CONSTRUCTING A PHYLOGENY WITH FEW OBVIOUS CHARACTERS: AN EXAMPLE USING LATE CRETACEOUS BACULITIDS (AMMONOIDEA)

Ammonites are among the best studied fossil groups in terms of understanding their biostratigraphy, biogeography, and paleoecology. Despite this extensive knowledge, there has been limited success in using phylogenetic approaches to understanding this important group’s evolution. To a large degree, this stems from the perceived notion that ammonites have a paucity of characters and great number of homoplasies, which are thought to reflect functional convergence rather than common ancestry. These problems are exacerbated in the baculitid ammonites, which have much simpler shells than the more typical planispiral forms that have been elongated into compressed cones that can reach up to two meters in length in certain species.

Here, we show that a robust phylogeny can be reconstructed for an ammonite group with a simple morphology using discrete and continuous character data. Character data were collected from various Campanian-Maastrichtian Baculites morphospecies in the Western Interior and Gulf Coastal Plain and analyzed using the software TNT (Tree analysis using New Technology) to produce a single most parsimonious phylogenetic tree.

Our results reveal new insight into the evolutionary relationships of Baculites. The Baculites in the Gulf of Mexico were characterized by several distinct clades that spanned most of the Campanian and Maastrichtian. In the Western Interior Seaway, there was a single endemic lineage of Baculites that persisted throughout the Campanian but went extinct prior to the Maastrichtian. Following the extinction of the Campanian lineage, there were two sequential immigration events into the Western Interior Seaway by Baculites from the Gulf of Mexico resulting in a distinct early Maastrichtian lineage and a separate, late Maastrichtian lineage. The late Maastrichtian lineage represents the last members of this important group in the Western Interior prior to the K/Pg mass extinction event that resulted in the loss of all ammonites.