TOWARDS A MODEL FOR SPECIATION IN AMMONOIDS: DEVELOPMENTAL TIMING AND ECOLOGICAL SPECIATION MAY PLAY COMPLEMENTARY ROLES IN PROMOTING RAPID DIVERSIFICATION

Two ideas are prevalent in the species-level systematics of ammonoid cephalopods: an emphasis on changes in developmental timing as diagnostic traits and the recognition of frequent parallelism. The model presented here addresses both ideas in a framework based on contemporary concepts of speciation. The model highlights the dual processes of ecological divergence and changes in developmental timing as keys to the rapid production of new species. In a generalized scenario, an ancestral species moves into a newly accessible habitat. Small, random changes in the developmental program produce individuals that may vary substantially in adult shell form and/or body size. These forms are then sorted by natural selection into different ecological niches or microhabitats. The variants persist via assortative mating and disruptive selection, producing divergence and eventually reproductive isolation. Due to developmental constraints, we would expect similar anatomical variants to be produced in related groups, which, if speciating in similar habitats, would create the widespread parallelism we see in ammonoids.

A case study from the Cretaceous Western Interior Seaway (WIS) of North America illustrates the model. Acanthoceratid ammonites moved into the newly formed WIS during the Cenomanian and underwent a rapid endemic radiation over the next two million years. Here I focus on one WIS acanthoceratid clade, the genus Metoicoceras and its dwarf offshoots. The earliest Metoicoceras species was derived from a species of Plesiacanthoceras; juveniles of the two groups are nearly identical, but diverge in the timing of appearance and disappearance of shell ornament traits during ontogeny. Later species of Metoicoceras also differ largely in the timing of appearance of traits during development. The degree of sympatry between contemporary Metoicoceras species increased as the radiation proceeded while the amount of geographic range symmetry decreased. These patterns suggest that the relative importance of geographic isolation changed during the course of Metoicoceras’ radiation—the initial move into the seaway may have followed an allopatric speciation model, with sympatric speciation driven by developmental changes and ecological divergence increasing in importance as the radiation proceeded.