THE ORIGIN OF HIGHER TAXA DURING THE CENOMANIAN RADIATION OF AMMONITES WITHIN THE WESTERN INTERIOR SEAWAY OF NORTH AMERICA

The Cretaceous Western Interior Seaway offers a spectacular laboratory for analyzing the links between environment and evolution, including the mechanisms underlying evolutionary radiations. Decades of painstaking, field-based stratigraphic and paleontological work by Bill Cobban and his colleagues, combined with Bill’s novel insights into the nature of morphological variation in ammonoids, have provided a rich, multi-faceted dataset that will be used for years to come to explore paleobiological questions. As an example, careful dissection of species-level geographical and morphological patterns reveals details of how environment and developmental lability can work together to fuel diversification. During the Middle and Late Cenomanian, acanthoceratid ammonites entered the newly-formed Western Interior Seaway and underwent a rapid radiation within this new habitat. Speciation events that established new endemic genera within the Western Interior Seaway preferentially occurred within a more northern region, centered in what is today eastern Wyoming. On the other hand, speciation events that established new genera that eventually emigrated out of the seaway (e.g., Metoicoceras) occurred in the south (New Mexico and Texas). Later speciation events within these genera are more geographically widespread. Genera arising within the northern endemic center show a pronounced size divergence, being predominantly either progenetic dwarfs (adult diameter <2 cm) or relatively large forms (adult diameter up to 40 cm). This shift away from intermediate sizes towards small and large extremes is the opposite pattern to that predicted by the “island rule,” which argues that restricted resources and/or habitat space tends to drive species to intermediate sizes. Rather, the size divergence seen in the endemic center may reflect trophic specialization. The morphological changes associated with most endemic speciation events involved shifts in the developmental timing of characters, rather than the appearance of entirely new traits. The northern “endemic generation machine” shut down after the Dunveganoceras pondi Zone (earliest Late Cenomanian), after which most speciation events filled in pre-existing genera rather than producing new genera.