A late Paleocene probable metatherian (?deltatheroidan) survivor of the Cretaceous mass extinction

Deltatheroidans are primitive metatherian mammals (relatives of marsupials), previously thought to have become extinct during the Cretaceous mass extinction. Here, we report a tiny new deltatheroidan mammal (Gurbanodelta kara gen. et sp. nov.) discovered at the South Gobi locality in China (Xinjiang Province) that is the first Cenozoic record of this clade and renders Deltatheroida a Lazarus taxon (with a new record 10 million years younger than their supposed extinction). The vertebrate fauna associated with Gurbanodelta is most similar to that from the slightly older late Paleocene Subeng locality in Inner Mongolia. The upper molars of Gurbanodelta exhibit a broad stylar shelf with one prominent cusp (stylocone), and a paracone that is sharp and significantly taller than the metacone. The lower molar tentatively assigned to Gurbanodelta has a very small talonid without an entoconid. This combination of these features is known only in deltatheroidans. Phylogenetic analysis places Gurbanodelta as the sister taxon of the North American latest Cretaceous Nanocuris. Gurbanodelta is the smallest-known deltatheroidan, and roughly the same size as the smallest living marsupial. It is likely that the Gurbanodelta lineage dispersed between Asia and North America as part of known intercontinental mammalian dispersals in the late Paleocene, or possibly earlier.

talonid (as compared to Gurbanodelta) with well-differentiated hypoconids, hypoconulids and entoconids on the lower molars.
Deltatheridium and Deltatheroides are closely related taxa and their morphology is well known 12,13 . In comparison to the smaller Gurbanodelta, both Deltatheridium and Deltatheroides have proportionally broader buccal stylar shelves that occupy more than half of the tooth's width. The stylar shelf in Gurbanodelta occupies about one-third of the tooth's width. The ectoflexus of Deltatheridium and Deltatheroides are very deep, and the depth increases from M1 to M3. The ectoflexus of Gurbanodelta is moderately deep, and it becomes shallower from M2 to M3. The paracone and metacone of Deltatheridium and Deltatheroides closely approach each other, and their bases almost fuse together. Both cusps are slender. The paracone is only slightly taller than the metacone. The bases of the paracone and metacone are not completely fused together in Gurbanodelta, but they approach each other. The paracone and metacone are trenchant in shape, with the former much higher than the latter. The postmetacrista of M2 in Deltatheridium and Deltatheroides is very long and strong. A small notch is present on this crista, making it like a carnassial shearing blade. The postmetacrista of M2 in Gurbanodelta also is very strong, but those teeth lack such a notch. Relative to the preparacrista, the postmetacrista of M3 is reduced in Deltatheroides, and very reduced in Deltatheridium. In Gurbanodelta, the postmetacrista of M3 is as long as the preparacrista. The m1 (known in Deltatheridium but not in Deltatheroides) has a paraconid higher and larger than the metaconid. The hypoconid of the tooth is quite projecting. The m1 of Gurbanodelta has a paraconid lower and smaller than metaconid. Its hypoconid is low and small, and barely projects above the talonid. The recently described Lotheridium is very similar to Deltatheridium and Deltatheroides. The differences present between Gurbanodelta and Deltatheridium (plus Deltatheroides) are the same differences between Gurbanodelta and Lotheridium.
Sulestes has very broad stylar shelves on the upper molars, very strong postmetacristae with carnassial notch-like structures on M2 and a reduced metacone and postmetacrista lobe on M3 (similar to Deltatheridium and Deltatheroides but different from Gurbanodelta) 9 . The protocone of Sulestes is large and bears strong paraconule and metaconule. For a deltatheroidan, this morphology is quite unique. The ectoflexus of Sulestes is relatively shallow and broad, and the width of the stylar shelves decreases from M1 to M3. These two features are similar to Gurbanodelta. The m1 of Sulestes has a very large paraconid, larger and taller than the metaconid. A deep carnassial notch is present on the paracristid. Corresponding to the broad protocone, the talonid in Sulestes is also relatively large. The hypoconid and hypoconulid are prominent, and a rudimentary entoconid is always present. These lower molar characters are in a sharp contrast to those of Gurbanodelta.
Oklatheridium is a small deltatheroidan, but still larger than Gurbanodelta. The protocone in this species is relatively better developed than in Gurbanodelta. Its trigon basin is broader, and the conules are larger than those of Gurbanodelta. Its preprotocrista is relatively low. It extends to the buccal side past the mesial side of the paracone, but it is not elevated and closely approaches the base of the paracone. In Gurbanodelta, the preprotocrista is elevated and well separated from the paracone. The relatively weak preprotocrista in Oklatheridium likely is coupled with more emphasis on the postvallum/ prevallid shearing than in Gurbanodelta. The postmetacrista of M2 in Oklatheridium is long and bear deep carnassial notches. The ectoflexus of the M2 is much deeper than the state in Gurbanodelta. The M3 of Oklatheridium has a reduced metacone lobe with very short postmetacrista, similar to that in Deltatheridium and Deltatheroides, but different from Gurbanodelta. The lower molar of Oklatheridium has a paraconid slightly larger and higher than the metaconid. Small cuspids e and f are present. The talonid is better developed than in other deltatheroidans, and both the hypoconulid and entoconid are present. These features of Oklatheridium are very different from the m1 of Gurbanodelta.
Similar to Gurbanodelta, the slightly larger Atokatheridium has a moderately broad stylar shelf, a mesiodistally compressed protocone, well-separated paracone and metacone, salient buccal expansion of preprotocrista, a shallow ectoflexus and a welldeveloped postmetacrista on M3. In both taxa, the postmetacrista lacks a carnassial notch. This absence may be related to their small body size and a lessened reliance on the postvallum/prevallid shearing. The parastyle and the stylocone in Atokatheridium are not twinned. The parastyle is more lingually positioned relative to the stylocone, and lower than the stylocone. The stylocone itself is quite blunt. The buccal part of the paracrista is low, and weakly connected to the stylocone. In Gurbanodelta, the parastyle and stylocone are twinned cusps. Both are buccally positioned, and are similar in height and size. The stylocone is conical. The paracrista extends buccally and connects the stylocone with a high ridge. The protocone of Atokatheridium is buccolingually broad, and proportionally wider than that of Gurbanodelta. The m1 of Atokatheridium has a paraconid slightly smaller than the metaconid, but the two cusps are similar in height. In Gurbanodelta, the paraconid is much lower than the metaconid.
The larger upper molars of Nanocuris have a proportionally longer crown outline than in Gurbanodelta. In Nanocuris, the protocone is relatively smaller. In mesial or distal view, it is significantly lower than the paracone and metacone. In Gurbanodelta, the protocone is slightly lower than the paracone and metacone. The paracone of Nanocuris has a rounded lingual border. In contrast, the lingual side of the paracone of Gurbanodelta forms a blunt ridge. The preparacrista of Nanocuris is relatively weak, and much shorter than the strongly distobuccally expanded postmetacrista. In Gurbanodelta, the two cristae are almost equally developed. The postmetacrista of Nanocuris does not have the postmetacrista cusp and does not form a carnassial notch. This feature is very similar to Gurbanodelta. The stylocone and parastyle in Nanocuris are fused together, forming the only dominant cusp along the stylar shelf border. The lower molar of Nanocuris has a large paraconid that is bigger and higher than the metaconid. A salient carnassial notch is developed between the paraconid and protoconid. The talonid of the lower molar in Nanocuris is relatively larger than that in Gurbanodelta. The talonid has a small hypoconid, hypoconulid and a cingulid-like entoconid, and the small talonid basin is enclosed by these three cusps. A strong cristid obliqua also is present in Nanocuris, and it extends mesially up to the tip of the metaconid. That feature is not present in other deltatheroidans.
The larger Tsagandelta is represented by a single a jaw fragment preserving m2 and part of the crown of m3 6 . Tsagandelta has a paraconid that is larger than its metaconid, a sharp carnassial notch on the paracristid and a large mesiobuccal cuspid f. Those features are absent in Gurbanodelta. In addition, the talonid of Tsagandelta is relatively broader than that in Gurbanodelta, and the hypoconid and cristid obliqua are better developed than those in Gurbanodelta.

Phylogenetic Analysis
We added Gurbanodelta kara to the dataset of Luo et al. (2011 14 to examine the systematic position of Gurbanodelta to Deltatheroida within a broader sample of mammals, and the recent dataset of Rougier et al. (2015) 6 to examine the phylogenetic relationships between Gurbanodelta and other deltatheroidans. The strict consensus of the most parsimonious trees is in S- Figure 1 The m1 (IVPP V 22804) assigned to Gurbanodelta kara has a relatively small paraconid and relatively weakly-developed paracristid. These features are not "typical" for a deltatheroidan. When this lower molar of Gurbanodelta kara is not scored into the data matrix, the phylogenetic relationship between Gurbanodelta and other deltatheroidans remains unchanged (S- Figure 2, 3).
S- Figure 1. The strict consensus tree derived from 215 equally-parsimonious trees (2170 steps) resulting from the analysis of the dataset from Luo et al. (2011) 14 . The numbers before the slashes are the Bremer Support values; numbers after the slashes are Relative Supports 15,16 . Internodes without Bremer Support values indicate polytomies.Deltatheroidan taxa are indicated in red.