Eobarbourula delfinoi, Folie & Rana & Rose & Sahni & Kumar & Singh & Smith, 2013

Folie, Annelise, Rana, Rajendra S., Rose, Kenneth D., Sahni, Ashok, Kumar, Kishor, Singh, Lachham & Smith, Thierry, 2013, Early Eocene frogs from Vastan Lignite Mine, Gujarat, India, Acta Palaeontologica Polonica 58 (3), pp. 511-524 : 512-516

Eobarbourula delfinoi sp. nov.

Figs. 2–5 .

Etymology: Named for Italian paleontologist Massimo Delfino, who alerted us to the presence of a particular articulation similar to a zygosphene−zygantrum complex on the vertebrae of this taxon.

Holotype: GU/RSR/VAS 5001 ( Fig. 2A ), nearly complete left ilium.

Type locality: Vastan Lignite Mine, Surat District, Gujarat, India.

Type horizon: Cambay Shale Formation,middle Ypresian, lower Eocene.

Referred material.—58 ilia: GU/RSR/VAS 5002 ( Fig. 2B ), GU/RSR/VAS 5003 ( Fig. 2C ), GU/RSR/VAS 5004–5059; 129 vertebrae: GU/RSR/VAS 5060–5065 ( Fig. 4A–F ), GU/ RSR/VAS 5066–5188; and 12 urostyles: GU/RSR/VAS 5189 ( Fig. 4G ), GU/RSR/VAS 5190–5200.

Diagnosis.—Differs from the other Bombinatoridae by having less laterally expanded sacral diapophyses on the vertebrae (antero−posterior/lateral length ratio, 1.1) and by having the tuber superius more developed dorsally, more asymmetric in lateral view and placed more anteriorly compared with the acetabulum. Differs further (in adults) from Bombina by being about twice as large, and by having a tuber superius situated anteriorly to the acetabulum; from Barbourula by being 1.5 times smaller and in having a more developed tuber superius.

Description.—In lateral view, the iliac shaft is almost straight and lacks a dorsal crest ( Fig. 2A ). The tuber superius is a long, prominent, asymmetric and slightly anterodorsally oriented tubercle, anteriorly situated with respect to the acetabulum ( Fig. 3D ). The junction between the acetabular area and the shaft is only slightly constricted. There is no subacetabular or supraacetabular fossa. The pars ascendens is short and posteriorly oriented. It forms an open angle (about 160°) with the tuber superius. The pars descendens is slightly developed and is obscured by the large acetabulum, which extends onto the pars descendens ventrally. The acetabulum is deep, slightly bell−shaped and bordered by a sharp rim. A shallow but well−marked protuberance is present near the anterior border of the acetabulum. In medial view, the acetabular area is bordered by shallow ridges. Between them, a triangular and medially prominent interiliac tubercle is developed. In dorsal view, the medial part of the acetabular area is prominent and the tuber superius projects laterally. This general shape is also visible on specimens that are nearly half the size of the first specimens ( Fig. 2C ). However, in lateral view, they differ from the first specimens by both less well−marked tuber superius and weaker protuberance near the anterior border of the acetabulum. The junction between the acetabular area and the shaft is slightly more constricted. In medial view, the interiliac tubercle is less developed. In dorsal view, the tuber superius is less laterally projected ( Fig. 2A 4, B 4, C 4 ). Other specimens with the same morphology and presenting intermediate sizes between these two extreme sizes ( Fig. 2B ) are attributed to the same single taxon.

The vertebrae ( Fig. 4 ) are imbricate (the elongate posterior part of the neural arch overlaps the anterior border of the subjacent vertebra like tiles on a roof), opisthocoelous, and present a particular articulation similar to the zygosphene−zygantrum complex of snake vertebrae ( Fig. 5 ). The zygosphene−zygantrum articulation was defined by Romer (1956) as an accessory articulation composed of a dorsally tapered wedge of bone bearing articular surfaces on either side (zygosphene) that projects forward from the neural arch into a corresponding wedge−shaped cavity (zygantrum). However, on the Vastan specimens, the structure resembling the zygosphene looks like a pair of anterior processes separated by a notch and well differentiated from the neural arch ( Fig. 5A ) and the zygantrum is the corresponding cavity ( Fig. 5B ). Such a structure can also be found in the anuran Bombina , the salamander Salamandrina and some lacertilians such as lacertid, teiid, and cordylid lizards ( Sanchìz 1988). A similar structure, the hyposphene−hypantrum complex, is also present in some dinosaurs and on cervical vertebrae of primitive birds ( Rauhut 2003). The atlas of the Vastan taxon is fused with V2 and presents two large reniform cotyles narrowly separated and forming two distinct articular surfaces. This morphology is characteristic of type II of Lynch (1971). Posteriorly, a zygantrum−like cavity is slightly developed on V2, and the neural arch is postero−dorsally elongated and ends with three expansions or spikes, the medial one being the spinous process, and the two lateral expansions being the postzygapophyses ( Fig. 4A 3 –A 5 ). From the atlas to the fourth vertebra ( Fig. 4B, C ), the neural

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canal decreases in diameter, the transverse processes become more strongly developed, and the neural arch becomes flatter but is posteriorly well developed, presenting an acute spinous process. The neural arches of the other vertebrae up to the sacral are shorter posteriorly and become more rectangular in dorsal view ( Fig. 4D, E ). The sacral vertebra is biconvex and the sacral diapophyses are dorsoventrally flattened and moderately expanded laterally ( Fig. 4F 4 ). The urostyle presents an anterior cotyle, a low dorsal crest, and two laterally and slightly anteroposteriorly elongated transversal processes ( Fig. 4G ; see Table 1).

Discussion.—All the bones described above are grouped together based on their morphology, size and relative abundance. The small form of ilium ( Fig. 2C ) is here interpreted as a juvenile stage of the larger form ( Fig. 2A ). Ilia of intermediate size, such as GU/RSR/VAS 5061 ( Fig. 2B ), are also present, supporting the interpretation of these as ontogenetic stages of the same species. The morphological characters of all these ilia are the same, with the exception of the tuber superius, which shows ontogenetic variability by being better developed and slightly inclined laterally in adult forms than in the juvenile forms, as can be seen in the posterior view of GU/RSR/VAS 5060 ( Fig. 2A 4, B 4, C 4 ).

The presence of an open angle between the tuber superius and the pars ascendens ( Rage and Hossini 2000) and. the imbricate vertebrae indicate that the Vastan taxon belongs to “Archaeobatrachia” and/or “Mesobatrachia” because they are only known in these groups ( Prasad and Rage 2004). However, it does not belong to the most common “ Discoglossus group”, including the genera Discoglossus , Eodiscoglossus , Paradiscoglossus , Latonia , and Paralatonia ( Duffaud and Rage 1999; Rage and Hossini 2000) because the Vastan taxon lacks the dorsal crest on the iliac shaft ( Rage and Hossini 2000) and the bicondylar sacro−urostylar articulation ( Clarke 1987). It is similar to gobiatids, including the ilium attributed to?Gobiatinae from the Late Cretaceous intertrappean beds of Naskal, India ( Prasad and Rage 2004: fig. 1D), in having an asymmetric tuber superius ( Roček and Nessov 1993: fig. 25). However, gobiatids present an acetabulum not extending over the pars descendens on the ilium and amphicoelous vertebrae ( Roček and Nessov 1993; Prasad and Rage 2004), whereas the Vastan species presents a ventrally extensive acetabulum that obscures the pars descendens on the ilium and has opisthocoelous vertebrae.

Within the Discoglossoidea , the Vastan species closely resembles the Bombinatoridae . This family includes only the genera Bombina and Barbourula ( Cannatella 2007) . The presence of an articulation similar to a zygosphene−zygantrum complex and an interiliac tubercle are resemblances to Bombina ( Sanchìz 1988; Gardner et al. 2010). However, in Bombina orientalis and Bombina bombina , the tuber superius

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is situated above the acetabulum (the anterior border of the acetabulum extends further to the anterior border of the tuber superius; Nokariya 1983a; Gardner et al. 2010; Fig. 3A, B ), than in E. delfinoi in which case the anterior border of the acetabulum reaches only a third to a half of the length of the tuber superius ( Fig. 3D ). Moreover, the adult size of the ilium of E. delfinoi , measured at the level of the posterior suture with the ischium and pubis, is about twice that of B. orientalis ( Nokariya 1983a) or B. bombina ( Gardner et al. 2010) . Little osteological information is available about the extant genus Barbourula . The Vastan specimens are close to Barbourula busuangensis in the general shape of the ilia and vertebrae ( Clarke 1987; Přikryl et al. 2009). Indeed, both taxa have an ilium without a dorsal crest, a long prominent tuber superius, imbricate and opisthocoelous vertebrae, neural spines decreasing posteriorly in height, a monocondylar sacro−urostylar articulation, and a pair of well−developed transverse processes on the urostyle. However, the ilia of adult E. delfinoi are about 1.5 times smaller than those of B. busuangensis ( Clarke 1987) . Finally, the Vastan bombinatorid differs from both Bombina and Barbourula in having the tuber superius situated above the acetabulum (the anterior border of the acetabulum extends further to the anterior border of the tuber superius; Nokariya 1983a; Gardner et al. 2010; Fig. 3A–C ), than in E. delfinoi in which case the anterior border of the acetabulum reaches only a third to a half of the length of the tuber superius ( Fig. 3D ). Moreover, in E. delfinoi , the tuber superius is more developed dorsally and more asymmetric in lateral view on the ilium and have sacral diapophyses that were likely less dilated laterally, like butterfly wings, than in Bombina and Barbourula ( Clarke 1987; Gardner et al. 2010; Přikryl et al. 2009; Nokariya 1983a). In Barbourula busuagensis ( Clarke 1987) , the ratio of antero−posterior to lateral length in the sacral diapophyses is 2.1; in Bombina orientalis ( Nokariya 1983a) , the ratio is 2.2, and in E. delfinoi , 1.1. Based on these comparisons, the Vastan taxon closely resembles the extant genus Barbourula .

The morphologies of the ilia and vertebrae correspond also exactly to those described from Vastan by Bajpai and Kapur (2008) as Discoglossidae indet. Indeed, the ilia IITR/SB/ VLM−LV/203–204 show the tuber superius as a swelling of the dorsal margin of the iliac shaft, the ventral margin of the acetabulum overhanging the pars descendens, and medially, the ilium bears an interiliac protuberance. The vertebra IITR/ SB/VLM−LV/208 is similar to the anterior trunk vertebra VAS 5061 in being opisthocoelous, bearing a dorso−ventrally flattened centrum, and presenting three posterior spikes on the neural arch. The same may be concluded for ilium IITR/SB/ VLM−LV/209 described from Vastan by Bajpai and Kapur (2008) and attributed to Leptodactylidae indet. based on general resemblances with the genera Telmatobius , Lymnodynastes , Kyarranus , and Philoria .

It is noteworthy that similarities were also observed with the genus Hatzegobatrachus from the latest Cretaceous of the Haţeg Basin, Romania, and this has been already compared with bombinatorids ( Venczel and Csiki 2003). They both share “a well−developed and undivided dorsal prominence” and a small pars descendens, but Hatzegobatrachus shows a thicker dorso−lateral margin and it also lacks “any waisting between the acetabular region and the iliac shaft”. Therefore, the Vastan taxon cannot be attributed to the Romanian genus Hatzegobatrachus .

Stratigraphic and geographic range.— Type locality and horizon locality only.

Figures

Fig. 2. Nearly complete ilium of frog Eobarbourula delfinoi gen. et sp. nov., early Eocene, Vastan Lignite Mine, Gujarat, India. A. GU/RSR/VAS 5001, holotype, left ilium in lateral (A1), posterior (A2), medial (A3), and dorsal (A4) views. B. GU/RSR/VAS 5002, left ilium in lateral (B1), posterior (B2), medial (B3), and dorsal (B4) views. C. GU/RSR/VAS 5003, right ilium in medial (C1), posterior (C2), lateral (C3), and dorsal (C4) views.

Fig. 3. Comparative drawings of bombinatorid ilia in lateral view. A. Bombina bombina (Linnaeus, 1761), specimen UMMZ 152271, Recent, Germany (modified from Gardner 2010). B. Bombina orientalis (Boulenger, 1890), specimen TMP 2010.30.21, Recent, captive raised specimen (modified from Gardner et al. 2010). C. Barbourula busuagensis Taylor and Noble, 1924, specimen FMNH 50999, Recent, Philippines (modified from Clarke 1987). D. Eobarbourula delfinoi gen. et sp. nov., specimen GU/RS/ VAS 5001, early Eocene, Vastan Lignite Mine, Gujarat, India. Arrows represent the distance between the anterior border of the acetabulum and the inflexion point where the tuber superius starts anteriorly.

Fig. 4. Vertebrae of frog Eobarbourula delfinoi gen. et sp. nov., early Eocene, Vastan Lignite Mine, Gujarat, India. A. GU/RSR/VAS 5060, atlas in anterior (A1), posterior (A2), left lateral (A3), dorsal (A4), and ventral (A5) views. B. GU/RSR/VAS 5061, anterior trunk vertebra in anterior (B1), posterior (B2), left lateral (B3), dorsal (B4), and ventral (B5) views. C. GU/RSR/VAS 5062, trunk vertebra in anterior (C1), posterior (C2), left lateral (C3), dorsal (C4), and ventral (C5) views. D. GU/RSR/VAS 5063, trunk vertebra in anterior (D1), posterior (D2), right lateral (D3), dorsal (D4), and ventral (D5) views. E. GU/RSR/VAS 5064, presacral vertebra in anterior (E1), posterior (E2), right lateral (E3), dorsal (E4), and ventral (E5) views. F. GU/RSR/VAS 5065, sacral vertebra in anterior (F1), posterior (F2), left lateral (F3), dorsal (F4), and ventral (F5) views. G. GU/RSR/VAS 5189, fragmentary urostyle in anterior (G1), left lateral (G2), dorsal (G3), and ventral (G4) views.

Fig. 5. Close up views of vertebrae of frog Eobarbourula delfinoi sp. nov., early Eocene, Vastan Lignite Mine, Gujarat, India. A. GU/RSR/VAS 5064, presacral vertebra in dorsal (A1) and anterior (A2) views. B. GU/RSR/VAS 5063, trunk vertebra in posterior view.