HICKMAN, Carole S., Museum of Paleontology and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780
The earliest calcified portion of the gastropod shell preserves valuable information about ontogeny and the evolution of development. The ability to recognize different developmental types and life history strategies in the fossil record has led paleontologists to important insights in gastropod paleoecology, biogeography, and macroevolution. The larval shell encodes considerably more information, ranging from non-adaptive information about the biomineralization process to records of interactions with predators in the plankton. A program of research focusing on the total information content of larval shell morphology is based on the study of living veliger larvae from the Hawaiian plankton. Using a comparative and integrative approach, I explore the interplay of (1) features that are purely constructional and emerge from specific biomineralization processes and growth rules; (2) features that are ecological; (3) features that are phylogenetically shared as innovations in specific clades; and (4) features that are adaptive in terms of close fit to engineering paradigms for performance advantage. Some of the most surprising results emerge from experimental studies of veligers in culture in the absence and presence of predators. Repeated patterns of larval shell breakage and subsequent repair, documented with SEM, shed light not only on the nature of predation attempts but also on antipredator adaptations in shell construction. Use of the paradigm method provides equally powerful evidence of a set of features of larval shell microsculpture that serve to retard breakage at particularly vulnerable points on the larval shell. Finally, there are microarchitectural features that suggest a heretofore unsuspected role of remote biomineralization and self assembly in construction of the larval shell. The outstanding fossil record of larval and early juvenile shells is hereby opened to assessment of new phenomena in the broader framework of geologic time.