Meeting Abstract
An integral step to understanding the evolution of the Acanthomorpha (spiny-rayed teleostean fishes) is identifying their earliest diverging extant members. A major impediment to understanding them is a lack of information about the ten species of the ‘living fossil’ genus Polymixia, the only survivor of a Late Cretaceous radiation, and the most-often cited primitive acanthomorph genus. Although often used as an outgroup in phylogenetic analyses, its morphological and genetic diversity are poorly understood. The type species Polymixia nobilis (Stout Beardfish) and all six species named since 1970 were described on external morphology and meristics. The only osteological treatment was a comparison of another species (P. lowei) to the Beryciformes, a group to which Polymixia is not closely related. In our interdisciplinary study, we have examined the osteology and molecular phylogeny of all available species of Polymixia. A critical first step has been a detailed osteological analysis of the type species, P. nobilis, using cleared-and-stained specimens and µ-CT scanning, with comparisons to other species. We obtained new specimens from the type locality of P. nobilis and comparative material of other species from throughout the known range of the genus. A second step has been a phylogenetic analysis based on DNA sequences. This yielded evidence for two clades within Polymixia, each represented by several valid species, as well as probable species-level synonymies and cryptic species. Importantly, although the phylogenetic position of Polymixia is often stated by the authors of broad-scale molecular phylogenies to be controversial or uncertain, our study provides strong evidence for one particular relationship.
