Fossil frogs of the genus Palaeobatrachus (Amphibia: Anura)

Roček, Zbyněk and Rage, Jean-Claude and Venczel, Márton (2021) Fossil frogs of the genus Palaeobatrachus (Amphibia: Anura). Abhandlungen der Senckenberg Gesellschaft für Naturforschung, 575. pp. 1-151. ISSN 1868-0356

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Abstract

Extinct frogs of the genus Palaeobatrachus are osteologically uniform, apparently because of their permanent life in water, similar to that of pipids. This, together with the fact that some species were based on articulated, mostly compressed skeletons, while others were based on disarticulated, 3D preserved bones, caused problems with their taxonomy. Another source of taxonomic misinterpretations was found in that frontoparietals of Palaeobatrachus have a complex inner structure (upper and basal layers of compact bone separated by a layer of cancellous bone), which can be fossilised separately, leaving morphologically different imprints in matrix. To avoid such inaccuracies, we used in our taxonomic comparisons three standardised osteological units called the character modules (frontoparietal, presacral vertebral column, and synsacrum) which were defined in advance according to a unified scheme and then sought in each specimen. These three modules were assessed as an interrelated complex termed here the morphotype. Comparisons of morphotypes allowed to make reasonable conclusions even if some important diagnostic characters were not preserved. For the same reason, we assessed the development of characters, in order to restrict comparisons to fully-grown individuals. It turned out that some tiny individuals, reaching an SVL length of about 30 mm or even less, may be considered fully-grown adults, based on the complete ossification of carpals, epiphyses of the long bones, and the synsacral wings. Special attention was paid to the synsacrum, which is a complex of two or three vertebrae that fuse with each other to various degrees in different taxa. Except for Palaeobatrachus gigas, the vertebral column of Palaeobatrachus consists of nine vertebrae, the anterior two (V1+2) and posterior two (V8+9) are fused together; and V7 may remain entirely independent of V8+9, or may completely (including the transverse processes) fuse to V8+9. A large number of investigated specimens enabled the statistical evaluation of morphometric data. As a general basis for taxonomic comparisons, we first gathered diagnostic features common to all hitherto recognised species of Palaeobatrachus, in order to differentiate it from other anuran genera. Then, we reviewed all hitherto recognised species, if they were based on reasonably well preserved and accessible type specimens, trying to find those features which differentiate them from the other species. Using the above-mentioned comparisons of morphotypes, 18 species are recognised, two of them introduced as new. Several other taxa (e.g. those from the late Eocene of Kučlín, and from the late Oligocene of Enspel) remain unnamed because of the scarcity of material, or for other reasons. Comparisons of samples extending from the middle Eocene through middle Pleistocene spanning about 40 Ma also made it possible to recognise some evolutionary trends.

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