Phylogenetic significance of the amphicoelous frogs, Ascaphidae and Leiopelmatidae

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Abstract
The most morphologically primitive of all frogs are those with amphicoelous vertebrae. These frogs are Ascaphus from North America, the six known fossil and Recent species of Leiopelma from New Zealand, and the Jurassic fossils Nolobatrachus and Vieraella from South America. Previous consideration of the evolutionary importance of these frogs has been almost exclusively concerned with their primitive characteristics relative to other frogs. Yet virtually no shared derived characters have been found among the genera. Genetic and cytogenetic divergence is extremely high. Present biochemical estimates of genetic divergence are at or beyond the limits of many of the techniques used. Although the living genera are very plesiomorphic, Ascaphus has such unique characteristics as an intromittent organ, high chromosome number (2n = 46), and oocyte gene amplification via eight endomitotic germinal vesicles. Derived features of Recent Leiopelma include inscriptional ribs, a reduced or absent tadpole stage, and low chromosome numbers (2n = 18 or 2n = 22 male/23 female). Notobatrachus and Vieraella may be difficult to assign to either Ascaphidae or Leiopelmatidae.