A comparative atlas of selected skeletal elements of European urodeles (Amphibia: Urodela) for palaeontological investigations

Abstract

The osteology of European urodeles was studied extensively in the past, but comparative analyses of isolated bones are rare, despite being the most useful tool for the identification of fossil remains. The present work is focused on the most robust skeletal elements (therefore, common in the fossil record: otic–occipitum complexes and vertebrae) and provides taxonomically significant diagnostic characters at the genus level for European genera of urodeles, including some taxa whose osteology was poorly known (e.g. Euproctus and Calotriton). Characters subject to wide variability are identified, and their use is discouraged for diagnoses of extinct species. A selection of diagnostic characters is used herein to build an identification key for each studied skeletal element and for a phylogenetic analysis to understand whether these osteological elements can convey a genuine signal. The analysis under Bayesian inference resulted in a well-supported Salamandridae clade and Pleurodelinae tribe. Salamandrina was recovered as part of the Pleurodelinae, in agreement with previous morphological phylogenetic analyses, but in contrast to the conclusion of molecular studies.

Biogeographic history of Palearctic caudates revealed by a critical appraisal of their fossil record quality and spatio-temporal distribution

“…One of these vertebrae differs from the Koalliella type material (as well as the other Cernay vertebra) in that it has posterior basapophyses. However, this is perhaps not surprising, given that several salamandrid species are characterized by posterior basapophyses in the first or second caudosacral vertebrae, representing the first bulge of the haemal arch [ 95 ]. This interpretation of vertebral position is supported by the transverse processes in the Cernay specimen, which are dorsoventrally developed (forming a vertical lamina in lateral view) and lack articulations for ribs, as is typical for non-rib-bearing caudosacral vertebrae.…”

“…[ 88 ], further supporting the presence of that clade in the Paleocene of Europe. However, it is highly unlikely that this specimen is referrable to Salamandra , as the ventral lamina of the vertebra is expanded, contrasting with the absent or reduced ventral crest that typically characterizes vertebrae of Salamandra [ 95 ]. The material is too fragmentary to provide a generic attribution and is herein regarded as an indeterminate specimen belonging to Salamandridae.…”

“…pliocenicus . Given considerable variation in this feature in extant species of Salamandrina [ 95 , 153 ], it is possible that this is also the case in Chelotriton . However, considering the stratigraphic distance between C. paradoxus and C. pliocenicus , and the absence of a zygosphene/zygantrum complex in specimens of C. paradoxus (see [ 116 ]), we retain C. pliocenicus as a valid species.…”

“…[ 30 , 115 , 187 ]). Furthermore, taphonomic biases influence the preservation of disarticulated remains, with the largest and most robust elements more often represented than other parts of the skeleton in palaeontological collections [ 95 , 153 ]. As such, limb and girdle elements are often found as fragmentary bones that do not bear diagnostic features and are thus consequently identified as indeterminate caudate or vertebrate remains, whereas vertebrae are more commonly identified, described and attributed to a specific taxon ( figure 7 ).…”

“…The extinct species Mertensiella mera has been reported from the Early Miocene of the Czech Republic [110], the Early Pliocene of Slovakia (from where it was originally described; [156]) and the Late Pliocene of Poland [155]. This species is regularly found in sympatry with Chioglossa meini, and it seems likely that the characters differentiating these two species derive from intraspecific variation [95]. Following Macaluso et al [95], we therefore suggest that Mertensiella mera is a junior synonym of Chioglossa meini.…”