Eopelobates Parker 1929

EOPELOBATES Parker, 1929

cf. Eopelobates sp.

Figure 19 A-J

Material. Hambach 11: one sacral vertebra (IPB-HaR 2179); one humerus (IPB-HaR 2154); two ilia (IPB-HaR 2084, IPB-HaR 2103).

Description. IPB-HaR 2179 (Figure 19 A-D) is a moderately small sacral vertebra and has an amphicoelous and cylindrical centrum. The neural canal is circular and the dorsal surface of the neural arch is smooth. Both the prezygapophyses and the transverse processes are broken off. The latter are anteroposteriorly extended.

IPB-HaR 2154 (Figure 19 E-F) is a poorly preserved humerus provided with a curved diaphysis (though it is broken and misses the proximal portion) and an eminentia capitata that is shifted laterally compared to the main axis of the bone. A deep fossa cubitalis ventralis is present; it opens on the lateral side. There are no cristae medialis and lateralis.

Ilia (Figure 19 G-J) lack a dorsal tubercle and a dorsal crest. They have an acetabular fossa provided with a strong anteroventral rim. The dorsal acetabular expansion is rather short. No supraacetabular fossa, preacetabular fossa, interiliac groove, or interiliac tubercle are visible. The spiral groove is not distinct. The posterior end of the bone is slightly eroded in both specimens, but very light striae are visible on their posteromedial surface.

Remarks. Few elements from Hambach 11 are attributed to Pelobatidae because of the following combination of features (Bailon, 1999; Roček, 2013): curved diaphysis of the humerus; laterallyshifted eminentia capitata; deep fossa cubitalis ventralis, which is open laterally; ilia with no dorsal crest and no dorsal tubercle; no preacetabular and supracetabular fossae; striae on the medial side of the ilial body. Within pelobatids, the absence of a deep spiral (or oblique) groove is used to distinguish Eopelobates from Pelobates Wagler, 1830 (Böhme, 2010; Syromyatnikova, 2019), and thus these fossils are here assigned to the former genus. However, this identification is only considered tentative here, because not all authors deem isolated postcranial elements sufficient for genus level discrimination (Rage and Roček, 2003). Known Eopelobates species are all based on articulated material (Roček et al., 2014), making comparison with the disarticulated specimens from Hambach difficult. Furthermore, most of the diagnostic features of the species are on cranial elements. Nevertheless, at least Eopelobates deani Roček et al., 2014, and Eopelobates grandis Zweifel, 1956, seem to differ from the Hambach ilia in the low dorsal tubercle and the low dorsal crest in the anterior portion of the shaft respectively. The sacral vertebra IPB-HaR 2179 is also tentatively attributed to cf. Eopelobates sp. because of the cylindrical centrum and the extended transverse processes, as well as the presence of the spinal foramina. In pelobatids, sacral vertebrae not fused to the urostyle are present in both Eopelobates and Pelobates (Bailon, 1999; Roček et al., 2014; Syromyatnikova, 2017). However, vertebrae of pelobatids are usually procoelous. Amphicoelous vertebrae, followed by a cartilaginous disk, are known only in E. grandis (even though its referral to Eopelobates is called into question by some authors also because of this feature; Roček et al., 2014). The holotype and only known specimen of E. grandis does not preserve the sacral centrum or the anterior part of the urostyle, and so an amphicoelous condition of the former cannot be evaluated. Amphicoelous sacral centra are shown by Ascaphus Stejneger, 1899, and some extinct Mesozoic frogs (Reilly and Jorgensen, 2011), but accounting for the absence of any existing evidence supporting the possible presence of the North American ascaphids in Europe at any moment in time as well as the highly unlikely circumstance of a survival of an early-branching Mesozoic anuran lineage in the Pliocene of the continent, we here consider more probable that IPB-HaR 2179 could represent a pelobatid with either a peculiar vertebral morphology or an anomalous condition due to ontogenetic or pathologic circumstances.