Variable patterns of phenotypic evolution among canonical 'living fossil' lineages

Abstract

Coelacanths, lungfishes, and holosteans represent three emblematic living fossil lineages, thought to be united by similar patterns of phenotypic change through time. While past studies suggest that diverse evolutionary patterns occur within these groups, it is unclear whether these reflect biological differences or arise from contrasting analytical approaches. Here, we examine these lineages under a common framework to assess variation in the evolution of discrete characters, and morphometric shape data, to test whether living fossils show comparable patterns of phenotypic evolution. Our results suggest different evolutionary modes occur, both among and within lineages, as a function of data type. For lungfishes, rates in discrete characters are highest in the Devonian and monotonically decline over time. Coelacanth rates show multiple early peaks followed by a decline toward the recent. Holostean rates show modest peaks but are broadly comparable over time. Patterns of body shape evolution also differ among clades, with strong support for declining rates over time for coelacanths but mixed evidence for similar dynamics in the other groups. Our results imply idiosyncratic processes of evolutionary change among traditional examples of living fossils and indicate a need to explicitly quantify patterns of change rather than apply informal, often qualitative, macroevolutionary classifications.