LITTLE EVIDENCE FOR ENHANCED PHENOTYPIC EVOLUTION IN EARLY TELEOSTS RELATIVE TO THEIR 'LIVING FOSSIL' SISTER GROUP

Neopterygian fishes represent over half of all extant vertebrate species, and comprise the diverse teleost lineage with ~29 000 species and their taxonomically depauperate holostean sister group with 8 species. This pattern of extreme contrast has fuelled notions of teleosts superiority and spawned a plethora of hypothesised drivers for teleost success, including innovations in jaw mechanics, reproductive biology, and most fashionably, the duplicate genomes of teleosts. Importantly, all of these scenarios presuppose enhanced phenotypic diversification in early teleosts.

To test this key assumption, we quantified evolutionary rate and capacity for innovation in size and shape for the first 160 million years (Permian–Early Cretaceous) of neopterygian history. This required the construction of novel size and shape datasets representing almost 500 Mesozoic neopterygian species. Supertrees of over 600 fossil neopterygian species were then constructed from 120 literature topologies to permit phylogenetic comparative analysis. Size and shape rates were quantified using a univariate Bayesian approach, and a multivariate simulation approach, respectively. Phenotypic innovation was examined by quantifying phylogentic signal in reference to rate information.

Early teleosts in sum do not show enhanced phenotypic rates and innovation relative to holosteans. On the contrary, holostean rates and innovation often match or even exceed those of stem-, crown- and total-group teleosts, belying the living fossil reputation of their extant representatives. Furthermore, we find some evidence for heterogeneity within the teleost lineage. Stem teleosts (which may or may not possess duplicate genomes) excel at discovering new body shapes, while early crown teleosts (unequivocally in possession of duplicate genomes) often display higher rates of shape evolution. Importantly, the latter result reflects low rates of shape evolution in stem teleosts relative to all other neopterygian taxa, rather than an exceptional feature of early crown teleosts. Our findings complement those emerging from studies of extant teleosts, which generally fail to detect an association between genome duplication and shifts in rates of lineage diversification.