DOES THE ECOLOGY OF CLADE IMBALANCE EXPLAIN THE "LIVING FOSSIL" PHENOMENON WITHIN A MEGA-PHYLOGENY OF RAY-FINNED (ACTINOPTERYGIAN) FISHES?

Depressed rates of speciation and persistence over long timescales appear to distinguish the evolutionary histories of some organisms from that of their sister clades. More than 150 years after the Origin of Species (1859), it is still unclear whether these taxa, “living fossils,” result from a distinct blend of historical processes, or simply a biased perception of the fossil record. Here, we test a set of ecological hypotheses for the imbalance of diversity between sister clades on a mega-phylogeny of ray-finned fishes. We hypothesize that clade imbalance will coincide with major ecological differences, such as likely diet and habitat preference, between sister clades. Our predictions are based on the traits of “dead clades walking” fishes after mass extinction events. As both “living fossils” and “dead clades walking” are notable for persistence at low taxonomic diversity, it is likely they represent overlapping sets of phenomena in the living and fossil records. Indeed, both “dead clades” and Darwin’s original living fossils, the non-teleost fishes, share traits such as freshwater habitat, large size, and filter-feeding. However, testing these assumptions phylogenetically requires high taxon sampling and high resolution. A recently assembled molecular, fossil calibrated mega-phylogeny of extant ray-finned fishes (~30,000 tips) allows for a robust test. Preliminary analysis shows that the ray fin tree is significantly imbalanced compared to null simulations, under a constant rate model of evolution, implying either persistence and/or biased extinction is common. The subsequent analysis of environmental parameters between sister groups will use data from Fishbase. The next generation of phylogenetic trees, with nearly comprehensive coverage of extant groups, provides an opportunity to revisit living fossils and persistence in the record, and other problems that appeared intransigent, using new methods.