BIOGEOCHEMICAL VARIATIONS IN A SCLEROCHRONOLOGIC CONTEXT: DECIPHERING SKELETAL RECORDS OF ONTOGENY, ENVIRONMENT, AND EVOLUTION

The mineralized skeletons of fossil organisms often contain rhythmic banding patterns that reflect incremental growth in response to a hierarchy of environmental stimuli. The most pervasive of such growth patterns is that caused by the seasonal cycle of change, resulting in annual band formation. Counting the number of annual bands yields age and longevity estimates while band size can be used to estimate age-specific growth rates. Techniques borrowed from dendrochronology have proven directly applicable to the analysis of rhythmic growth bands, regardless of the periodicity involved. Such sclerochronological contexts represent important opportunities within which to interpret the record of variation in biogeochemical components such as trace and minor elements and stable isotopes. Research over the last two decades has demonstrated the precision, fidelity, and utility of these biogeochemical records in reconstructing paleoenvironmental conditions and climatic variations of the ancient past, as well as in deciphering the mode of life and life history characteristics for specific fossil organisms. As technology improves, the future remains bright for improving the resolution and expanding the spectrum of such paleoecological studies in space and time. Perhaps brighter, however, is the promise of exploiting fossil bioarchival data to elucidate the pattern and process of evolutionary change. Biogeochemical and sclerochronological analyses of fossil populations within evolving lineages have the potential to reveal changes in developmental timing which are at the very heart of heterchrony, and hence, evolution. Combining phylogenetic analyses with investigations of bioarchival data has the potential to emancipate the study of heterochrony from its dependence on size as a proxy for age or developmental stage, enabling paleontologists to tease apart the fabric of evolutionary change in fossil sequences.