Helanshania deserta, Vianey-Liaud & Rodrigues & Marivaux, 2010

Vianey-Liaud, Monique, Rodrigues, Helder Gomes & Marivaux, Laurent, 2010, A new Oligocene Ctenodactylinae (Rodentia: Mammalia) from Ulantatal (nei Mongol): new insight on the phylogenetic origins of the modern Ctenodactylidae, Zoological Journal of the Linnean Society 160 (3), pp. 531-550 : 534-535

HELANSHANIA DESERTA SP. NOV. ( FIG. 2 )

Etymology: Derived from the geographical position of the Ulantatal localities at the border of the Ting- Ko-Li desert.

Type locality, stratigraphical range: Ulantatal area, Inner Mongolia ( China); late early Oligocene (UTL1) to early late Oligocene (UTL4).

Holotype: Left second lower molar (m2), UTL1-50 . Paratype: UTL1-51 , upper M1 .

Other material: UTL4-70: upper M2 and UTL4-71: upper M3.

Diagnosis: Molars more hypsodont than any member of the Tataromyinae ; less hypsodont than the other Ctenodactylinae . Lower molar with a shallow trigonoid structure limited anteriorly by an incomplete metalophulid I and posteriorly by a short metalophulid II. This is connected to the ectolophid, which appears swollen by the presence of a minute mesoconid; entolophid central; hypoconid and protoconid compressed mesiodistally, forming oblique transverse crests; mesosynclinid and posterosynclinid of equal size; hypoconulid incipient. Upper molars unilaterally hypsodont; hypocone and protocone equal in size; deep mesosyncline, curved posteriorly; antero- and posterosyncline open, even on worn teeth; sinus shallow and nearly symmetrical, whereas it is deeper and directed obliquely forward in the other Ctenodactylinae ; protoloph transverse, connected to the protocone; metaloph inflated, orientated obliquely posteriorly, connected to the posterior part of the hypocone.

Description of the holotype: The type specimen is a completely unworn m2 [ Fig. 2A; L (1.53 mm) ¥ W (1.34 mm), HLabial (1.12 mm), HLingual (0.85 mm)], with incipient hypsodonty. Its protoconid and hypoconid are compressed anteroposteriorly and ridgelike. The posterolingual arm of the protoconid is rather robust. It descends toward the centre of the tooth, and is connected to a short ectolophid that is enlarged by the presence of a small mesoconid. A short posterior arm of the metaconid is directed towards the mesoconid. Distally, both posterior arms of metaconid and protoconid (in position of the metalophulid II) and metalophulid I (mesially) delimit a shallow trigonoid structure. The posterolingual arm of the hypoconid is strong. It is orientated obliquely and continues into an equally strong and curved posterolophid, the lingual end of which reaches the lingual border of the tooth. There is not a strong inflation of the posterolophid at the location of the hypoconulid. The entoconid is cuspidated and not buccolingually compressed and occupies a central position on the lingual border of the tooth. Because of the entoconid position, the posterosyncline and the mesosyncline are about the same size. The sinusid (hypoflexid) penetrates lingually into the tooth in an anterolingual direction, and is in line with the posterosyncline (delimited by the entoconid and the posterolophid). The metaconid is compressed.

Paratype: A greatly worn left upper M1 [UTL1-51, Fig. 2B, L (1.24 mm) ¥ W (1.24 mm), HLabial (0.51 mm), HLingual (0.83 mm)] from the same locality corresponds to the holotype in its size and overall morphology and as such is assigned to the same species. The shape of the crown, the weakness of the buccal roots and the strongly enlarged lingual root are already typically modified in the direction of unilateral hypsodonty. As is typical for molars with incipient hypsodonty, the thickness of the enamel band varies in the occlusal pattern. It is broader on the anterolingual side of the protocone and hypocone, and narrower on the side of the mesosyncline. As in other slightly hypsodont species, wear affects the crown surface not only horizontally but also obliquely, producing both plane and sloping wear surfaces. Despite the advanced wear of the tooth, the main morphological details can still be recognized. The occlusal pattern is similar to that of the Tataromyinae but the hypocone is nearly as strong as the protocone and the sinus is nearly symmetrical. It is very shallow and only slightly inclined posteriorly. The anterosyncline and the posterosyncline are very narrow, and it is likely that they did not extend beyond the sagittal middle axis of the tooth crown in the pristine teeth. The mesosyncline is slightly curved and directed posteriorly. The metaloph, thick, is oblique backward and connected to the posterior arm of the hypocone. The bent anterior border of the protocone continues into a narrow anteroloph. There is no trace of an anterocone. The posteroloph is particularly narrow.

Additional material: A moderately worn right upper molar of the locality UTL4 [ UTL4-70 , Fig. 2D, L (1.71 mm) ¥ W (1.67 mm), HLabial (0.73 mm), HLingual (1.51 mm)] is considered to be a M2 because of its size larger than M1. Its morphology is close to that of the upper molars described above. The hypocone is even slightly larger than the protocone. The sinus is shallow and symmetrical. As the tooth is less worn, the para and meta synclines are somewhat larger than in the M1. The mesosyncline is strongly curved backwardly, owing to the connection of the metaloph with the posterior arm of the hypocone, at the beginning of the posteroloph. The metaloph is wider, at the location of a metaconule, than in the M3. The anterolingual arm of the protocone, slightly curved, forms an angle at its junction with the anteroloph. As in the holotype, the enamel thickness is considerably increased on the lingual side of the protocone and hypocone (compared to their labial side). As the tooth is less worn than the M1, its unilateral hypsodonty becomes particularly evident. A slightly worn tooth of the same locality [ UTL4-71 , Fig. 2C, L (1.38 mm) ¥ W (1.32 mm), HLabial (0.62 mm), HLingual (1.26 mm)] is assigned to a M3, as indicated by the posterior slant of the roots, and the asymmetrical lingual profile of the crown. It is smaller than the M2 and bigger than the M1, and shows a similar morphological arrangement. The metaloph is connected to the middle of the hypocone .

Comparisons: The pattern of m2 is reminiscent of that of the m2 of Karakoromys , but the tooth is more hypsodont and lophodont, the trigonoid is more reduced, and the ectolophid seems slightly more lingual. This structure could be the result of an increasing hypsodonty mainly on the labial flank of the crown. The hypoconulid is more elongated than the posterolophid, and there is only a faint undulation at the location of the hyposinusid. On the upper teeth, clearly more hypsodont than that of Karakoromys , the metaloph is shorter and as such, appears directed posteriorly. In Karakoromys as in Helanshania , the lingual connection of the metaloph is the same, either to the middle of hypocone (M3), or to its posterior arm (M1-2).

The dental patterns of lower and upper molars of Helanshania are close to that of molars of Prosayimys flynni Baskin (1996) . The main difference, apart from the smaller size, is in the lesser degree of hypsodonty and lophodonty. The latter is notably marked by the mesiodistal compression of the paracone, metacone, and metaloph, and the mesiodistal pinching of the lingual part of protoconid and hypoconid. On lower molars of Prosayimys , the ectolophid remains at the midline of the teeth as in Helanshania . On upper teeth, the anterosyncline and posterosyncline are shallow, but not as much as in Helanshania . The endoloph is moved labially, so that the sinus is deeper. This displacement can be the result of an increasing asymmetrical hypsodonty, where the lingual wall increases more than the labial one.

Figures

Figure 2. Helanshania deserta gen. et sp. nov. Ulantatal area (Inner Mongolia, China). ULAN-I and II (lower and beginning of upper Oligocene). A, UTL1-50, left m1–2, Ulantatal 1, HOLOTYPE; A1, occlusal view, A2, labial side, A3, lingual side. B, UTL1-51, left M1, Ulantatal 1; B1, occlusal view, B2, lingual side, B3, labial side; C, UTL4-71, left M3? Ulantatal 4; C1, occlusal view, C2, lingual side, C3, labial side; UTL4-70, right M2, Ulantatal 4; D1, occlusal view, D2, lingual side, D3, labial side. Scale bar = 1 mm.