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.
