The specimen studied by Lemierre et al. (2023) was discovered by Yves Milon on July 7, 1922 in a lime quarry at Chartres-de-Bretagne (Ille-et-Vilaine). This study celebrates the 100 years of the discovery, carefully recorded in the researcher’s field books at the Musée de geolologie of the University of Rennes. The specimen was found in the collections and the information was cross-referenced with the museum’s historical archives (Figure 1). It is indeed rare to obtain such precise information on a specimen found a century ago.
It is very difficult to distinguish species among the fossil remains of anurans. Since their bone anatomy has been preserved over time, the specimen has been attributed to a species complex morphologically close to the current species Pelophylax kl. esculentus. In Europe, Ranidae frogs are composed of two genera: Rana (brown frogs) and Pelophylax (green frogs). Although a few rare bone fragments as ancient as these exist, the specimen described is the oldest occurrence of an articulated skeleton of a green frog in the world, dating back about 30 million years (Lower Rupelian).
It is also very close to Pelophylax aquensis, a species common in sites with exceptional preservation a little younger, of the upper Rupelian, in the south of France. These sites bear witness to ancient lakes where the remains of frogs, fish, water lily leaves, seeds have been deposited. Tissues are also preserved, including the imprint of the skin (Figure 2).
Green frogs would have appeared in Asia 35 million years ago and would have conquered western Europe very quickly, with this new witness in Brittany 5 million years later. This rapid diversification would be linked to the "Great Break" to the Eocene-Oligocene transition during which an important faunal renewal occurs within amphibians, taking advantage of the ecological niches released by extinct species.
Figure 1: Frog specimen collected from the "fish clay" indicated by the black arrow (view of the quarry). Bottom right: Yves Milon and two workers during his excavations in the Oligocene of Chartres-de-Bretagne in July 1922.
Figure 2: Green frog (Pelophylax kl. esculentus) of the Chartres-de-Bretagne oligocene. A) Articulated individual, coll. Museum of Geology, University of Rennes; B) Osteological interpretation; C) Reconstruction of the animal with white elements preserved (postcranial skeleton).
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.”
Lemierre Alfred, Gendry Damien, Poirier Marie-Margaux, Gillet Valentin & Vullo Romain. 2023 The first European 'true' Ranidae ? An exceptionally well-preserved Pelophylax from the earliest Oligocene of Chartres-de-Bretagne (NW France). Journal of Vertebrate Paleontology, article e2191663, doi:10.1080/02724634.2023.2191663