IS THERE A RELATIONSHIP BETWEEN SEA LEVEL AND EVOLUTIONARY PATTERN AMONG MACROINVERTEBRATES?

One of the most enduring and vexing issues dealt with by paleobiologists revolves around documenting and interpreting evolutionary patterns – especially dealing with the ‘competing' hypotheses of phyletic gradualism versus punctuated equilibrium. By and large, the approach to delving into this debate has centered on gathering various types of morphologic data from individual lineages throughout the fossil record. The vast majority of these studies, however, have focused on documenting temporal trends in relative isolation. Here, we present a model examining one potential control on the temporal distribution of evolutionary patterns in macroinvertebrates: sea level. Because fossil macroinvertebrates are dominantly retrieved from a broad range of shelfal environments, sea level has the potential to play a substantial role in regulating/controlling a range of aspects associated with the nature and stability of the environments found in such settings and, potentially, their evolutionary response to changes in those environments. Rather than focus on absolute highs and lows associated with past sea-level variation, we instead focus on rate of change. From a theoretical perspective, during icehouse intervals associated with the development of continental ice sheets, sea level should respond with pronounced, short-term fluctuations mediated by variability in the Milankovitch orbital parameters; this results in rates of sea-level change that are substantially faster than most documented evolutionary responses. Concomitantly, greenhouse intervals should be dominated by slower rates of sea-level change largely regulated by tectonic processes that are more in line with documented gradualistic evolutionary responses. Therefore, this model suggests that the dominant evolutionary modes should trend more towards punctuated and gradualistic responses during ice- and greenhouse climate modes, respectively, as a response to the rate of environmental variation mediated by sea-level change. The available data, albeit representing a limited set of macroinvertebrate case studies, show that the vast majority of cases where gradualism has been documented occur during greenhouse intervals.