Trionychinae Gray, 1825

Danilov, Igor G., Vitek, Natasha S., Averianov, Alexander O. & Glinskiy, Vadim N., 2015, A new soft-shelled trionychid turtle of the genus Khunnuchelys from the Upper Cretaceous Bostobe Formation of Kazakhstan, Acta Palaeontologica Polonica 60 (1), pp. 155-161 : 156-159

Trionychinae Gray, 1825

Genus Khunnuchelys Brinkman, Nessov, and Peng, 1993

Type species: Khunnuchelys erinhotensis Brinkman, Nessov, and Peng, 1993 , Northeast of Erinhot, Inner Mongolia, People’s Republic of China, Iren Dabasu Formation, Upper Cretaceous.

Emended diagnosis. —A trionychid in which the orbits face strongly forwards; the suborbital region of the maxilla is deep; the palate is vaulted; the triturating surfaces of the maxilla and dentary are beak-like; the triturating surfaces of the maxillae meet at the midline, forming a mid-ventral ridge; the internal nares are located far posteriorly and are roofed by the palatines and maxillae; the anterolateral corner of the palatine and posterolateral corner of the maxilla are greatly enlarged to form about half of the triturating surface; the maxilla and jugal are swollen to form a broadly convex surface in dorsal view; the postorbital contacts the maxilla, excluding the jugal from the margin of the orbit; the roof of the skull is constricted to expose the palatine in dorsal view; the descending process of the parietal does not participate in the processus trochlearis oticum; a posteriorly facing occipital surface is bordered dorsally by a rounded ridge formed by the opisthotic and supraoccipital; a concave surface is present on the occiput above the fenestra postotica and lateral to the foramen magnum; the occipital surface of the exoccipital faces posteriorly; nervi hypoglossi exit via a single foramen in the exoccipital. Similar to Conchochelys in the presence of a deeply excavated palate, extensively developed secondary palate with broad contact of the maxillae at the midline, grooves extending backwards from the internal choanal openings, and short basioccipital region. Similar to Dogania in that the prefrontal makes a small contribution to the orbit margin between the frontal and maxilla. Emended from Brinkman et al. (1993) and Vitek and Danilov (2013).

Geographic and stratigraphic range.—Asia ( China, Kazakhstan, and Uzbekistan), Upper Cretaceous (Iren Dabasu Formation, Santonian; Bissekty Formation, upper Turonian; Bostobe Formation, Santonian–lower Campanian).

Khunnuchelys lophorhothon sp. nov.

1957 Trionychid; Khosatzky 1957: 18.

1967 Trionychid; Rozhdestvensky and Khosatzky 1967: 87.

1994 Trionychidae indet.; Kordikova 1994: 345.

1995 cf. Lophorhothon ; Nessov 1995: 107.

1997 Khunnuchelys sp. ; Nessov 1997: 110.

2008 Khunnuchelys sp. ; Glinskiy and Danilov 2008: 19.

2008 Khunnuchelys riabinini ( Kuznetsov and Chkhikvadze, 1987) ; Glinskiy 2008: 23.

2009 Khunnuchelys sp. ; Danilov and Vitek 2009: 53.

2010 Khunnuchelys sp. ; Vitek and Danilov 2010: 390.

2012 Khunnuchelys sp. 1 ; Danilov and Vitek 2012: 9.

2012 Khunnuchelys sp. 2 ; Danilov and Vitek 2012: 9.

Fig. 2. Holotype of the trionychid turtle Khunnuchelys lophorhothon sp. → nov. from Baybishe , northeastern Aral Sea region, Kazakhstan, Bostobe Formation , Santonian– lower Campanian, Upper Cretaceous; ZIN PH 5/55, a partial skull; in dorsal (A), ventral (B), lateral (C), and anterior (D) views. Photographs (A 1 –D 1), explanatory drawings (A 2 –D 2).

Etymology: Named after Lophorhothon , the hadrosaur, to which the holotype of this species was originally attributed (see Introduction).

Holotype: ZIN PH 5/55, a partial skull from Baybishe.

Type locality: Baybishe and Baykhozha, northeastern Aral Sea region, Kazakhstan.

Type horizon: Bostobe Formation, Santonian–lower Campanian (see Nessov 1997).

Material.—ZIN PH 1/146, a partial skull from Baykhozha.

Diagnosis.—A species of Khunnuchelys which can be differentiated from other species of this genus in that the palatines not only form the lateral walls of the grooves that open posteriorly from the apertura narium interna, but also form part of the floor of these grooves, leaving the grooves less ventrally exposed. In addition, it can be differentiated from K. erinhotensis in that the upper margin of the external nares is located dorsal to the ventral orbit margins, and in that the foramen jugularis posterius is confluent with the fenestra postotica.

Description.—The holotype (ZIN PH 5/55; Fig. 2), consists of mostly complete maxillae, palatines, and the vomer, as well as much of the parietal and part of the jugal. The parietals and palatines are broken anterodorsal to where the ventral margin of the skull and the foramen nervi trigemini would be. Sutures are mostly obliterated, either from damage to the skull or co-ossification of the bones. Therefore, many of the sutures are shown in illustrations by dotted lines, indicating uncertainty. This skull fragment is approximately 16 cm long. Based on the proportions of the fragment, the condylobasal length was probably 20–24 cm, and the complete skull (including the supraoccipital crest) was probably 30–40 cm long at the midline.

The second skull (ZIN PH 1/146; Fig. 3 ) is smaller and only partly preserved. It is missing parts of the maxillae and prefrontals, the jugals, the basioccipitals, the supraoccipital crest, and the left quadrate. Preservation of this skull is poor and sutures between bones are not visible. The length of the preserved partial skull is approximately 9 cm. Calculations suggest an estimated condylobasal length of 11 cm and an estimated total skull length of about 17 cm. The skull is unique among specimens of Khunnuchelys in that the ventral margin of the skull is generally well preserved. It indicates that the skull is arched antero-posteriorly.

In ZIN PH 5/55 (Fig. 2), the preserved maxillae do not extend anteriorly as far as the margins of the external nares. In dorsal view (Fig. 2A), they articulate with the jugals posterolaterally and the postorbitals posteromedially near where the margin of the orbits would be. The relative snout length is unclear, but the cheek region of the maxillae is short, deep, and broadly flared. The palate is vaulted and expanded both anteriorly and posteriorly (Fig. 2B), causing the maxillae to bulge posterolaterally (Fig. 2C). The palate is similarly vault- ed in ZIN PH 1/146. In that specimen, the ventral margin of the right maxilla is preserved, indicating a single, flattened beak with no separate triturating surface of any kind ( Fig. 3A ). The lack of a separate triturating surface is only known in other species of Khunnuchelys and in Conchochelys admiribalis Hay, 1905 . The more complete facial portion of ZIN PH 1/146 is short ( Fig. 3B ). Although the dorsal margin of the external nares in ZIN PH 1/146 is broken, the segment of the margin that is still complete extends dorsal to the ventral orbit margins ( Fig. 3C ).

Parts of the maxillae and prefrontals are broken off in ZIN PH 5/55, exposing the nares and the palate dorsally, including the midline narial ridge formed by the vomer (Fig. 2A). The nares are floored by both the maxillae and vomer. Laterally, they are walled by the maxillae. Anteriorly, the vomer has a dorsal fork that would most likely have contact- ed the prefrontals in the roof of the nares. Along the posterior midline of the nares, the vomer forms a dorsal midline ridge that arches posterolaterally to enclose the narial canals and contact the maxillae. In ventral view, the vomer is exposed posterior to the maxillae on the midline but does not articulate with the palatines. This lack of contact was potentially present in the complete skull but this area of the skull is not well preserved. Part of the external nares is preserved in ZIN PH 1/146, but the dorsal margin is broken and potential emargination cannot be discerned.

In ZIN PH 5/55, the maxillae meet at the midline in ventral view, forming a median ridge similar to that described in K. erinhotensis . The preserved margins of the apertura narium interna are formed by the maxillae and palatines in ZIN PH 5/55. Like other species of Khunnuchelys , the palate has two grooves that open posteriorly from the apertura narium interna. The grooves meet at the midline and continue as a single groove posteriorly. The palatines not only form the lateral walls of these grooves, but also form part of the floor for the grooves, leaving the grooves less ventrally exposed than is apparent in other species of Khunnuchelys .Anteriorly, this secondary floor of the palatine may have contacted the maxillae, fully closing the grooves at certain points. In the smaller specimen, ZIN PH 1/146, no internal midline ridge or grooves can be identified from photographs. However, observations by AOA indicate that part of the palatal surface is covered in matrix. In lateral view, the palatines do not contribute to the wall of the braincase, instead contributing to the posterolateral bulge of the palate in ZIN PH 5/55.This contribution is the same as the jugal process described by Brinkman et al. (1993).

The jugal and postorbital are both preserved as fragments sutured to the left maxilla (ZIN PH 5/55). Although the orbit margins are not preserved, it is clear that the jugal contacts the postorbital medially and the jugal and postorbital contact the maxilla anteroventrally. This arrangement indicates that the postorbital prevented the jugal from contributing to the orbit margin. The jugal makes up about half of the posterolateral bulge of the palate (Fig. 2A).

In dorsal view, the skull roof is constricted anteriorly, leaving the dorsolateral surface of the palate exposed (Figs. 2A, 3B). Within the lower temporal fossa, the anterior margin of bone formed by the prootic and parietal is bent at an angle similar to other specimens of Khunnuchelys ( Brinkman et al. 1993) . In lateral view (ZIN PH 1/146; Fig. 3C ), bone completely surrounds the incisura columella auris, and a thin ridge of bone protrudes from the structure within the cavum tympani.

In posterior view, a dorsal rounded ridge is present and separates the dorsal and posterior surface of the skull (ZIN PH 1/146; Fig. 3D ). Below this ridge, lateral to the foramen magnum, is a concave surface. Both features are considered diagnostic for Khunnuchelys . Each exoccipital has a single foramen nervum hypoglossum. The foramen jugularis posterius is confluent with the fenestra postotica, as in Khunnuchelys kizylkumensis but unlike K. erinhotensis .

Figures

Fig. 3. The trionychid turtle Khunnuchelys lophorhothon sp. nov. from Baykhozha, northeastern Aral Sea region, Kazakhstan, Bostobe Formation, Santonian–lower Campanian, Upper Cretaceous; ZIN PH 1/146, a partial skull; in ventral (A), dorsal (B), lateral (C), and posterior (D) views.