COMPARING MORPHOLOGICAL VARIATION IN UPPER CRETACEOUS AMMONITE SPECIES CO-OCCURRING IN A LARGE AND SMALL BIOGEOGRAPHIC REALM

Evolutionary biologists have long debated the influence of size and shape of biogeographic realms on evolutionary histories. Perhaps the two most fundamental questions concerning the relative fates of clades within differently sized biogeographic units deal with rates of speciation and extinction of otherwise similar species in small and large biogeographic units. But there are other questions as well, including degree of intraspecific variability to be found in species inhabiting small to large biogeographic units. The recent discovery (Ward et al, 2008) that the same species of Upper Cretaceous ammonites within the genera Baculites, Nostoceras, and Didymoceras co-occur in a relatively small (the North American Western Interior Province) and a very large province (the Indo-Pacific Province) allows direct comparison of characteristic intraspecific morphological variability in the same species. Analyses of cross sectional shape, conch shape, shell ornament, and sutural morphological characters from over 1000 specimens of Baculites from four co-occurring species (B. inornatus Meek, B. subanceps Haughton, B. gregoryensis Cobban, and B. occidentalis Meek, all found in both provinces) demonstrate that the larger province contains populations with a far higher degree of morphological variation than from the Western Interior. Sadly, regional parochialism, and unrealistically narrow views of species definitions largely based on ignorance of modern cephalopod biology causes such studies to be viewed with suspicion, with the result that the rich potential of using Upper Cretaceous ammonites for evolutionary studies remains relatively untapped. Sadly, the continued paradigm of using ammonites “species” as time markers in the absence of meaningful understanding of characteristic morphological variation increasingly makes this rich group biostratigraphically as well as paleobiologically irrelevant.