Class Mammalia Linnaeus, 1758

Subclass Tribosphenida McKenna, 1975

Infraclass Metatheria Huxley, 1880

Order Deltatheroida Kielan-Jaworowska, 1982

Family Deltatheridiidae Gregory & Simpson, 1926

Gurbanodelta kara gen. et sp. nov.

Etymology

The genus name is derived from the Gurbantunggut Desert where the holotype was found and delta, a common component of deltatheroidan names. The specific epithet is from ‘kara,’ meaning black in the local Kazakh language.

Holotype

Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), V 22802 (Fig. 3F–J), a right M2.

Figure 3 Dentition of Gurbanodelta kara gen. et sp. nov. (A–E) Right M2, IVPP V 22801. (F–J) Right M2, IVPP V 22802, holotype. (K–O) Right M3, IVPP V 22803. (P–T) Right m1, IVPP V 22804. From left to right, the images are in occlusal, mesial, distal, buccal and lingual views, respectively. Scale bar indicates 1 mm. Full size image

Hypodigm

IVPP V 22801, a right M2; IVPP V 22803, a right M3; IVPP V 22804, a right m1.

Locality and horizon

South Gobi mammalian fossil locality, Kalabulegen Town, Xinjiang Province, China (Fig. 1), late Gashatan ALMA in the late Paleocene.

Diagnosis

Smallest known deltatheroidan (Table 1), much smaller than all other deltatheroidans. Differs from all other deltatheroidans in having a stylar shelf narrower than the half of the width of the tooth, and a paraconid smaller and lower than the metaconid on m1. Differs from all deltatheroidans (except Atokatheridium) in lacking a buccal cingulum on the upper molars, and having a fully developed postmetacrista on M3. Differs from all deltatheroidans (except Tsagandelta and Deltatheroides) in having a narrow talonid that occupies half of the molar’s width, and differs from Tsagandelta, Sulestes and Deltatheroides in lacking a sharp mesial keel below the paraconid.

Table 1 Measurements of the specimens. Full size table

Description

The upper teeth (identified as M2–3) all have a triangular crown with three trenchant, buccally leaning cusps (the protocone, paracone and metacone). The mesial border of the tooth is straight. The buccal border is concave between the paracone and metacone (i.e., the ectoflexus is deep). Distally, the border between the protocone and metacone also has a shallow indentation. The protocone is a trenchant cusp, and the cusp is quite small relative to the tooth size, as in all deltatheroidans. The cusp is mesiodistally very compressed and buccolingually quite narrow. Both the preprotocrista and postprotocrista of the protocone are very sharp and strong. The former extends all the way to the buccal side and terminates at the large parastyle. The latter is very short, and it extends to the lingual side of the metacone only, not passing the base of the metacone. The pre- and postprotocristae connect to each other at the tip of the protocone at a narrow angle. The buccal surface of the protocone (bordered by those two strong ridges) is therefore narrow and concave. The lingual surface of the protocone is very narrow and forms a round ridge. There is no cingulum developed on the mesial, lingual or distal surfaces of the protocone. The paracone is smaller, but taller than the protocone. It has a convex and ridged lingual surface, and has a narrow and concave buccal surface. The preparacrista of the paracone is very long. It extends mesiobuccally for a short distance, and makes a sharp turn to the buccal side, becoming parallel to the buccal part of the preprotocrista. Eventually, the preparacrista terminates at the stylocone, distal to the parastyle. The postparacrista is short and low, and extends distally to meet the premetacrista of the metacone. The metacone is much smaller and lower than the paracone. The premetacrista of the metacone is short and low, and has the same form as the postparacrista. The postmetacrista is very strong, with its buccal part even longer and higher than the preparacrista; this feature is related to the postvallum-prevallid shearing mechanism.

The protocone, paracone and metacone enclose a very narrow but deep trigon basin. Buccal to the paracone and metacone, the tooth has a very broad stylar shelf. Buccolingually, the shelf occupies more than one-third of the tooth’s total width. The bottom of the stylar shelf is very smooth and bowl shaped, and its buccal edge has a poorly developed cingulum.

The conules and styles of the upper molars are moderately developed. Both the paraconule and metaconule are present as small swollen nodules on the preprotocrista and postprotocrista, and they do not project more ventrally than the two cristae. The paraconule is larger than the metaconule. A weak postparaconule crista extends to the base of the paracone. The metaconule does not have cristae. On the buccal side, only the parastyle and stylocone are relatively well developed, and they are present as twinned cusps located at the mesiobuccal corner of the tooth, barely projecting above the cristae connected to them. Near the parastyle, the buccal part of the preprotocrista has a tiny swollen nodule that forms a small cusp-like structure. Two similar tiny nodules also are present on the buccal border of the stylar shelf, and may be equivalent to the stylar cusps in other metatherians.

The three upper molars are quite similar to each other. IVPP V 22801 and V 22802 are identified as M2s, because the protocones of the two teeth are mesiodistally symmetrical. The distobuccal ends of the postmetacristae in the two teeth are elevated. In buccal view, the elevated part looks like a metastyle. V 22801 is smaller than V 22802. The twinned parastyle and stylocone of V 22801 are very close to each other, whereas the two cusps in V 22802 are well separated. V 22803 is slightly larger than the M2s and identified as an M3. The protocone of this M3 is mesially tilted, and as a result, its distal surface is slightly bigger than its mesial surface. The ectoflexus of the buccal tooth border of M3 is shallower than that of M2. The twinned parastyle and stylocone of M3 are well separated as in M2 (V 22802, but not V22801). The postmetacrista of M3 is quite straight, and its distobuccal end is not elevated.

The tentative attribution of the lower molar (IVPP V 22804) to Gurbanodelta kara is based on its very small size, and its combination of a triangular trigonid, well developed and widely separated paraconid and metaconid, trenchant protoconid, and long but narrow talonid. The eutherian insectivores from the same locality, such as Bumbanius ningi and Asionyctia guoi, are much larger than G. kara. The molars of these taxa have a more fully developed talonid with a large hypoconid and entoconid, and a broad talonid basin. The premolars of A. guoi and similar eutherian mammals are roughly similar to V 22804 in size, but the premolars of these mammals have a much weaker paraconid and metaconid, and a much shorter talonid than V 22804. Some eulipotyphlan placentals, such as Plagioctenodon, have molariform premolars. In these eutherians, the paraconid and metaconid of p4 are quite large, but differ from V 22804, with the metaconid more mesially positioned, and the paraconid and metaconid closer to each other. The carnassial notch between the protoconid and metaconid is absent in those eulipotyphlans. The p4 talonid is proportionally wider, and the difference in height between the trigonid and talonid is lower than in V 22084 and the molars of other deltatheroidans. In some basal marsupialiformes metatherians (= “traditional” definition of Marsupialia, e.g. in ref. 1), such as Alphadon and Peradectes, the trigonid of the lower molars has large, well-separated paraconids and metaconids, and notched shearing crests between the paraconid and protoconid and between the protoconid and metaconid. However, the talonids in these marsupialiform metatherians are proportionally much broader. The premolars in Alphadon or Peradectes-like marsupialiform are buccolingually narrow, with very weak paraconids and metaconids.

As a molar of a deltatheroidan, the small size, very open trigonid and low paraconid of V 22804 suggests that this tooth probably is an m1. The trigonid of the tooth is very tall. The protoconid is shaped like a triangular pyramid. The paraconid and metaconid are conical in shape, and all located near the lingual border of the tooth. The paraconid is smaller and lower than the metaconid. The enamel of the paraconid is chipped off; as a result, the cusp appears smaller than it would have been when intact. Both the paraconid and metaconid are much lower than the protoconid. The paraconid is widely separated from the metaconid, leaving the lingual side of the trigonid completely open. The distolingual side of the paraconid and the mesiolingual side of the metaconid are smooth, and no ridge connects the two cusps. Two blunt crests (the paracristid and protocristid) run down from the tip of the protoconid and connect to the paraconid and metaconid, respectively. The paracristid, linking the protoconid and paraconid, is long and straight. In mesiobuccal view, the paracristid is shaped like a checkmark. At the bottom of the checkmark, a rudimentary carnassial notch is developed (a lower molar feature related to the postvallum/prevallid shearing mechanism). The protocristid is short and has a shallow carnassial-notch-like structure. Along the distolingual margin of the metaconid, a weak but long distal metacristid is present. This ridge extends distally and continues to the lingual margin of the talonid.

The talonid of the lower molar is much narrower, shorter and lower than the trigonid. The hypoconid is the dominant cusp, and it barely projects more dorsally than the very short cristid obliqua. The cristid obliqua is low and weak. It extends from the mesial side of the hypoconid to connect to the distal wall of the metaconid. A tiny swollen nodule is developed on the lingual end of the hypocristid, and it can be interpreted as the rudimentary hypoconulid. The talonid basin is small and shallow with its lingual side is completely open. Many non-marsupial metatherians have mesial cingulid cusps at the base of the paraconid and protoconid, known as cuspid e and f. The enamel on the mesial side of V 22804 is broken, but the remaining dentine is smooth, and so it is unlikely that a cingulid and cingulid cusp were present.

The hypertrophy of postvallum-prevallid shearing in deltatheroidans results in a paraconid that is taller and larger than metaconid. A deep carnassial notch is always present on the paracristid. The talonid in deltatheroidans is small, but the hypoconid and hypoconulid are usually prominent. The lower molar (V 22804) referred to Gurbanodelta, has a large paraconid, but it is lower than the metaconid. The carnassial notch on the paracristid of this tooth is very shallow. The talonid is proportionally as small as those unequivocal deltatheroidans, but with a less projecting hypoconid and hypoconulid.