Discoglossus, OTTH, 1837

Rage, Jean-Claude, 2016, Frogs (Amphibia, Anura) From The Eocene And Oligocene Of The Phosphorites Du Quercy (France). An Overview, Fossil Imprint 72 (1 - 2), pp. 53-66 : 54-55

Discoglossus group

( Text-fig. 2 : 1– 6)

The Alytidae are currently present in Europe, northwesternmost Africa and the western Middle East. They include three living genera, Alytes WAGLER, 1830 , Discoglossus OTTH, 1837 and Latonia VON MEYER, 1843 ( Biton et al. 2013). These frogs, with two other living genera ( Bombina OKEN, 1816 and Barbourula TAYLOR et NOBLE, 1924 ) have been traditionally referred to as Discoglossidae GÜNTHER, 1858 . However, Frost et al. (2006) split the Discoglossidae into two families, the Alytidae and the Bombinatoridae GRAY, 1825 . The name Discoglossidae disappeared and, in Frost et al.’s classification, its equivalent is Costata LATASTE, 1879. However, the name Discoglossidae is still widely used, mainly in paleontology (e.g., Roček et al. 2010, Szentesi and Venczel 2012, Gardner and DeMar 2013, Venczel and Hír 2013).

The Alytidae first occur in the Middle Jurassic (Bathonian) of England ( Evans et al. 1990). They range among the earliest known anurans (Triassic forms being salientians, not anurans) and, from the Middle Jurassic onwards, they have been present in Europe. Most fossils belong to the ‘ Discoglossus group’, more specifically they are related to the Discoglossus - Latonia assemblage ( Roček 1994).

In the Phosphorites, Alytidae are rare, especially in the Eocene. In his unpublished thesis, Duffaud (2000) reported a possible scapula from Sainte-Néboule (MP 18), but the specimen was neither described nor figured and now is missing from the collections. For the time being, a single fragment of urostyle from Rosières 1 (MP 19) demonstrates that alytids were present in the Phosphorites before the Oligocene ( Text-fig. 2 : 1). However, outside of the Phosphorites, alytids were more frequent, although never numerous, in the European Eocene ( Rage and Ford 1980, Milner 1986).

In the Oligocene, alytids occur from MP 21 to MP 25 and in MP 28. Alytids from the Oligocene were all referred to as Discoglossus cf. giganteus by de Bonis et al. (1973: table 2–5). D. giganteus WETTSTEIN- WESTERHEIMB, 1955 is now included in the genus Latonia , as L. gigantea ( Roček 1994, Rage and Hossini 2000). Before Roček’s revision of Latonia ( Roček 1994) , various alytids from the Cainozoic were erroneously assigned to D. giganteus . However, there is likely more than one alytid taxon in the Oligocene of the Phosphorites. The few alytids from the early Oligocene (MP 21 and MP 22) appear to be about twice the size of the younger specimens ( Text-fig. 2 : 2, 3) and, based on urostyles, their size is similar to that of the only specimen known from the Eocene. This difference in size may correspond to a taxonomic distinction, but this cannot be demonstrated on the basis of the available material. In addition, in MP 23, large and smaller alytids coexisted. Duffaud (2000) suggested that Latonia vertaizoni FRIANT, 1944 might be present in the Oligocene of the Phosphorites. The latter taxon is known by a single specimen from Vertaizon, a locality relatively close to the Quercy and whose age is regarded as late Oligocene, although an early Miocene age cannot be rejected ( Gaudant 1993). Unfortunately, comparisons are almost impossible because the holotype of L. vertaizoni is an articulated skeleton whose bones are poorly preserved ( Roček 1994: fig. 19).

The Alytidae from the Phosphorites cannot be identified to genus level. However, various features observable on the available bones demonstrate that these alytids belong to the ‘ Discoglossus group’ (presence of a dorsal crest on the ilium, tuber superius formed by a thickening of the posterior part of the latter crest, vertebral centrum cylindrical, neural spine projecting posteriorly beyond the level of the postzygapophyses, distal part of the humerus relatively expanded transversely; Text-fig. 2 : 4–6).

Figures

Text-fig. 2. Alytidae and Pelobatidae from the Phosphorites du Quercy. 1: Alytid indeterminate, urostyle UM-ROS1-601, from the late Eocene of Rosières 1 (MP 19), in anterior (a) and dorsal (b) views. 2: Alytid indeterminate, urostyle UM-ABL1-1901, from the earliest Oligocene of Aubrelong 1 (MP 21), in anterior (a), dorsal (b), and left lateral (c) views. 3: Alytid indeterminate, urostyle UM-BEL 1513, from the middle Oligocene of Belgarric (MP 25), in anterior (a), dorsal (b), and left lateral (c) views. 4: Alytid indeterminate, left ilium UM-GAO3-01, from the early/middle Oligocene of Gardiol 3 (MP 23), in lateral view. 5: Alytid indeterminate, presacral vertebra UM-BEL 1512, from the middle Oligocene of Belgarric (MP 25), in anterior (a), ventral (b), and dorsal (c) views. 6: Alytid indeterminate, left humerus UM-BEL 1514, from the middle Oligocene of Belgarric (MP 25), in ventral view. 7: Pelobatid indeterminate, right ilium UM-RAV 3500, from the earliest Oligocene of Ravet (MP 21), in lateral view. 8: Pelobatid indeterminate, sacral vertebra UM-ECC 2547, from the late Eocene of Escamps (MP 19), in dorsal (a), ventral (b), and posterior (c) views. Each scale bar = 3 mm.