Maastrichtian strata. They are highly valued by paleontologists and stratigraphers for their biostratigraphic utility, which results from a combination of their short temporal ranges as well as their abundance across a wide range of marine facies. In spite of extensive knowledge of their biostratigraphy, biogeography, and more recently their paleoecology, comparatively little is known regarding their phylogeny or evolution.

To better understand their phylogeny and evolution, we examined the relationships between the various Campanian to Maastrichtian Baculites morphospecies from the Western Interior Basin (WIB) utilizing a cladistic approach to compare against earlier, pre-cladistic evolutionary hypotheses. We examined evolutionary relationships between the various morphospecies using a variety of both discrete and continuous characters. This matrix was then analyzed using the software TNT (Tree search using New Technology) to reconstruct a phylogenetic tree for the Campanian to Maastrichtian Baculites in the Western Interior.

Results provide new insight into the origin, evolution, and phylogeny of Baculites in the WIB, which also support as well as revise pre-cladistic evolutionary hypotheses. The most significant revision to pre-cladistic view, which supports a separation between Campanian and Maastrichtian species in the WIB into discrete lineages, indicates that Maastrichtian forms were a less-derived sister group to the endemic Campanian Baculites in this region. These results also support a Gulf Coastal Plain (GCP) origin for the various lineages of Baculites in the WIB. Results also maintain the interpretation that a single endemic lineage of Baculites persisted in the WIB throughout the Campanian and that species derived from the GCP periodically emigrated into the WIB and coexisted with the endemics. Finally, our data also support the view that there were two successive immigration events into the WIB by Baculites from the GCP resulting in a distinct early Maastrichtian lineage and a separate, but closely related, late Maastrichtian lineage.