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Gigantism in tadpoles of the Neogene frog Palaeobatrachus

Zbynk Roek¹, Ronald Böttcher² and Richard Wassersug³

¹ Zbynk Roek. Department of Paleobiology, Geological Institute, Academy of Sciences, Rozvojová 135, CZ-165 00 Prague, Czech Republic, and Department of Zoology, Charles University, Vininá 7, CZ-128 44 Prague 2, Czech Republic. Rocek{at}gli.cas.cz
² Ronald Böttcher. Staatliches Museum für Naturkunde, Rosenstein 1, D-70191 Stuttgart, Germany. boettcher.smns{at}naturkundemuseum-bw.de
³ Richard Wassersug. Department of Anatomy and Neurobiology, Sir Charles Tupper Medical Building, 5850 College Street, Dalhousie University, Halifax, Nova Scotia B3H 1X5, Canada. tadpole{at}dal.ca

We describe three giant palaeobatrachid fossil tadpoles of the genus Palaeobatrachus (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 giant Palaeobatrachus tadpoles 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., certain Rana and Telmatobius) from tropical species, such as Pseudis paradoxa, whose tadpoles normally achieve large size in temporary seasonal ponds. The paleoecology of Randecker Maar suggests that Palaeobatrachus tadpoles lived in a permanent semitropical lake, but one with few predators.