The oldest articulated ranid from Europe: a Pelophylax specimen from the lowest Oligocene of Chartres-de-Bretagne (N.W. France)

Abstract

Ranids represent an important part of the extant anuran diversity of Europe. One of the best-known genera is Pelophylax (green-water frog). This genus is considered to have arrived in Europe during the Eocene/Oligocene transition, with numerous occurrences of the genus throughout European Oligocene sites. Unfortunately, most of the specimens are isolated bones, hampering our understanding of the diversity and evolution of the genus during this time. We here present the description of an incomplete but articulated anuran skeleton from the lowest Oligocene of Chartres-de-Bretagne (western France). This specimen, missing its head, preserves almost all postcranial bones articulated and skin impressions. The osteological description allows to assign this specimen to Pelophylax kl. esculentus, making it one of the oldest known occurrences of the genus. We also suggest that specimens assigned to the late Oligocene "Rana" aquensis should be referred to the genus Pelophylax. The presence of a Pelophylax in western Europe during the early Oligocene indicates that the genus had already spread throughout Europe, no later than 5 Ma after its emergence in the eastern part of the continent. It suggests that Pelophylax benefitted from the extinction of ranoids during the "Grande Coupure." SUPPLEMENTARY FILES--Supplementary files are available for this article for free at www.tandfonline.com/UJVP.

Piecing the barcoding puzzle of Palearctic water frogs (Pelophylax) sheds light on amphibian biogeography and global invasions

“…The phylogeography of nearly all Palearctic amphibians has been relatively well‐examined within their western and eastern ranges, and specific regions therein (e.g., Busack, 1986; Dufresnes & Litvinchuk, 2022; Ehl et al., 2019; Oosterbroek & Arntzen, 1992), but causes for the early emergence of distinct clades between these ranges received a more sporadic attention. Ranid frogs originate from Asia (Yuan et al., 2016) and putatively gained access to Europe in the Early Oligocene (~30 Mya), a timing confirmed by German and French fossils from the Paleogene zone MP22 (32.5–30.9 Mya, Lemierre et al., 2023). This timeframe suggests that the closing of the Turgai Sea—a large body of salt water that previously fragmented Eurasia since the Cretaceous (Akhmet'ev, 2011; Briggs, 1995; Duellman & Trueb, 1994)—opened the route for colonization (a in Figure 10).…”

“…Four analyses were conducted in BEAST using most recent common ancestor (MRCA) priors according to competing hypotheses of the Mediterranean taxa that originated during the Messinian Salinity Crisis (MSC), namely P. saharicus (Nicolas et al., 2015), P. cretensis (Lymberakis et al., 2007), P. (ridibundus) cypriensis (Plötner et al., 2012) or P. (ridibundus) cerigensis (Beerli et al., 1996). Fossil evidence indicates that the genus had already dispersed across Eurasia by the early Oligocene (Paleogene zone MP22, 30.9–32.5 Mya; Lemierre et al., 2023; Rage & Roček, 2003), suggesting that its MRCA is at least this old. In all time trees, a first calibration was thus enforced by a log‐normal MRCA prior of all Pelophylax sequences, with offset = 30, M = 3 and S = 1.…”