Species delimitation and coexistence in an ancient, depauperate vertebrate clade.

backgroundA major challenge to understanding how biodiversity has changed over time comes from depauperons, which are long-lived lineages with presently low species diversity. The most famous of these are the coelacanths. This clade of lobe-finned fishes occupies a pivotal position on the vertebrate tree between other fishes and tetrapods. Yet only two extant species and fewer than 100 extinct forms are known from the coelacanth fossil record, which spans over 400 million years of time. Although there is evidence for the existence of additional genetically isolated extant populations, a poor understanding of morphological disparity in this clade has made quantifying coelacanth species richness difficult.

resultsHere, we quantify variation in a sample of skulls and skeletons of the Triassic eastern North American coelacanth †Diplurus that represents the largest assemblage of coelacanth individuals known. Based on the results of these quantitative comparisons, we identify a diminutive new species and show that multiple lacustrine ecosystems in the Triassic rift lakes of the Atlantic coastline harbored at least three species of coelacanths spanning two orders of magnitude in size.

conclusionsConceptions about the distribution of species diversity on the tree of life may be fundamentally misguided when extant diversity is used to gauge signals of extinct diversity. Our results demonstrate how specimen-based assessments can be used to illuminate hidden biodiversity and show the utility of the fossil record for answering questions about the hidden richness of currently species-poor lineages.