First occurrence of the eutherian mammal Asioryctes nemegtensis from the Upper Cretaceous Djadokhta Formation, Gobi Desert, Mongolia, and a revised alpha taxonomy based on the skull and dentition

ABSTRACT We report on the discovery of new skulls and dentitions documenting a broader geographic and stratigraphic range for the eutherian mammal Asioryctes nemegtensis. Originally discovered in the Upper Cretaceous Barun Goyot Formation of the Gobi Desert, Mongolia, A. nemegtensis has been hypothesized to be absent from the Djadokhta Formation, another Upper Cretaceous Gobi Desert formation. This distinction has previously contributed to an emphasis on faunal and age dissimilarity between the two sets of Cretaceous rocks. Here we report four new specimens of A. nemegtensis from the Djadokhta Formation and document its first co-occurrence with its sister taxon, Ukhaatherium nessovi. The new specimens reveal additional details about the cranial and dental anatomy of asioryctitheres and prompt an alpha taxonomic revision and emended diagnosis for A. nemegtensis. Among the new specimens are a nearly complete skull and several articulated jaws, which provide new information on tooth occlusion. These discoveries are an additional datapoint indicating that some Upper Cretaceous faunal assemblages in Mongolia are not as distinctly different in composition and age as has been previously hypothesized.


INTRODUCTION
The diminutive mammal Asioryctes nemegtensis from Upper Cretaceous rocks of the Gobi Desert, Mongolia, is hypothesized to be one of several rare eutherian fossils on the stem to the crown clade Placentalia (Wible et al., 2007). Knowledge of the anatomy of such extinct species is critical for interpreting the morphology of the ancestral placental and for calibrating the age of the clade Placentalia. Asioryctes nemegtensis has, until now, only been known from a single Gobi Desert formation and a restricted set of localities (Kielan-Jaworowska, 1975, 1977; see also faunal list in Jerzykiewicz et al., 2021).
Asioryctes nemegtensis was discovered and named by Kielan-Jaworowska (1975) as a member of Palaeoryctidae. However, upon discovery of its sister taxon, Ukhaatherium nessovi, Novacek et al. (1997) erected a new clade "Asioryctitheria," to include Asioryctes, Ukhaatherium, and Kennalestes, each a monotypic genus, from Upper Cretaceous Gobi Desert strata. Novacek et al. (1997) identified several cranial features shared among the three constituent asioryctithere genera, including the presence of a fusiform auditory bulla. A more recent combined data phylogenetic analysis, however, suggested that "Asioryctitheria" may be paraphyletic unless Leptictidae is nested within it (Velazco et al., 2022).
The original descriptions of Asioryctes nemegtensis and Kennalestes gobiensis were based on the discovery of several specimens that permitted an in depth characterization of the skull and dentition (Kielan-Jaworowska, 1975). These discoveries were subsequently supplemented with a description of a very complete skeleton of A. nemegtensis (Kielan-Jaworowska, 1977). Conspicuous dental features of A. nemegtensis distinguishing this taxon from K. gobiensis include the presence of four, rather than three, lower incisors, and more transverse upper molars that lack pre-and post-cingula (Kielan-Jaworowska, 1975). Novacek et al. (1997) observed that the asioryctitheres Ukhaatherium and Asioryctes share, to the exclusion of Kennalestes, several premolar and molar characters as well as a bifurcate or sigmoid contact between the jugal and maxilla (illustrated in Kielan-Jaworowska, 1975: fig. 1B). Those authors also noted that Ukhaatherium has the autapomorphies of an enlarged, single-rooted upper and lower canine and an extensive diastema between the last upper incisor and canine for the occlusion of a lower canine, among other dental and cranial autapomorphies. Kielan-Jaworowska (1975) originally recognized Asioryctes nemegtensis as occurring at three Gobi Desert localities each now assigned to the Barun Goyot Formation: Khulsan, Nemegt (Southern Monadnocks), and Khermeen Tsav II (see review in Jerzykiewicz et al., 2021:table 2). The first two are located in the Nemegt Basin, and the third is 20 km southwest of the Nemegt Basin (Figs. 1, 2). The attribution of Khermeen Tsav II to the Barun Goyot Formation is a more recent hypothesis (Jerzykiewicz et al., 2021). Each of these localities contains red beds with broadly similar lithology (Gradzinśki & Jerzykiewicz, 1972;Jerzykiewicz et al., 2021;Kielan-Jaworowska, 1975). These discoveries led to a description of A. nemegtensis as restricted to the Barun Goyot Formation and part of a faunal signature distinguishing the Barun Goyot and the Djadokhta Formations (Gradzinśki et al., 1977:table 4). Subsequently, a general lack of faunal overlap between the vertebrates of the Barun Goyot Formation and those of the Djadokhta Formation has been emphasized (Gradzinśki et al., 1977;Kielan-Jaworowska et al., 2003). Noted rare exceptions present in both places are Deltatheridium pretrituberculare, a marsupial relative (Kielan-Jaworowska & Nessov, 1990;Rougier et al., 1998;Velazco et al., 2022) and the multituberculate Kryptobaatar dashzevegi (Kielan-Jaworowska et al., 2003). Gradzinśki et al. (1977:307) further stated that the comparative composition of mammals and dinosaurs indicated that the Djadokhta Formation is older than the Barun Goyot Formation.
We report here, however, the first occurrences of Asioryctes nemegtensis in the Djadokhta Formation, another Upper Cretaceous fossil-bearing unit in the Gobi Desert. This formation is exposed in several localities, many of them widely separated (Fig. 1). These localities are not laterally continuous with the Barun Goyot beds, greatly complicating an understanding of their relative ages despite significant effort (Dashzeveg et al., 2005;Dingus et al., 2008;Gradzinśki et al., 1977;Jerzykiewicz et al., 2021). Two of these Djadokhta Formation localities have yielded A. nemegtensis: Ukhaa Tolgod and Udan Sayr. Since its discovery in 1993, Ukhaa Tolgod has yielded thousands of fossil dinosaurs, lizards, and mammals (Clark et al., 1999;Dashzeveg et al., 1995;Norell et al., 1994;Novacek et al., 1997). Similarities between the faunas of Ukhaa Tolgod and those of other Djadokhta Formation localities have been noted for more than 20 years (Dashzeveg et al., 2005;Kielan-Jaworowska et al., 2003) and detailed study of the geology and paleontology of this locality has led to its current assignment to the Djadokhta Formation (Dingus et al., 2008) and to the newer concept of the Djadokhta lithobiotope (Jerzykiewicz et al., 2021:table 2). The locality Udan Sayr has been studied in less detail; however, recent work has led to its preliminary assignment to the Djadokhta Formation as well based on geology and paleontology (Watabe et al., 2010).
The new specimens permit closer comparisons between Asioryctes nemegtensis and its sister taxon Ukhaatherium nessovi that prompt a revision of the diagnosis of A. nemegtensis. Initial descriptions of asioryctitheres predated the availability of high-resolution CT imaging techniques now available, and we apply this technology to describe newly discovered skulls and dentitions, revealing new morphological details and a clearer view of tooth occlusion. In addition, we discuss below how the new occurrence of A. nemegtensis bears on concepts of the faunas of the Barun Goyot and Djadokhta Formations and more general arguments regarding the relative ages of the Gobi Cretaceous vertebrate faunas.

Barun Goyot Formation Localities
History-The three localities assigned to the Barun Goyot Formation (or Baruungoyot lithobiotope of Jerzykiewicz et al., 2021) yielding Asioryctes nemegtensis are Khulsan, Nemegt (Southern Monadnocks), and Khermeen Tsav II (Figs. 1, 2; Table 1). These localities have been known for over 50 years (Kielan-Jaworowska, 1975). Russian expeditions of the late 1940s first described these facies but because they did not find fossils in them referred to them as the barren deposits (Efremov, 1949(Efremov, , 1954(Efremov, , 1955. They were later named the "lower Nemegt Beds" by Gradzinśki et al. (1969:37) and were redefined and formally designated as the Barun Goyot Formation by Gradzinśki and Jerzykiewicz (1974a:116). The Polish-Mongolian Expeditions in the 1960s (Kielan-Jaworowska & Dovchin, 1968) recovered numerous vertebrate fossils in the Barun Goyot Formation as have many subsequent expeditions (reviewed in Kielan-Jaworowska et al., 2000). Kielan-Jaworowska (1975) reported no specific level within the Barun Goyot Formation yielding fossils of Asioryctes nemegtensis; however, in their description of Barun Goyot Formation lithology at Khulsan, Gradzinśki and Jerzykiewicz (1974a:140) stated that while many mammal specimens were found as float, those that were in situ were in the "diversely stratified sandstones." The Mongolian Academy of Sciences-American Museum of Natural History Joint Paleontological Expedition (MAE), in various seasons beginning in 1990, collected fossils at the locality Khulsan that the senior author here observed as weathering out in nodules or found in situ, primarily in massive sandstones lacking cross-bedding. The A. nemegtensis-producing localities were not known to be laterally continuous at the time of their description nor have continuous beds been discovered since (Gradzinśki & Jerzykiewicz, 1972;Jerzykiewicz et al., 2021;Kielan-Jaworowska, 1975).
Lithology and Age-Khulsan is the type locality of the Barun Goyot Formation (Gradzinśki et al., 1977) and the type locality of the newer concept of the Baruungoyot lithobiotope (Jerzykiewicz et al., 2021). As noted above, Asioryctes nemegtensis was also reported from the locality Nemegt (Kielan-Jaworowska, 1975) where the Barun Goyot Formation either interfingers with or lies directly below an Upper Cretaceous fossiliferous unit, the Nemegt Formation (Fanti et al., 2018). The Barun Goyot Formation consists of red continental rocks more than 110 m thick containing dinosaur, mammal, and lizard fossils at all levels (Gradzinśki & Jerzykiewicz, 1974b). The original description of Gradzinśki and Jerzykiewicz (1974b:111) noted five distinct lithologies: "The lower part … is characterized by mega cross-stratified units, interpreted as dune deposits; they are intertonguing with water-deposited sediments laid in interdune areas. Channel deposits, attributed to intermittent streams are subordinate; massive sandstones, probably of various origin are predominating [see also Figs. 2A,3A]. The upper part of the profile of the formation is characterized by the predominance of flat-bedded sandstone units which were probably deposited in an intermittently flooded takyr-like area." The Barun Goyot Formation has been described as Campanian; however, its age remains very uncertain because the formation lacks beds amenable to radiometric dating or high resolution biostratigraphy (Gradzinśki et al., 1977;Jerzykiewicz et al., 2021).
Depositional Environment- Jerzykiewicz et al. (2021:844) considered these rocks to represent the "marginal zone of a sand sea covered by aeolian dunes, and ephemeral interdune streams and ponds."

Djadokhta Formation Localities
History-The two localities where Asioryctes nemegtensis has now been identified are Ukhaa Tolgod and Udan Sayr. Ukhaa Tolgod has been placed in the Djadokhta Formation (Dingus et al., 2008;Jerzykiewicz et al., 2021:table 2). The Djadokhta Formation is represented by several isolated and widely separated localities in the Nemegt Basin, including Ukhaa Tolgod, as well as localities within the Ulan Nur Basin: Bayn Dzak and nearby Tugrugyin Shireh (Fig. 1). Bayn Dzak, known informally as "The Flaming Cliffs" and famous for having produced the first known dinosaur egg nests (Andrews, 1932), is the site of the type section (Gradzinśki et al., 1977). The rocks of the Djadokhta   Figure 3. Cross-bedded strata interpreted as ancient aeolian dunes predominate in the Barun Goyot Formation, whereas structureless sandstones interpreted as stabilized dunes and sandslides predominate in the Djadokhta Formation. Fossiliferous sandstones at both localities are dominantly moderate reddish orange (10R 6/6) to pale red (10R 6/2) to moderate orange pink (10R 7/4) in color. No lateral continuity among these formations has yet been established. Abbreviations (after Dingus et al., 2008): Kc, Cretaceous conglomerate; Ke, Cretaceous crossbedded eolian sandstone; Ks, Cretaceous structureless sandstone; Kwp, Cretaceous white caliche pebble unit (indicated by dashed line); Qc, Quaternary alluvium; X, fossil locality.
Formation are not laterally continuous with those of the Barun Goyot Formation, greatly complicating correlation among these fossiliferous beds (Gradzinśki et al., 1977;Dashzeveg et al., 2005;Dingus et al., 2008;Jerzykiewicz et al., 2021). In fact the northern China (Inner Mongolia) locality Bayan Mandahu situated over 340 km to the southeast of Bayn Dzak (not shown in Fig. 1) has been noted for its lithological and faunal similarity to the Djadokhta Formation despite its distance from the type locality (Jerzykiewicz et al., 1993); it is currently assigned to the Wulansuhai Formation (Wei et al., 2005; see also Hone et al., 2010).
Ukhaa Tolgod, one of the two Djadokhta Formation localities that has produced Asioryctes nemegtensis, is located in the Nemegt Basin 183 km to the southwest of Bayn Dzak (Figs. 1, 2B). The locality was discovered in 1993 by the MAE and proved to be extremely rich in medium-sized and smaller dinosaurs, mammals, lizards, and dinosaur nesting sites (Dashzeveg et al., 1995;Dashzeveg et al., 2005;Novacek et al., 1997;Dingus et al., 2008). Despite the much greater abundance of vertebrates, the faunal assemblage at Ukhaa Tolgod resembles the basic composition of assemblages at other Djadokhta Formation localities in that they all preserve smaller vertebrates and lack large dinosaurs (Dashzeveg et al., 2005;Dingus et al., 2008;Jerzykiewicz et al., 2021). This preservational profile is also common to the Barun Goyot Formation localities (Jerzykiewicz et al., 2021).
Udan Sayr, the other Djadokhta Formation locality that has, as reported here for the first time, produced Asioryctes nemegtensis, is a cluster of scattered outcrops of low relief located outside the Nemegt Basin 126 km northeast of Ukhaa Tolgod and 63 km west of Bayn Dzak (Fig. 1). The locality was discovered by the Soviet-Mongolian Paleontological Expeditions in the 1980s (Ivakhnenko & Kurzanov, 1988). The site was also prospected by the MAE and the joint Mongolian-Japanese Expeditions (HMNS-MPC). Both expeditions focused on the southern area called Zhinst Tolgoi (Watabe et al., 2010;Rougier et al., 2016). Udan Sayr was first assigned to the Barun Goyot Formation (Jerzykiewicz & Russell, 1991;Kielan-Jaworowska et al., 2003; also followed by Rougier et al., 2016) and more recently to the Djadokhta Formation (Handa et al., 2012;Ivakhnenko & Kurzanov, 1988;Watabe et al., 2010), an assignment we follow. Watabe et al. (2010:92) mainly based this assignment on the occurrence of Protoceratops andrewsi and Pinacosaurus sp., taxa likewise well-represented in the Djadokhta Formation at Bayn Dzak as well as at Ukhaa Tolgod and Bayan Mandahu, but also described features of the lithology of the upper part of the Udan Sayr beds that very closely resemble those of the Bayn Dzak Member of the Djadokhta Formation at the type locality (Dashzeveg et al., 2005). Czepinśki (2020:481), in a study of protoceratopsid dinosaurs from Udan Sayr and related localities, also recognized that the Djadokhta Formation taxon P. andrewsi was "abundant" at Udan Sayr, an observation that would seem to corroborate prior work cited above assigning the locality to the Djadokhta Formation. However, based on his identification of a single specimen of another ceratopsian more common to the Barun Goyot formation (Bagaceratops rozhdestvenskyi), Czepinśki (2020) argued that Udan Sayr should be considered temporally  intermediate between the Djadokhta and Barun Goyot Formations. This deduction, however, conflates temporal information and faunal signatures. As Jerzykiewicz et al. (2021) argued, the relative ages of Gobi Desert rocks are uncertain and the Barun Goyot Formation may not be younger than the Djadokhta Formation, which would be highly problematic for the proposal of Czepinśki (2020), and we do not follow that idea here. Lithology and Age-Bayn Dzak was named the type locality of the Djadokhta Formation (Gradzinśki et al., 1977) and is also the type Djadokhta lithobiotope (Jerzykiewicz et al., 2021). The two members of the Djadokhta Formation are the lower Bayn Dzak Member exposed at Bayn Dzak and the Upper Tugrugyin Member exposed at Tugrugyin Shireh (Dashzeveg et al., 2005). The formation is at least 70 m thick in the Bayn Dzak region (Gradzinśki et al., 1977) with the exposure at the main "Flaming Cliffs" scarp of more than 35 m in thickness (Dashzeveg et al., 2005: fig. 2) and approximately 75 m thick in the Ukhaa Tolgod region. Exposures at Ukhaa Tolgod belong exclusively to that of the Bayn Dzak Member (Dingus et al., 2008). Watabe et al. (2010) divided the stratigraphic section at Udan Sayr into two parts: a lower section 10 m thick or greater with fluvial facies that preserve casts of dinosaur footprints, and an upper section 50 m thick with cross-bedded and massive, structureless sandstones that closely resemble the facies of the lower Bayn Dzak member at the type Djadokhta section and preserve small dinosaurs, mammals, and other vertebrates.
Through the use of biostratigraphy and magnetostratigraphy, the Djadokhta Formation has been recognized as Campanian and older than the Barun Goyot Formation, which has also been considered Campanian (Dashzeveg et al., 2005;Gradzinśki et al., 1977). However, in a recent review, Jerzykiewicz et al. (2021) stressed that due to the graben-dominated nature of these Cretaceous formations it remains extremely difficult to date them either absolutely or relatively using any modern method.
Depositional Environment-The lithology of the Djadokhta facies suggests that fossils were entombed in stabilized dunes that likely collapsed during intermittent rainstorms and floods (Loope et al., 1998). The paleoenvironment comprised migrating dunes, stabilized dunes, and interdunes where mudstones and siltstones suggest the presence of ephemeral ponds and lakes that formed when corridors between the dunes were flooded (Dashzeveg et al., 2005;Dingus et al., 2008). In contrast to the aeolian and alluvial environments of the Barun Goyot Formation described above that indicate more reliable sources of water, the Djadokhta Formation is interpreted as containing predominantly aeolian rocks (Jerzykiewicz et al., 2021).

Material
Several new specimens of Asioryctes nemegtensis collected by the Mongolian American Expedition (MAE) are described and compared with previously known and new specimens of Ukhaatherium nessovi, also collected on those expeditions ( Table 1). The repository for the specimens described is the PSS-MAE collection of the Mongolian Academy of Sciences and the specimens were collected by the MAE between 1990 and 2019.

Methods
Manual Preparation-Excess matrix covering the delicate mammal fossils was manually removed, often on only one side, so as to reveal the skull or skeleton without disturbing the skeletal elements in their fossilized position.
Digital Preparation-All specimens were scanned at Yale University on a Nikon XT H 225 ST system (Supplemental Table 1). Segmentation and rendering were performed in VGSTUDIO MAX 3.5.1. Tooth measurements were taken from anatomically aligned CT scans using Fiji (Schindelin et al., 2012). Repositioning of certain elements in Ukhaatherium nessovi PSS-MAE 102 was performed in VGSTUDIO MAX 3.5.1 and such instances are noted in the captions. Specifically, the lower jaws are displaced relative to the cranium; thus, to bring the teeth into approximate occlusion, we digitally repositioned the left lower jaw to align its dentition with that of the upper jaw, and we replaced the left p4, which is preserved within the oral cavity just anterior to the canines, in its alveolus.
Revised Generic and Specific Diagnosis-Emended from Kielan-Jaworowska (1975), Novacek et al. (1997), and Archibald and Averianov (2006). Small eutherian mammals resembling the Late Cretaceous Mongolian monotypic taxa Kennalestes (Kielan-Jaworowska, 1969) and Ukhaatherium (Novacek et al., 1997) in having sharp, well-developed upper canines, a P4 (traditional P3) with a large shearing paracone, a semi-molariform P5 with a paracone, metacone, and protocone, last lower premolar and molars with high, sectorial trigonids, a foramen for the postglenoid vein exiting within rather than posterior to a postglenoid buttress, which is developed medially as an entoglenoid process, a well-developed fusiform auditory bulla, a pronounced caudal tympanic process of the petromastoid connected by an interfenestral ridge, and a large piriform fenestra in the anterior roof of the tympanic cavity. Asioryctes and Ukhaatherium share the following characters that differ from the condition in Kennalestes: second upper premolar (P2) smaller than P1, upper molars more strongly elongate transversely, absence of pre-and postcingula and distinct metaconules (both present in Kennalestes), lower molars with smaller paraconids and more mesio-distally compressed trigonids, mastoid rectangular in outline, and anterior contact of the jugal with maxilla strongly bifurcate. Asioryctes differs from Ukhaatherium in the following characters, where the condition in Ukhaatherium is described in parentheses: two mastoid foramina (one), hypoglossal foramen larger than jugular foramen (smaller than jugular foramen), postglenoid foramen much larger than the fenestra vestibuli (subequal), dentary with four mental foramina (two), smaller upper canine with two roots (distinctly larger and single-rooted), absence of a large diastema between I5 and the upper canine for the occlusion of the large lower canine (diastema present), P1/p1, P2/p2 larger relative to P4/p4, P1 with a recurved main cusp and a small distal cusp, more robust P4, with a more salient protocone, M3 relatively wider transversely, as wide or wider than M2, trigonid cusps on m1-3 more salient and recurved, and m3 talonid relatively more elongate mesio-distally with more salient hypoconulid. In addition, Asioryctes is about 15% smaller than Ukhaatherium (see Tables 2, 3).
Description and Comparisons-The most complete of the newly identified specimens of Asioryctes nemegtensis, PSS-MAE 366, is a skull with articulated lower jaws and a full dentition (Figs. 4, 5, 7). The skull, however, is laterally compressed and damaged particularly in the area of the skull roof, orbit, and basicranium. Accordingly, some of the cranial features originally described for the taxon by Kielan-Jaworowska (1975) cannot be identified in PSS-MAE 366, but other cranial and dental features matching those of Asioryctes can be seen. The anterior part of the snout is narrow and elongate and widens opposite P4 (traditional P3) (Fig. 5C). The infraorbital foramen is deep and positioned above P4. The zygomatic arch is only preserved on the left side of PSS-MAE 366 and, although much of the jugal is missing, this element appears to have been well-developed. The unfused dentaries are slender, and the coronoid process is prominent, with a strong anterolateral crest (Figs. 5A, 7A, C).
When describing the cranium of Kennalestes, Kielan-Jaworowska (1981) also provided extensive descriptions of the skull of Asioryctes nemegtensis in her comparisons. However, the fragmentary nature of the skull material of the new Asioryctes specimens limits additional comparisons. Published observations are of use here in contrasting with cranial features in the closely related Ukhaatherium. The cranium, dentition, and postcranial skeleton of Ukhaatherium were coded in the matrix of O' Leary et al. (2013), which was expanded to include additional Cretaceous and early Tertiary taxa in Velazco et al. (2022). Notable differences in skull structure between Asioryctes and Ukhaatherium follow. The letter C followed by a number in parentheses refers to that character for Ukhaatherium in the matrices of O' Leary et al. (2013) and Velazco et al. (2022).
Asioryctes was described as having two mastoid foramina; a more superior one situated relatively dorsomedially, and a lower mastoid foramen near the tympanic region (Kielan-Jaworowska, 1981:32 and fig. 2). Ukhaatherium has only one such foramen and it is situated in a relatively medial position on the mastoid (C1009, C1017). Although the auditory regions in both Asioryctes and Ukhaatherium share several distinguishing features (e.g., the expanded ectotympanic forming a fusiform bulla, the piriform fenestra, and the prominent caudal tympanic process of the petrosal), there are also auditory features in Ukhaatherium that differ from those described and figured by Kielan-Jaworowska (1981) for Asioryctes (Fig. 6). Asioryctes is shown in Kielan-Jaworowska (1981: fig. 3) as having a very large hypoglossal foramen, larger in area than the jugular foramen, but the hypoglossal foramen is much smaller than the jugular foramen in Ukhaatherium (C909; Fig. 6). The large size of the hypoglossal foramen in Asioryctes is indeed atypical; however, in the Cretaceous eutherian Maelestes, this foramen is subequal in size to the jugular foramen (Wible et al., 2009:fig. 15). The postglenoid foramen appears much larger than the fenestra vestibuli in Asioryctes, whereas the two openings are subequal in size in Ukhaatherium as well as in the Cretaceous eutherians Zalambdalestes and Maelestes (C532). The condition in Asioryctes is more characteristic of members of Lipotyphla (= Eulipotyphla of Waddell et al., 1999) such as Solenodon (McDowell, 1958;Novacek, 1986b).
Asioryctes also shows several distinct features in its dentition that can be seen in the newly referred specimens. Asioryctes has four lower incisors as in Ukhaatherium (Figs. 5, 7, 8), whereas Kennalestes only has three (Kielan-Jaworowska, 1975). In Asioryctes specimens PSS-MAE 366 and 616, P2 is smaller than P1 (Fig. 8). Moreover, the cheek teeth of Asioryctes and Ukhaatherium are highly sectorial, with strongly transverse upper molars lacking pre-and postcingula (Figs. 9, 10). Kielan-Jaworowska (1975) also noted that Asioryctes, unlike Kennalestes, lacks a metacone on P5, but that cusp is clearly preserved in one of the new Asioryctes specimens (PSS-MAE 114; Fig. 10). The newly referred specimens provide an unambiguous demonstration of dental characters that differ from those in Ukhaatherium as described by Novacek et al. (1997). In Asioryctes, the canines are double-rooted, in contrast to the very prominent single-rooted canines in Ukhaatherium (Figs. 5, 7, 8). In Ukhaatherium, a diastema is present between I5 and the upper canine for the occlusion of the large lower canine. In an Asioryctes specimen (PSS-MAE 366) the P4 (traditional P3) is more robust with a more salient protocone than that tooth in Ukhaatherium.
CT images of the newly referred specimens reveal several dental characters not described by Novacek et al. (1997)  that are diagnostic for separating Asioryctes from Ukhaatherium. In Asioryctes (PSS-MAE 616 and 366), P1 and P2 are relatively larger than these teeth in Ukhaatherium (i.e., compared with PSS-MAE 102, the holotype; Fig. 8). The P1 of Asioryctes is prominent enough to occlude with the lower dentition (Fig. 8), and the anterior edge of the crown of P1 is also distinctly recurved, and there is a well-developed distal cusp on P2 (Fig. 8). In the lower dentition, the p4 (traditional p3) in Asioryctes (PSS-MAE 616) has a more robust and salient protoconid than in Ukhaatherium. Additionally, Ukhaatherium has only two mandibular foramina (inferior to the p1 and p5) whereas Asioryctes has four (inferior to the c, p1, and p5, Fig. 7A, B).
The new specimens of Asioryctes (i.e., PSS-MAE 616, 366, and 114) moreover reveal new features of the molars that distinguish Asioryctes from Ukhaatherium. M3 is as wide as or transversely wider than M2, whereas M3 is clearly less wide than M2 in Ukhaatherium (Figs. 9, 10, 11, 12). The lower molars in Asioryctes have more sectorial and salient trigonids and the talonid on m3 is more elongate with a much more prominent hypoconulid (Fig. 11).
Thus, Asioryctes, Kennalestes, and Ukhaatherium are distinguished from each other by cranial and dental features and at the same time grouped by cranial and dental features that separate them from other eutherians. They are also distinguished from the proposed asioryctitheres Daulestes, Bulaklestes, and Uchkudukodon by dental features noted by Archibald and Averianov (2006): a P4 with three roots and a distinct protocone, a parastylar lobe anterior to the paracone in M1, a paraconule and metaconule (the latter present only in Kennalestes) closer to the paracone and metacone respectively than to the protocone, and a diastema between p1 and p2.
Remarks on Tooth Occlusion and Wear-Because specimens of Asioryctes nemegtensis (PSS-MAE 616 and 366) are fossilized with the teeth in occlusion or near occlusion and the dental morphology has been extracted using CT scans, we have a rare opportunity to visualize the relative positions of the trigonids and talonids as they contacted during mastication (Fig. 12). Designation of shearing surfaces here follows the standard terminology and description of tribosphenic molar occlusion established by Crompton (1971) and used in more recent papers (Davis, 2011). The lower jaw moves upward and medially during occlusion and all the images in Figure 12 represent  occlusion of the more lingual shearing surfaces of the upper molars at the end of tooth-to-tooth contact during the power stroke of the masticatory cycle (Crompton, 1971: fig. 2). The crest running anterolabially from the protocone, the preprotocrista, is the leading edge of a shearing surface in contact with the lingual area of the posterior wall of the trigonid, or shearing surface 5 of Crompton (1971). The premetacrista occludes against the posthypocristid or shearing surface 4 of Crompton (1971). A. nemegtensis (PSS-MAE 616) exhibits distinctive abrasion on the molar cusp apices: nearly all six major cusps on m1-m3 exhibit wear through the enamel to the underlying dentin (Fig. 12). A similar degree of wear is apparent in Ukhaatherium (PSS-MAE 102; Fig. 12). We also observe some differences in the mode of occlusion between Asioryctes and Ukhaatherium by comparing A. nemegtensis  with Ukhaatherium (PSS-MAE 102). The relatively more prominent molar cusps in Asioryctes allow for a more extensive prevallum/postvallid shearing surface in occlusion between the preparacrista and preprotocrista of the upper molars and the protocristid of the lower molars or shearing surface 1 of Crompton (1971).
Remarks on Classification-In her original description, Kielan-Jaworowska (1975) allocated Asioryctes to the Palaeoryctidae, a family of small eutherians with sectorial dentitions known primarily from the lower Cenozoic of North America. Both Asioryctes and Kennalestes were long associated with different lineages of early eutherians, the Palaeoryctoidea and the Leptictoidea, respectively (Kielan-Jaworowska, 1969, 1981. Kielan-Jaworowska (1981) also included Asioryctes within the Palaeoryctidae. With the description of the monotypic genus Ukhaatherium, Novacek et al. (1997) later diagnosed Asioryctitheria, a new clade endemic to Asia that included Asioryctes, Kennalestes, and Ukhaatherium. The association of these three taxa was also subsequently supported by a few dental characters in a phylogenetic analysis of 16 Cretaceous taxa known from China, Mongolia, and Uzbekistan (Archibald and Averianov, 2006). More recently, a combined data phylogenetic analysis suggested that "Asioryctitheria" may be paraphyletic within the higher eutherian clade Tamirtheria (Velazco et al., 2022). The latter study, however, did not include Asioryctes, and is currently limited to six Cretaceous eutherian taxa. A study that incorporates Asioryctes and more taxa of relevance in the very large, combined matrix originally published by O' Leary et al. (2013) would be an important test of this finding and is beyond the scope of this paper. Pending such a study we have indicated "Asioryctitheria" in quotes for its potential paraphyly. Novacek et al. (1997) also grouped Asioryctes and Ukhaatherium in the family Asioryctidae by the characters in the above diagnosis, to the exclusion of Kennalestes. Kielan-Jaworowska et al. (2004) distinguished two families within "Asioryctitheria," Asioryctidae and Kennalestidae, but Archibald and Averianov (2006) recognized the latter as the junior subjective synonym of the former. Those authors grouped Asioryctes and Ukhaatherium within the Asioryctinae and added Kennalestes to form the higher group Asioryctidae (Archibald & Averianov, 2006:table 4). That refinement of the classification is accepted here. Archibald and Averianov (2006) also proposed that the Cretaceous Uzbek taxa Daulestes, Bulaklestes, and Uchkudukodon were asioryctid relatives within "Asioryctitheria," but acknowledged that the evidence for this association offered only "weak support" (Archibald & Averianov, 2006:373). They surmised that this result might reflect the retention of a large number of "ancestral eutherian traits" in the Uzbek asioryctitheres, and the fact that their analysis was limited to dental traits.
The relationships of asioryctitheres to other eutherians requires broader and deeper comparisons. The phylogenetic analysis of Wible et al. (2004) showed a close relationship between asioryctitheres and zalambdalestids, but this study did not include zhelestids. The phylogenetic results of Archibald and Averianov (2006) indicated that zhelestids and zalambdalestids may be more closely related to each other than either is to asioryctitheres, but that study was limited to 33 dental and jaw characters. Wible et al. (2007), in a description of a new Cretaceous eutherian, the monotypic Maelestes gobiensis, sampled 19 non-placental eutherians that included several zhelestids scored for 408 morphological characters. In this study, asioryctitheres, albeit solely represented by Asioryctes, were nested with Maelestes and two early Cenozoic North American taxa, Batodon and Cimolestes. As noted, Velazco et al. (2022), in a combined data analysis that included asioryctitheres but not Asioryctes, showed that "Asioryctitheria" was paraphyletic unless lepticids were included, and that Ukhaatherium, Zalambdalestes, and Maelestes formed a subclade that excludes Kennalestes. That analysis is notably rich in morphological character information: fossil species were scored for any of the 4,541 phenomic characters in O' Leary et al. (2013) that were preserved-for example, Kennalestes alone was scored for 1138 character states. However, as noted, this study excluded several eutherian taxa of relevance, including zhelestids as well as Asioryctes. Thus, published studies are currently limited either in taxon or character sampling and a more comprehensive assessment is needed.

Barun Goyot and Djadokhta Formation Faunal Comparisons: Biostratigraphic Implications
The important fossil discoveries of the Polish-Mongolian teams in the 1960s yielded a new assemblage of small dinosaurs, mammals, and other vertebrates from the beds at Khulsan, Nemegt, and Khermeen Tsav that prompted comparisons with assemblages from the type locality of the Djadokhta Formation, Bayn Dzak, first worked by the American Museum Expeditions in the 1920s (Andrews, 1932;Berkey, 1923;Gradzinśki et al., 1977;Gradzinśki & Jerzykiewicz, 1974a). Two initial conclusions emerged from this work: first, that the faunas of the two formations were significantly different, and second, that the fauna of the Barun Goyot Formation was younger than that of the Djadokhta Formation (Gradzinśki et al., 1977). The relevant evidence for the first distinction included the presence of only one mammal in common between the two formations, Deltatheridium pretrituberculare, and the absence of certain species in the Barun Goyot Formation, such as the dinosaur Protoceratops andrewsi and the multituberculate mammal Krypotobaatar dashzevegi, that are extremely well-represented in the Djadokhta Formation (Dashzeveg et al., 2005;Dingus et al., 2008). The relative age differentiation of the two units was primarily based on an argument that some of the Djadokhta vertebrates were more primitive, and thus older than Barun Goyot vertebrates, and that the Barun Goyot Formation was directly in contact with the superposed and younger beds of the Nemegt Formation (Gradzinśki et al., 1977). This approach to dating rocks is, however, extremely problematic for several reasons including that the definition of "primitive" typically goes unspecified. Moreover, such an approach relies on numerous undefined and unmet assumptions such as an ancestor-descendant relationship, rather than a sister taxon relationship, between fossil taxa, and the idea that a "primitive" taxon cannot endure in the fossil record longer than its sister taxon such that it might actually be found in younger rocks.
Despite the dramatic increase in recent years of information on the vertebrate fossils of the Upper Cretaceous Gobi Desert, the relationship and the relative ages of the relevant formations and their assemblages remain unclear. For example, Makovicky (2008) attempted to discover the biochronologic order between the Gobi Cretaceous faunas based on the phylogenetic (sister taxon) relationships of some of their constituent dinosaur, mammal, and lizard taxa. Results of this analysis, however, were ambiguous, showing that some Djadokhta Formation faunas were either younger than or contemporaneous with Barun Goyot Formation faunas. Jerzykiewicz et al. (2021:tables 1 and 2) compared the Djadokhta and Barun Goyot rocks using a novel concept of lithobiotopes. They noted that both formations are noteworthy for having at least 40 species of mammal fossils, 25% of which are in common between the two, with the Barun Goyot fauna being slightly less diverse. This comparison does not, however, provide any evidence bearing on the relative ages of these assemblages. Jerzykiewicz et al. (2021:848), in a review of decades of study of the complex geology of the Gobi Desert, stressed that localities of the Barun Goyot and Djadokhta Formations are part of a "graben-dominated system" in which sedimentation in any single graben was decoupled from that of others. This conclusion is based on extensive field and analytical work including the identification of Cretaceous extensional faulting (Cunningham et al., 2009) on the northwest margin of Altan Uul, the mountain range that borders the western Nemegt Basin in the area of the Nemegt Formation localities Altan Ula I-IV. Jerzykiewicz et al. (2021:848) concluded that traditional Cretaceous formations of the Gobi Desert could not be time-delimited with current methods and may even be "coeval" (Jerzykiewicz et al., 2021:848). The increasing faunal similarity we note here between Barun Goyot and Djadokhta Formation mammals is an additional datapoint that is consistent with that most recent assessment.

CONCLUSION
The four new skulls and dentitions described herein document a broader geographic and stratigraphic range for the eutherian mammal Asioryctes nemegtensis. In contrast to what has been previously hypothesized, A. nemegtensis is not absent from the Upper Cretaceous Djadokhta Formation of the Gobi Desert, Mongolia, but is instead present in that formation and in the Upper Cretaceous Gobi Desert Barun Goyot Formation where A. nemegtensis was originally discovered. This discovery documents greater faunal similarity between the two sets of Cretaceous rocks than has been previously recognized. Moreover, our report marks the first documentation of the co-occurrence of A. nemegtensis with its sister taxon, Ukhaatherium nessovi. Additional details on the cranial and dental anatomy of asioryctitheres are present in the new specimens and we revised the alpha taxonomy of A. nemegtensis herein and provided new details on tooth occlusion. Upper Cretaceous faunal assemblages in Mongolia are not as distinctly different in composition and age as once hypothesized and may indeed be coeval.