Zangerlia ukhaachelys, JOYCE & NORELL, 2005

JOYCE, WALTER G. & NORELL, MARK A., 2005, Zangerlia ukhaachelys, New Species, a Nanhsiungchelyid Turtle from the Late Cretaceous of Ukhaa Tolgod, Mongolia, American Museum Novitates 3481, pp. 1-20 : 4-7

Zangerlia ukhaachelys , new species

HOLOTYPE: IGM 90 /1 ( figs. 1–4 ), incomplete skeleton consisting of partial cranium, peripherals, plastron, and fragmentary other remains of the postcranium.

TYPE LOCALITY: Ukhaa Tolgod, just south of Xanadu, Omongov Aimag, Mongolia. The Ukhaa Tolgod beds have been considered to be roughly equivalent to Djadoktha ( Loope et al., 1998; Dashzeveg et al., 1995). Djadoktha beds elsewhere in Mongolia are considered to be Late Campanian ( Lillegraven and McKenna, 1986; Gao and Norell, 2000).

ETYMOLOGY: ‘‘Ukhaa’’, in reference to the fossil locality Ukhaa Tolgod, Mongolia, and ‘‘chelys’’, Greek for turtle.

DIAGNOSIS: Zangerlia ukhaachelys is diagnosed by a single autapomorphy, the presence of an anteromedial process of the hyoplastron that limits contact between the entoplastron and epiplastron. In addition, Zangerlia ukhaachelys is diagnosed by the following list of synapomorphies and symplesiomorphies: cranium with enlarged nasal cavity; wide fissura ethmoidalis; well­developed upper temporal emargination; frontals do not contribute to orbit and produce descending processes that almost surround sulcus olfactorius ventrally; lingual ridges absent; antorbital groove present along anteroventral rim of orbit; shell covered with deep pock­marks, deep nuchal notch formed by small and trapezoid nuchal and first peripheral; vertebral and pleural scutes do not overlap onto peripherals; entoplastron dissected by humero­pectoral sulcus; four pairs of inframarginals present which fully separate marginals from plastral scutes; marginal VI not expanded ventromedially; pectoral does not contribute to axillary notch; midline plastral sulci straight.

DESCRIPTION AND COMPARISON OF IGM 90/1

PRESERVATION

Unlike many other fossils from the locality of Ukhaa Tolgod, IGM 90/1 is only moderately well preserved and shows signs of pre­ or postdepositional decay due to scavenging and/or postburial insect activity. Most limb elements are absent, and only fragmentary remains of the girdles were found within the shell. No traces of vertebral column elements, including the neural elements of the carapace, are present. Despite the lack of these elements, parts of the skull remain intact and display details in morphology never seen before in a nanhsiungchelyid turtle. The skull was found within the carapace just above the entoplastron. This position, however, is not considered positive evidence for full neck retraction in NANHSIUNGCHELYIDAE , but may be coincidental, because the cranium is not in articulation with the neck and may have been moved before final deposition.

The shell is incompletely preserved and many areas that are useful in diagnosing NANHSIUNGCHELYID species are missing. Only the anterior and lateral peripherals remain of the carapace, thus obscuring the morphology of most carapacial scutes and the nuchal and pygal region. The plastron is more complete, but significant parts of the anterior lobe and the bridges are missing or they are heavily weathered. The larvae of large scavenging beetles likely produced the holes seen in the plastron ( figs. 1–4 ).

CRANIUM AND MANDIBLE

The most interesting aspect of IGM 90/1 is its uncrushed, partial cranium, which is the best preserved of any nanhsiungchelyid and clearly exhibits all cranial sutures ( figs. 1 , 2 ).

PREFRONTAL: The prefrontal of IGM 90/1 consists of a dorsal plate, which forms the roof of a broad fossa nasalis, and a vertical plate, which forms significant portions of the anterior orbit wall. In dorsal view, the prefrontal has a posteromedial contact with the frontal, a medial contact with its counterpart, and a small posterolateral contact with the postorbital, thus excluding the frontal from contributing to the orbital rim. The anterior rims of both prefrontals are weathered away, making it unclear if nasals were present anteriorly. In lateral view, the vertical plate of the prefrontal meets the ascending process of the maxilla along a short interdigitated contact to form the lateral wall of the fossa nasalis. An additional, elongate contact between these bones exists along the anteroventral rim of the orbit. Although the distal ends of the ascending process of the prefrontals are weathered, the left one is better preserved and shows the presence of a moderately sized foramen orbito­nasale. The fissura ethmoidalis and the sulcus olfactorius are apparent and are notably wide.

The prefrontal of IGM 90/1 closely resembles that of Zangerlia neimongolensis . It also resembles the prefrontals of Adocus sp. , but differs by having a contact with the postorbital and by covering a much larger proportion of the dorsal roofing of the skull between the orbits. The fissura ethmoidalis greatly resembles that of the TESTUDINIDAE in being wide and not keyhole­shaped.

FRONTAL: In dorsal view, the frontal of IGM 90/1 is flat and subtriangular. It meets the prefrontal anterolaterally, the postorbital laterally, the other frontal medially, and the parietal along an interdigitated suture posteriorly. It is a large element forming much of the skull roof, but it does not contribute to the orbital rim. The frontal generally has a smooth surface, but a faint sulcus crosses it in an arch. The morphology of the frontal is more complex in ventral view. Between the parietal and the prefrontal, it forms a significant portion of the sulcus olfactorius. From the rim of the sulcus, tapered processes ascend from both frontals that almost meet medially, practically rendering the sulcus olfactorius a canal. Transverse to the sulcus olfactorius, the prefrontal additionally forms a small ridge that runs parallel to its border with the parietal and forms the posterodorsal limit of the orbit.

The skull materials of Z. neimongolensis and N. wuchingensis are not prepared enough to assess the presence of a descending frontal process in these taxa. These processes are not known in Adocus sp. Among living turtles, descending frontal processes are known from a number of TESTUDINOID turtles, especially terrestrial forms of the TESTUDINIDAE . The frontals of Z. neimongolensis do not contribute to the orbital rim, as seen in IGM 90/1.

Figures

Fig. 1. Zangerlia ukhaachelys. IGM 90/1, holotype, Upper Cretaceous of Mongolia. Photographs were manipulated digitally to enhance sutures. A, Oblique dorsal view of mandible; B, dorsal view of mandible; C, dorsal view of cranium; D, ventral view of cranium. Abbreviations: desc proc, descending process of frontal; ex, exoccipital; fr, frontal; mx, maxilla; op, opisthotic; pa, parietal; pf, prefrontal; pm, premaxilla; po, postorbital; pro, prootic; qu, quadrate; rec lab op, recessus labyrinthicus opisthoticus; rec lab pro, recessus labyrinthicus prooticus; so, supraoccipital; vo, vomer;?, bone of uncertain homology.

Fig. 2. Zangerlia ukhaachelys, cranium of IGM 90/1, holotype, Upper Cretaceous of Mongolia. Photographs were manipulated digitally to enhance sutures. A, Left lateral view; B, right lateral view; C, frontal view; D, caudal view. See figure 1 for abbreviations.

Fig. 3. Zangerlia ukhaachelys, IGM 90/1, holotype, Upper Cretaceous of Mongolia. Ventral view of plastron.

Fig. 4. Zangerlia ukhaachelys, IGM 90/1, holotype, Upper Cretaceous of Mongolia. Ventral view of plastron. Abbreviations: AB, abdominal scute; AN, anal scute; ent, entoplastron; epi, epiplastron; FE, femoral scute; HU, humeral scute; hyo, hyoplastron; hyp, hypoplastron; IM, inframarginal scute; PE, pectoral scute; per, peripheral; xi, xiphiplastron.