IMPACT OF THE K-PG MASS EXTINCTION ON PHYLOGENETIC RELATIONSHIPS IN UPPER MAASTRICHTIAN-DANIAN (LATE CRETACEOUS-PALEOCENE) TURRITELLID GASTROPODS IN THE U.S. GULF COASTAL PLAIN

Turritellid gastropods are some of the most diverse and abundant macrofauna of benthic marine assemblages from the Late Cretaceous and Early Paleocene worldwide, including the U.S. Gulf and Atlantic Coastal Plains (CP). Of the more than a dozen turritellid species previously recorded in the CP from each the Maastrichtian and Danian, none have been reported to span the End-Cretaceous mass extinction itself, leaving an entirely new cast of species present in early post-K-Pg strata with uncertain ancestry in the Cretaceous. New phylogenetic analyses of the most well-preserved specimens on either side of the K-Pg boundary – specifically those with known protoconch characters and growth line traces – includes a total of roughly six Maastrichtian and twelve Danian species and supports the conclusion that several Late Cretaceous turritellid lineages did indeed survive the mass extinction and give rise to much of the diversity of Paleocene forms. In particular, two of the most widespread CP species in the latest Maastrichtian, T. vertebroides Morton and T. tippana Conrad, show close morphological affinity to the Paleocene “mortoni” and “rina” species groups of Allmon (1996).

The differential survival of Cretaceous turritellid lineages points to complexity in the K-Pg extinction dynamics of the family given that some, but not all, of the species with larger, multi-whorled protoconchs (associated with lengthier durations in the water column, and thus wider geographic distribution) appear to have survived the environmental catastrophe spawned by the asteroid impact 66 Ma. The frequency with which descendant Paleocene species seem to inhabit a geographically isolated subsection of their Cretaceous ancestral species’ range also suggests that the selective pressures placed on larval ecology during and after the mass extinction event played important roles in shaping the diversity and evolutionary history of the clade.