DYNAMICS OF MORPHOLOGICAL EVOLUTION IN THE NAUTILOIDEA: A COMPUTATIONAL GEOMETRIC APPROACH

The dramatic history of morphological innovation, fluctuation, and persistence found in the nautiloids provides an excellent opportunity to investigate the dynamics of morphological evolution during radiations and other evolutionary rate-changing events. 592 genera, including representatives from every nautiloid order, were sampled for 49 characteristics of the external shell, internal shell, and endo- and ecto-siphuncular structures. The morphological space resulting from the principal components analysis of this data set was then investigated using both established techniques and new methods developed from computational geometry. The first Late Cambrian nautiloids already exhibit a range that presages the forms that arise in the Arenigian (Lower Ordovician) "explosion" of nautiloid taxonomic diversity. The subsequent Middle Ordovician maximum morphologic range encompasses that found throughout the rest of the Ordovician, Silurian, and Devonian. Beginning in the Carboniferous, there is a dramatic shift towards previously unutilized morphologies-- primarily manifested by smooth, coiled shells with simpler endo- and ecto-siphuncular structures. The post-Paleozoic range of shell character combinations barely intersects that of their initial Paleozoic radiation. Mesozoic and Cenozoic morphological evolution consists only of small-scale fluctuations around shell-types similar to the single extant genus, Nautilus. New methods from computational geometry have been developed to circumscribe the generalized shape of the distribution of points through time and then to measure the volume of occupied morphospace weighted by density. A time-series at the stage level of these shapes provides insight into the patterns of morphological evolution of nautiloids and the mechanics of radiations in general.