PUNCTUATIONAL EVOLUTION IN LIGHT OF THE GEOLOGIC RECORD: THE TEST OF LIVING FOSSILS REVISITED

Living fossils, often defined as ancient survivors which undergo only minor anatomical changes over long stretches of geologic history, are somewhat of a conundrum in macroevolution. Phyletic gradualism considers living fossils as prime examples of bradytely, or unusually slow rates of evolution. Alternatively, the punctuational model proposes that evolution primarily occurs through speciation and that living fossils are simply long-lived taxa which escaped extinction. Consistently low diversities over great intervals imply sluggish speciation and therefore conserved morphologies (the so-called ‘test of living fossils’). Assuming that taxonomic diversification is correlated with morphologic evolution, an examination of large-scale patterns of generic (or ideally, species) diversity may provide a rough approximation of the rate of morphologic change. Analysis of the geologic histories of living fossil taxa at the generic level reveals four major patterns. 1) Rapid diversification associated with initial adaptive radiation, an abrupt decline in speciation, and a subsequent period of low diversity lasting hundreds of millions of years characterize the lungfishes, lingulid brachiopods, and monoplacophoran mollusks. 2) A similar pattern may be found in the Aplodontidae (mountain beavers) and tapirs, except the extended phase of low diversity is not present because these mammalian groups originated relatively recently in the Cenozoic Era. 3) Generally high diversity early in a clade’s history followed by repeated episodes of radiation and extinction results in a jagged diversity curve; horseshoe crabs, coelacanths, rhynchocephalian reptiles, nautilid cephalopod mollusks, and perhaps the sclerosponges exhibit this pattern. 4) Both mytilid and pinnid bivalve mollusks show an unexpected pattern of increasing diversity through time – contrary to the usual view of living fossils as the sole remnants of once diverse clades. These results suggest that living fossils do not share a common evolutionary pattern and offer predictions on the actual rate and magnitude of morphologic evolution in these taxa.