Skull

Skull ZIN PH 1/146, one of two skulls assigned to K. lophorhothon

Khunnuchelys is an extinct turtle is which many skulls are known from. In fact, is the only skull-based trionychid from the Cretaceous.^[2] The side of the maxillary makes up a beak-like structure, an unusual feature of trionychids.^[1] One skull, ZIN PH 1/146, is relatively small, and is only partly preserved. The maxilla and prefrontals of the specimen are only partly known, although the jugals, basioccipitals, supraoccipital crest, and left quadrate are close to complete. The preserved skull length is 9 cm (3.5 in), and the total approximate skull is 17 cm (6.7 in) long. The specimen is unique among Khunnuchelys, as the ventral edge is generally well-preserved. The ventral edge shows that the skull of Khunnuchelys is arched from the front to the back.^[2]

Distinguishing characteristics

A set of features unique among trionychids were identified by Brinkman et al. in 1993 during the description of Khunnuchelys erinhotensis and K. kisylkumensis. They are listed below:^[2]

• orbits facing strongly forwards;
• the suborbital region of the maxilla is deep;
• the palate is vaulted;
• the triturating surfaces of the maxilla and dentary are beaklike;
• 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;
• and nervi hypoglossi exiting via a single foramen in the exoccipital.

The characteristics would not be useful if Khunnuchelys was not a trionychid, but Brinkman et al. found that a classification as a trionychid was likely because of the features listed below:^[2]

• it is 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
• and it is similar to Dogania in that the prefrontal makes a small contribution to the orbit margin between the frontal and maxilla.

K. lophorothon is distinguished from the other species by the features listed below, found by Danilov et al. in its description:^[2]

• 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.

Both species can be differentiated from K. erinhotensis:^[2]

• 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.