NEHM, Ross H., Museum of Paleontology and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780
What processes are responsible for large-scale morphological patterns in the history of life? Integrative investigations of ontogenetic and morphogenetic patterns have great potential for elucidating the role of developmental processes in morphologic evolution within and among extinct clades. Fossil gastropods have figured prominently in studies of large-scale morphological trends, but the role of development as a generating or constraining force in these patterns remains poorly investigated. Prunum gastropods are particularly appropriate for studying developmental evolution: First, Prunum exhibits determinate growth, permitting the recognition of juvenile and adult shell forms; second, the adult shell preserves a complete record of ontogeny, which is easily examined using X-radiography, Scanning Electron Microscopy, and Hard-Tissue Histology; third, phylogenetic analyses have established the evolutionary relationships of the Marginellidae in general and Prunum in particular.
Multivariate morphospaces and phylogenetic analyses provide a rigorous framework for examining the developmental basis of macroevolutionary change in three Prunum clades. In order to examine the role of developmental processes in the generation, maintenance, and dissociation of morphological features, I examined spatial, temporal, and compositional components of shell ontogeny using Scanning Electron Microscopy, hard-tissue histology, and multivariate morphometry. SEM is used to examine the deposition and microstructural composition of shell layers and features through ontogeny; paleohistology is used to quantify the differential deposition of shell material through ontogeny; multivariate morphometrics are used to quantify both heterotopic and heterochronic trajectories. These methods independently demonstrate nearly identical patterns of developmental evolution in all three Prunum clades, including changes in microstructural composition of features, loss and reduction of features, and changes in the timing and order of appearance of features through ontogeny. These changes are a result of iterative paedomorphosis and demonstrate the role of developmental constraint in repeated patterns of large-scale morphological evolution.