--> --> Phylogenetic relationships and history of the Campanian and Maastrichtian ammonite Baculites Lamark 1799 in the Western Interior of North America

AAPG Rocky Mountain Section Meeting

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Phylogenetic relationships and history of the Campanian and Maastrichtian ammonite Baculites Lamark 1799 in the Western Interior of North America

Abstract

In the 1950’s, W.A. Cobban began to biostratigraphically subdivide the lithologically monotonous Campanian and Maastrichtian marine strata of the U.S. Western Interior (WI) using the heteromorphic ammonite Baculites Lamark 1799. This was considered by many contemporary ammonite researchers familiar with this clade to be a near impossible task because this group was thought to be too morphologically indistinguishable to break up into multiple species. However, over the next few decades Cobban was able to delineate multiple biostratigraphically important WI Baculites species, which could be used to correlate strata across the WI. Despite extensive knowledge of this clade’s taxonomy and biostratigraphy in the WI, there are still many questions remaining about their evolutionary relationships and history. To build upon Cobban‘s work and to better understand this group’s evolution, this study examines the phylogenetic relationships among middle Campanian to late Maastrichtian Baculites in the WI using a cladistic approach. This study used 31 continuous and 24 discrete characters to construct a single most parsimonious tree using the software TNT (Tree Analysis using New Technology). The resulting tree has excellent biostratigraphic congruence, which is interpreted as independent corroboration of the underlying evolutionary relationships among Baculites in the WI seaway. The most apparent divergence between the tree topology and the biostratigraphic order is represented by the late Maastrichtian Baculites, which plot as the basal-most branch on the phylogenetic tree. This peculiarity in the branching pattern among late Maastrichtian species is the result of this group’s distinct character suite and diminutive size, which is likely related to progenesis (i.e., the retention of ancestral juvenile traits in the mature descendant). The tree topology reveals that middle Campanian to early Maastrichtian Baculites belong to three closely related clades, which reflect successive extinctions of endemic lineages and replacement by new unrelated species that would evolve into new endemic lineages. The late Maastrichtian Baculites also likely belong to a fourth clade that entered into the WI seaway after the extinction of the early Maastrichtian lineage, but due to their position on the phylogenetic tree this remains to be fully resolved. The causes for repeated clade extinction are unknown, however, they were likely forced by the unique and varying environmental conditions of the epicontinental WI seaway. These results suggest that non-vertebrate groups, which have typically been assumed to have a limited number of available morphological characters, can be analyzed to establish a robust phylogeny with a careful selection of appropriate characters.