Abstract

Rapid diversification of clades within a restricted geographic area is fairly common, and has been well studied as a model of adaptive radiation. The monophyletic clades produced by this type of event have been called “species flocks.” An investigation of the Cenomanian radiation of acanthoceratid ammonites in the North American Western Interior Seaway reveals that this clade shows all the characteristics of a species flock: the acanthoceratid genera are speciose and diversified rapidly, and many of the species are geographically restricted within the Western Interior, possibly due to environmental barriers. A cladistic analysis shows many true polytomies, a high autapomorphy to synapomor- phy ratio, and much parallelism, all reflecting rapid diversification from a single ancestral lineage. These ammonites also show pronounced plasticity of developmental timing. Most differences in morphology among taxa are due to small changes in developmental timing, and an unusually high number of progenic dwarf spinoff taxa occur in this clade, indicating that larger-scale changes in ontogenetic timing are also common. It seems, then, that the developmental program of this ammonite clade was exceedingly labile during its radiation. This plasticity may have allowed the clade to radiate so rapidly and profusely. As developmental plasticity may be a characteristic of ammonoids as a whole, this internal factor, rather than sensitivity to external environmental conditions, may be the root cause of high speciation rates in ammonoids.