DISMANTLING "LIVING FOSSILS": A FRAMEWORK FOR PERSISTENCE, PROLONGED STASIS AND SLOW RATES OF CHANGE

How slow is slow and how persistent is persistent? We demonstrate that while "living fossils" and related concepts are something that everybody knows, their meanings have become convoluted and are often misappropriated. We parse the separate components of these concepts to devise a practical framework for analyzing patterns of long durations and prolonged stasis. The framework has two comparisons at its core: the relative sameness versus difference in a suite of traits or properties of some historical objects relative to other similar objects that are members of a higher level in the genealogical hierarchy, and the shorter or longer persistence among these members. These comparisons are then combined to determine patterns of co-occurrence in relative duration and relative deviation in morphology or other traits. We present results from an analysis of 66 published phylogenetic datasets for metazoan groups with sufficient fossil records to illustrate these comparisons. Contrary to the idea that very persistent lineages are special or unique, we show that they are often no different from most other lineages in terms of morphological distance from a clade average. These concepts are necessarily dependent on context, especially frames of geologic time and the level of the inclusive group in a geneological hierarchy. The branching (cladogenesis) of objects adds another complexity to understanding prolonged stasis. Intuitively, we would expect low rates of branching within long duration, static lineages, and higher rates among comparable lineages that are shorter-lived and more variable. However, a lineage may sustain high branching rates among its member objects, yet very narrow distances between sister objects and very long durations of these objects, thus yielding a long-lived, static pattern. The framework is also extensible to comparisons of relative durations and stasis among non-taxonomic historical objects: nucleotide, biomolecular and developmental modules. Putative explanations of prolonged stasis among both taxonomic and non taxonomic objects include ecological factors, selection, and genetic/developmental constraints. These are shown to have different relationships with components of duration, morphologic distance, and branching rate, and also to be dependent on context.