PATTERNS IN RATES OF DISCRETE-CHARACTER AND BODY-SHAPE EVOLUTION: ARE “LIVING FOSSILS” ALIKE?

Coelacanths, lungfishes, and holosteans represent three emblematic “living fossil” clades, with two of them cited by Darwin when introducing the concept in Origin. Since then, numerous qualitative and quantitative attempts to document their rates of morphological evolution have yielded conflicting results both among and within the groups. However, it is unclear whether this reflects genuine evolutionary differences or stems from contrasting analytical approaches. Here, these groups were examined under a common framework to investigate variation in rates of change of: (1) discrete characters derived from cladistic matrices and (2) shape based on geometric morphometric analysis of fossil specimens. For each clade, we used morphological matrices to infer phylogenetic relationships and jointly estimate branch divergence times and evolutionary rates under a Bayesian framework using the fossilized-birth death model. Branch-specific evolutionary rates generated during these analyses were then used to estimate rates of discrete character evolution over time. Additionally, we fit a series of explicit single and multi-modal models of phenotypic evolution (selected a priori based on hypotheses in the literature) for a subset of taxa with available shape data. The results indicate these “living fossil” groups do not show comparable patterns of morphological evolution over time. In the case of discrete traits, lungfishes show the highest rates in the Devonian and a monotonic decline in over time; coelacanths show peaks in the Devonian, Permian, and Triassic, with declining rates to the recent; and holosteans show peaks in the Late Permian, Jurassic, and Early Cretaceous, but are otherwise stable throughout. Patterns of body shape evolution also differ between each group; however, there are broad similarities between the patterns of discrete character and modeled shape evolution within groups (Early Burst for coelacanths and Ornstein-Uhlenbeck for holosteans). These results show that even among the type examples of “living fossils” there are no consistent patterns of phenotypic evolution and stress the significance of explicitly characterizing the patterns of morphological evolution within lineages rather than broadly applying imprecise—and potentially misleading—historical classifications.