LIVING FOSSILS AS A RESEARCH PROGRAM

Advances in quantitatively evaluating evolutionary modes among different characters in fossil lineages illustrates the value of rethinking living fossils in terms of parts (characters) and wholes (typically organisms, lineages, or clades) for advancing our understanding of evolutionary stasis. Too many definitional criteria apply in some instances, but not others. Instead of viewing the living fossil concept in terms of categorization (i.e., what counts as a living fossil), we argue that it is better to see its role as setting an agenda for research. First, different conceptions (and criteria) of living fossils derive from different explanatory expectations. Second, increased sampling of the fossil record changes how different conceptions apply to perceived rates and status. Third, molecular phylogenetic methods have introduced new 'parts' of lineages. What needs to be specified now are evaluative standards for deciding whether descriptions or explanations meet the aims of the relevant research communities. It is also necessary to distinguish different types of questions (e.g., geographical distribution of extant and extinct taxa versus geological duration of particular molecular or morphological parts or of lineages) and articulate thematic and dependency relationships between these questions (e.g., connections between questions about ecological stability and morphological stability or answers to questions about organismal stasis relying on answers to questions about stasis for proxy characters). Increasingly sophisticated computational models and comparative analyses are synthesizing evidence from molecular and morphological characters, and functional or ecological parameters relevant to rates of character evolution. Molecular phylogeography, niche modeling, paleoclimatology, and life history traits reveal more dynamic geographic histories than implied simply by the term "geographic relic." In sum, a viable research program on living fossils is comprised of focused questions about patterns in need of explanation and processes relevant to specific morphological and molecular character constellations and wholes for which they may serve in proxy roles.