Robustness in the History of Life?

Abstract

Abstract The definitions of robustness are many and varied, but most focus on the ability of a system to retain function in the face of an external perturbation. Assessing function in the fossil record is a difficult but not intractable task and such a definition poses considerable difficulty for considering robustness in the fossil record. Paleontologists have identified patterns of stability in the fossil record of diverse marine and terrestrial plants and animals through the past 600 million years in taxonomic diversity, in the stability of major morphological architectures, or body plans, and in the structure of ecological relationships. Inferring the extent to which the documented resilience (return to a previous state after a perturbation) of these patterns implies a robustness of function must always be indirect. The ability to infer ecological or developmental function in deep time is limited by the fact that the fossil record is primarily composed of durably skeletonized marine organisms, bones of various vertebrates, insects, and the often disarticulated remains of plants, but, of course, missing the soft anatomy. In almost all cases the many soft-bodied organisms, those without durable skeletons, are missing from the record. We are left with changing patterns in the number of taxa (generally families or genera, less commonly species) within clades, patterns of dominance within assemblages of organisms at a particular locality (assemblages that have often been accumulated over a span of time that can extend to hundreds or even thousands of years), and the conservation of morphological architectures. For many of these systems we may be able to make plausible arguments that stability reflects an underlying robustness, but identifying the functional attributes of this robustness can generally only be done by comparison to living organisms and extant ecosystems. This is an increasingly difficult task the farther back in time one goes.