Gigantism in tadpoles of the Neogene frogPalaeobatrachus

We describe three giant palaeobatrachid fossil tadpoles of the genusPalaeobatrachus(Nieuwkoop-Faber [NF] stages 60–64) from the Miocene of Randecker Maar, Germany. The largest was 150 mm at the beginning of metamorphosis (stage 60), whereas the smallest was 100 mm and approaching the end of metamorphosis (stage 64). In contrast, normal palaeobatrachid tadpoles and their pipid relatives, both extinct and extant, rarely exceed 60 mm in length. We review here both ecological and pathological conditions that are conducive to the development of gigantism in tadpoles. Tadpoles that lack a thyroid gland become exceptionally large and arrest development at early hindlimb stages (NF stages 53–56). However, the advanced metamorphic stages of the giantPalaeobatrachustadpoles indicate that they were able to metamorphose, and thus were not athyroid. Environmental factors—pond size and permanence, predators, duration of the growing season—may all contribute to tadpole gigantism in certain extant anuran species. We identify suites of ecological features that distinguish extant anurans with large tadpoles from high-latitude and high-altitude permanent lakes in temperate regions (e.g., certainRanaandTelmatobius) from tropical species, such asPseudis paradoxa, whose tadpoles normally achieve large size in temporary seasonal ponds. The paleoecology of Randecker Maar suggests thatPalaeobatrachustadpoles lived in a permanent semitropical lake, but one with few predators.