Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928

Mao, Fang-Yuan, Wang, Yuan-Qing & Meng, Jin, 2016, New specimens of the multituberculate mammal Sphenopsalis from China: Implications for phylogeny and biology of taeniolabidoids, Acta Palaeontologica Polonica 61 (2), pp. 429-454 : 433-447

Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928

Figs. 2–13 .

Type material: Holotype: AMNH 21736, a left M2 ( Fig. 2A ). Paratypes: Other specimens from the hypodigm, some reported and figured by Matthew et al. (1928), are considered paratypes following Article 72 in the International Code of Zoological Nomenclature (4 th Edition); these include an anterior part of the left m2 (AMNH 21713; Fig. 2B ) and an anterior part of m1 (AMNH 21715; Fig. 2C ). We present photographs of those specimens (except for some very fragmentary bits) to make the type series of the species more clearly illustrated. We also include two specimens associated with the holotype and paratypes in the collection that were neither mentioned nor figured in Matthew et al. (1928) ( Fig. 2D [AMNH 21719.001], E [AMNH 21719.002]).

Type locality: Shabarakh Usu , Mongolia .

Type horizon: Late Paleocene Gashato Formation.

Material.—IVPP V19025, fragments of a skull including a left partial maxilla bearing complete M1–2, partial rostrum (the right side) with I2–3 and broken root of P4, partial right M1, anterior portions of both dentaries with basal parts of the incisors, skull fragment with the orbital crest, skull fragment with anterior portion of the left middle ear cavity, including a partial promontory of the petrosal and other bone fragments. All the specimens referred to IVPP V19025 were from a single individual; they were collected from a small a square foot) pit where some elements were weathered out while others were preserved in situ in the sandstone. IVPP V19026, fragmentary right maxilla with M2 with damaged lingual cusp row; IVPP V19027, a tooth of uncertain identity tentatively identified as a right m2 at an early stage of development); IVPP V19028, fragmentary right dentary with m2; IVPP V19029, several parts presumably from a single individual (with a similar preservation condition of IVPP V19025) including left maxilla with M1–2, partial right maxilla with broken M1–2, partial left dentary with damaged m2, fragmentary skull roof with frontals and other elements, and fragmentary bone that appears to be part of an inflated vestibule of the petrosal; IVPP V19030, fragments presumably from a single individual (with a similar preservation condition of IVPP V19025) including an anterior segment of the left dentary with broken incisor and the symphysis, the fragmentary left dentary with m2 and alveoli for m1,

the left(?) mandibular condyle, proximal portion of the right scapulocoracoid, proximal portion of the left femur, the right femur with the proximal portion broken, nearly complete right tibia with the anterior side of the distal end broken, and various fragmentary vertebrae and bone chips; IVPP V19031, fragments presumably from a single individual, including distal left humerus, a segment of possible proximal portion of the left humerus with the head broken, left anterior part of the dentary with broken incisor (sectioned for enamel microstructure study), and partial right M2 (sectioned for enamel microstructure study); IVPP V19032, fragmentary left maxilla with M1–2 and left dentary with broken p4 and complete m1–2, from the same individual; IVPP V19033, left dentary with distal end of m1 and complete but deeply worn m2; IVPP V19034, anterior segment of the right dentary containing the incisor, with the tooth tip broken; IVPP V19035, partial left(?) lower incisor; IVPP V19036, fragments presumably from a single individual, including a partial right dentary with m2 and mandibular condyle with most of the neck; IVPP V19037, right dentary with the distal root of m1 and complete m2.

Emended diagnosis.—Differ from other multituberculates, including Lambdopsalis , in having the following combination of features: Largest known multituberculate in Eurasia (measurements in Table 2 and SOM 5); I2 and I3 subequal in size and shape and both incisors positioned at the anterior margin of the palate with I3 being slightly buccal; P4/p4 extremely small compared to the enlarged molars; a large incisive foramen on the palatal plane medial to the diastema between I3 and P4; M1 and M2 cusp formula 7:7:8 and 1:2:4; M1 lingual cusp row nearly as long as the medial one; humerus proportionally robust in relation to the femur. See Comparisons for more detailed differences between Sphenopsalis and Lambdopsalis .

Description.— Upper incisor: Of the two upper incisors of Sphenopsalis , I2 is slightly larger than I3 ( Fig. 3 ; see Table 2 for tooth measurements). Both incisors are single rooted and single cusped, are only slightly compressed transversely, and the crown of either is covered with enamel for most of the labial surface. The alveolus for I3 is primarily housed in the premaxilla, but its posterior wall is formed by the maxilla ( Fig. 3 ). The tip of I2 has a distinct wear facet, and although I3 is more procumbent than I2, its lingual side also bears a wear facet at the tip, but smaller than that of I2. These wear facets do not indicate cutting motion of the incisors. The I3 of Sphenopsalis is positioned at the edge of the premaxilla, distal and slightly buccal to I2.

Upper premolar: A right portion of the rostrum is preserved ( Fig. 3 ). On the ventral side of the rostrum a long diastema, about 20 mm in length, separates I3 and the alveolus for P4. The crown of the right P4 was broken but its root was preserved in the alveolus. From the circular cross-section of the root, it is clear that P4 is very small, simple and single-rooted. The maximum diameter of the root is about 3 mm.

Upper molars: Among the several upper molars referred here ( Figs. 4 , 5 , 8 ), the best preserved and most informative are from IVPP V19025, an extensively preserved specimen that includes the nearly complete left M1–2 ( Fig. 4 ), as well as the slightly damaged right M1 ( Fig. 8E ). All molars, including the lowers, have smooth enamel. The occlusal surface of the upper molars is concave ventrally when viewed buccally or lingually.

From slightly worn teeth ( Figs. 4 , 5 , 8E, F ) it is clear that the M1 crown is low and has a cusp formula 7:7:8. The buccal cusp row consists of seven cusps with the mesial two cusps being confluent but still recognizable. The mesial cusp continues mesiolingually as a ridge that connects to the mesial cusp on the medial row, and closes the furrow between the buccal and medial cusp rows. The distal cusp of the buccal row is the smallest, which is clear on the right M1 ( Fig. 8E ) but obscured by wear on the left M1 ( Fig. 4 ). Cusps 3–5 on the buccal row are well delimited, and because their lingual sides are worn, the buccal sides of the cusps are higher. Cusps of the medial row are larger than those of the buccal and lingual rows and are generally wider than long. The penultimate cusp of the medial row is the largest, and the remaining cusps decrease in size mesially. Wear is deepest at the distal end of the crown, such that the distal cusps of the three rows form a large concave facet when deeply worn. The wear on M1 decreases mesially. The buccal side of each cusp in the medial row has a short ridge that extends mesially so that the buccal sides of adjacent cusps are connected after wear, whereas the lingual sides of the medial cusps are separated by a narrow and transverse groove ( Fig. 4 ); this pattern remains even after the tooth was heavily worn. The wear facet on each cusp on the medial row can be divided into two parts: the buccal part continues on the buccal ridge, and is more extensive than the lingual part and faces ventrobuccally. The lingual part is primarily at the tip of the cusp but slightly inclines to face ventrolingually. The lingual cusp row is slightly shorter than the medial row; its cusps decrease in size mesially and are transversely narrow but mediodistally long. After wear the lingual cusps become almost confluent to form a narrow crest ( Fig. 5 ). The wear facets of the lingual cusps are inclined, facing ventrobuccally.

The M2 cusp formula is 1:2:4. The crown differs from that of M 1 in being wider but shorter and higher, and in having reddish pigmentation on the enamel surface, similar to that of Lambdopsalis ( Miao 1988) . The pigmentation is concentrated in a thin layer of the outermost parts of the enamel of the tooth, but its composition and mechanism of formation are unknown. In contrast to M1, the M2 cusps are more crescentic, and the medial and lingual rows form a broad trench that is broadly V-shaped in distal view.

The single cusp of the buccal row is distinctive and situated at the mesiobuccal corner of the tooth ( Figs. 4 , 5 , 8 ), and in all specimens at hand is smaller than its homologue in the holotype ( Fig. 2A ). The buccal cusp is crest-like and oriented mesiolingually-distobuccally, oblique to the long axis of the tooth. It has a convex mesiobuccal surface and a flat distolingual surface, and is separated from the mesial cusp of the medial row by a deep groove. Unlike the M1, where two furrows were comfined by three cusp rows, the M2 has only one central furrow, which is aligned distally with the furrow between the medial and lingual cusp rows of M1. The medial cusp row forms most of the buccal side of the M2. The mesial cusp on the medial row is situated slightly distolingual to the buccal cusp and extends mesiolingually as a strong, crescentic ridge that connects with the ridge from the mesial cusp on the lingual cusp row. The connection of the ridges mesially closes the furrow between the medial and lingual cusp rows. In the specimens at hand this cusp is relatively smaller than that of the holotype. The distal cusp of the medial row is higher than the mesial one, and it similarly extends distolingually as a strong ridge that is parallel to that of the mesial cusp. The two cusps are completely separated by a deep and narrow valley. The mesial cusp and ridge are worn more deeply than the distal one. In both cusps, the wear facet is divided into the buccal and lingual parts. The buccal part is primarily on the cusp tip and faces slightly ventrobuccally, and the lingual facet is primarily on the elongate ridge that is much larger and oblique, facing ventrolingually ( Fig. 4 ). In more deeply worn M2s, such as the holotype, these two wear facets are confluent. The wear facet on the single cusp of the external (buccal) row is aligned with the buccal wear facets of the medial cusps; thus, although the single buccal cusp is considered as from a different cusp row, it functions as if it was a mesial cusp of the medial cusp row.

The lingual cusp row of M2 consists of 4 cusps that are smaller and lower than the two medial cusps ( Figs. 4 , 5 , 8 ). The base of each lingual cusp, except for the distal one, extends mesially, but they do not form strong ridges in the case of the medial cusps. In the holotype, the mesial cusp is small and subconical, whereas in IVPP V19025 , the cusp is larger and continues mesiobuccally as a curved ridge to join the ridge of the mesial cusp of the medial row. The second cusp is the largest and tallest of the four lingual cusps. The distal cusp in the holotype is large and long, but in IVPP V19025 the homologue cusp is the smallest and in a form of transverse crest at the distolingual end of the tooth. Wear facets develop exclusively on the buccal side of each lingual cusp, but particularly on the mesial three cusps. These wear facets are oblique to the long axis of the crown, parallel to one another, but are not connected. The central furrow is wider mesially than distally and is open distally .

Lower incisor: The lower incisor is elongate with enamel restricted to the buccal side of the crown ( Fig. 6 ). The tip of the incisor was broken on all specimens in collection. From the cross section revealed by broken incisors (IVPP V19025) and the relatively complete incisor with its medial side exposed (IVPP V19034), the incisor is determined to be nearly circular in cross section, being only slightly compressed transversely. The thickest portion of the lower incisor is within the region of the symphysis, and the tooth gradually tapers mesially and distally. The distal end of the tooth is open, revealing a hollow pulp cavity. The enamel does not extend the entire length of the incisor, but rather is restricted to the buccal side of the anterior portion of the tooth, with part of the enamel extends on the incisor body that is in the jawbone. There is no distinct boundary between the crown and the root, but the distal portion bearing no enamel may be considered as the root. The posterior extent of the incisor within the dentary cannot be accurately determined, but it likely extended to the level of p4. The enamel bears a reddish pigmentation, similar to that seen on the permanent incisor of Lambdopsalis (Miao 1986) . The incisor has the gigantoprismatic enamel, which will be presented in a separate study.

Lower premolar: An incomplete p4 is preserved in IVPP V19032 ( Fig. 7 ). The mesial part of the tooth is broken, but the remainder indicates that p4 is a small, single-rooted tooth with a simple crown that is transversely compressed and aligned with the lingual cusp row of m1.

Lower molars: In the original report by Matthew et al. (1928), a fragmentary left m1 (AMNH 21715) was reported ( Fig. 2C ), and a second incomplete left m1 (AMNH 21719.002; Fig. 2E ) was also included in the hypodigm but was not figured. Because only the anterior half of m1 is preserved in both specimens, the cusp formula has until now remained unknown. The m1 (IVPP V19032) in the current collection is complete and has a cusp formula 5:4 ( Fig. 7 ). The buccal cusps are smaller and lower than the lingual ones, but the size difference is not so dramatic when compared to those on the buccal and lingual cusp rows of m2. The mesial cusp of the buccal row is small; it is clearly present in AMNH 21715 and 21719.002 ( Fig. 2 ), which are lightly worn, but is obscured by wear and poor preservation in IVPP V19032 Fig. 7 ). Cusps 2–4 of the buccal row are subequal and each has a short ridge at the distolingual side that extends distolingually along the central valley. The cusps of the lingual row increase in size and height distally to the fourth cusp, which is of similar height to the mesial cusp on the lingual row of m2. The blunt ridges from the lingual cusps extend distobuccally along the central furrow. At the distal end of the tooth there is a distally convex ridge connecting the distal lingual and buccal cusps. Compared to m2, m1 is much narrower, as in Lambdopsalis (Miao 1986) , and it is also slightly shorter than m2, differing from those in other taeniolabidoids.

The current collection contains several m2s that are well preserved ( Figs. 7 , 8 ). The m2 has a cusp formula 3:2 ( Fig. 7 ) and in occlusal view narrows distally. Similar to M2, m2 also has the reddish pigmentation. The occlusal surface of m2 is not flat, but rather forms a broad trench that in distal view is V-shaped. In relatively unworn m2s (e.g., IVPP V19030; Fig. 8B ), the cusps of both rows decrease in size and height distally and the central furrow is deep and narrow; with wear, the cusps become subequal in height and the occlusal surface becomes relatively flat (IVPP V19033; Fig. 8G ). As seen in IVPP V19033, in which the lingual part of the dentary has been broken away, m2 has two robust roots that are positioned mesiodistally and gradually taper ventrally (not figured). The buccal cusp row of m2 has three cusps, of which the mesial one has a ridge that extends distally along the central furrow. The second cusp is the largest in the row and bears a mesial ridge that extends to the distobuccal base of the mesial cusp, and the two cusps are separated by a deep, diagonal groove. Distally, the second cusp is connected to the ultimate cusp by both lingual and buccal ridges, such that a concavity is formed between the two cusps (see seen in Fig. 8A, D 1 , I). The lingual side of each buccal cusp has an oblique wear facet that faces dorsolingually. The longitudinal groove is narrow and is closed distally by a low ridge that connects the distal cusps of the lingual and buccal rows, as in Lambdopsalis ( Miao 1988; F-YM personal observations).

The lingual row of m2 consists of two large cusps that account for about two thirds of the tooth width ( Figs. 7 , 8 ). The two cusps are strongly crescentic with buccal crests extending distally along the central furrow. The mesial lingual cusp is higher and larger than the distal cusp, but the distobuccal crest of the latter is longer and extends distally and buccally; it eventually joins the base of the distal buccal cusp and closing the distal end of the narrow central furrow. A pocket is formed at the distolingual side of the distal cusp.

Among the specimens of Sphenopsalis from Erden (Urtyn) Obo, there is a peculiar tooth (IVPP V19027; Fig. 8F ) of uncertain identity. The tooth is of light color and bears thin enamel. Because this tooth was entirely within bone and excavated by preparation, it is likely a tooth at a very early stage of development, and the morphology and arrangement of cusps and crests suggest that it is most likely a right m2, but unlike m2 of Sphenopsalis , the tooth is much shorter and lacks the reddish pigmentation. The lingual cusp row, as interpreted here, is much longer than the buccal one, The cusps and crests are sharp and are unworn. Similar to m2, the lingual cusps are at least twice the width of the buccal ones. The mesial cusp is a sharp crest that extends more mesially than the buccal cusps, which is similar to that of m2 (IVPP V19037; Fig. 8A ) but at a greater degree. Unlike m2, there is a sharp, isolated cusp at the distolingual end of the tooth. The width of the tooth is close to that of an adult m2 but its length is only a half of an adult m2.

Dentary and skull fragments: Several fragmentary dentaries and skull bones included in the current collection provide limited but informative anatomical information on the cranium of Sphenopsalis . The symphysis, preserved on the fragmentary left dentary (IVPP V19030) and several additional specimens ( Fig. 6 ), is unfused, implying mobility between the two dentaries in life. The anterior end of the coronoid process is positioned at the buccal side of m2 as shown by its broken base in IVPP V19032 ( Fig. 7 ). Additional, incomplete dentaries (not figured) demonstrate that, the dentary has a long condylar process that continues posteriorly to the mandibular condyle (preserved in two fragmentary dentaries, IVPP V19036). The condyle has a smooth surface with its dorsal portion being bulbous. In posterior view, the condyle is not symmetrical, with the lateral side being dorsoventrally thicker. The medial surface of the condylar process is smooth, whereas the buccal surface bears a ridge that runs nearly parallel to the ventral border of the mandible.

The partial rostrum (most of the right half) (IVPP V19025; Fig. 3 ) is short, broad and low, as in Lambdopsalis and other taeniolabidoid multituberculates where the skulls were known ( Kielan-Jaworowska and Hurum 2001; Kielan-Jaworowska et al. 2004). The width of the right part of the rostrum, measured between the curving point of the rostrum anterior to the P4 alveolus and the medial border of the maxilla on the palate, is about 16 mm. The premaxilla and maxilla are thick and nearly fused, although a vague suture is still discernible laterally. The premaxilla supports an anterior process, most of which is broken away, except for a small lateral part which projects anterodorsally. On the dorsal side of the rostrum and posterior to the premaxilla process, there is a concave area marking the point of articulation with the nasal; the bone is broken posterior to the concave area, revealing the termination of the I2 root. The premaxilla-maxilla suture intersects at the posterior side of I3 alveolus.

On the lateral side of the rostrum, the maxilla forms a knob-like protuberance posterodorsal to the I3 alveolus. The infraorbital canal is not preserved, but a groove anterior to its presumed position runs anteriorly on the lateral surface of the maxilla toward the protuberance. The groove was likely left by the infraorbital blood vessels and nerve. The anterior root of the zygoma appears to have originated at the anterior half of M1; this condition is also supported by the left maxilla bearing M1–2 ( Figs. 4 , 5 ). A short suture is developed on the dorsolateral edge of IVPP V19025, and probably represents the maxilla-nasal suture, or possibly the maxilla-frontal suture; if it is the former, then the nasal would have been extensive, extending posteriorly to the level of the anterior root of the zygoma.

In ventral (palatal) view, the premaxilla is rugose medial to the incisors. The most distinctive feature of the palate of Sphenopsalis is the large incisive foramen ( Fig. 3 ), contrasting with the relatively small incisive foramen in Lambdopsalis ( Miao 1988) and Prionessus (Meng et al. 1998) . The foramen is immediately posteromedial to I3 and is primarily located within the maxilla, with only a minor contribution of the premaxilla at its anterior wall. The incisive foramen of Sphenopsalis measures 13.4 mm long and 7.4 mm wide at its maximum. The lateral wall of the incisive foramen, formed by the maxilla, is a steep but smooth surface that extends dorsally and faces ventromedially. The medial side of the incisive foramen is a narrow bar, which is thick and has a rounded surface. The enlarged incisive foramen is not equivalent to any of the palatal vacuities that occur in some multituberculates, such as Sloanbaatar ( Kielan-Jaworowska 1969) , Bulganbaatar and Nemegtbaatar ( Kielan-Jaworowska 1974) , Stygimys ( Sloan and Van Valen 1965) and Ptilodus ( Simpson 1937) . These palatal vacuities usually occur more posteriorly and medial to the premolars and molars, and where the suture is known the palatal vacuities are commonly between the maxilla and the palatine. The palatal vacuities do not occur in Prionessus Matthew et al. 1928 ) and Lambdopsalis ( Miao 1988) . The posterior part of the palate was broken. The P4 alveolus was partly preserved in the broken maxilla at the lateral-most edge of the preserved specimen ( Fig. 3 ).

The current collection includes a fragmentary skull roof with an incomplete orbital region (IVPP V19025; Fig. 9A ). In dorsal view, the orbital region is characterized by having narrow strip that is delimited medially by a suture on the roof of the skull fragment. This bony stripe is broken anteriorly so that it is not certain about its nature. By position, it could be part of the nasal or the maxilla, but more likely the latter. This bony stripe cannot be part of the frontal. Posterior to the broken “postorbital process” is a rounded notch that leaves a shallow grooved surface on the bony roof; we tentatively identify the notch as part of the postorbital foramen. The posterior part of this skull fragment should be the parietal, which is a thick bone.

A second specimen, IVPP V19029, from a different individual, includes part of the skull roof with what are interpreted here as the posterior parts of the frontals and anterior parts of the parietals ( Fig. 9B ). The frontals converge posteriorly as in other taeniolabidoids, and the anterolateral parts of the parietals curve ventrally, forming a blunt ridge that defines the anterodorsal border of the temporal region of the skull. If our interpretation is correct, then the narrowest part of the skull roof occurs at the anterior temporal region.

A third skull fragment, IVPP V19025, preserves the left portion of the pterygoid region and anterior part of the middle ear cavity ( Fig. 10 ). The pterygoid forms the posterior portion of the lateral wall of the choanae. The preserved portion of the promontorium is straight and bulged, with its anterior pole extending anteromedially. The ventral surface is smooth, with steep medial and gently sloped lateral sides. The promontorium was broken at its posterior part, revealing the thick-boned cochlear house that in life enclosed the cochlea of the inner ear. The endocast shows a straight cochlear canal with a rounded cross-section. On the medial side of the broken posterior part of the promontorium is an incomplete foramen for the facial nerve. Owing to the breakage, the foramen can be traced to the dorsal side of the petrosal on the floor of the braincase. On the anterolateral side of the promontorium is a large rounded bony surface: based on its anatomical position, we interpret this recess as the fossa for the tensor tympani muscle ( Wible and Rougier, 2000). This recess is much more expanded than that in other multituberculates where the middle ear cavity is known (e.g., Kryptobaatar ; see Wible and Rougier 2000) and is significantly different from the middle ear of Lambdopsalis . Notwithstanding the several cracks in the bone, there is no fenestration in the tensor tympani fossa, and the middle ear cavity in this region is completely roofed by bone. The anterior edge of this concave area is extended more anteriorly than the anterior tip of the promontorium. The composition of the bony roof for the tensor tympani fossa is unclear, but it must be primarily from the petrosal, judging from its relationship with the promontorium.

A final fragment, IVPP V19029, preserves an inflated vestibule, similar to that of Lambdopsalis ( Fig. 10B ). This bony element has a rough external surface and a roughly spherical inner surface. The best interpretation of this element is that it represents the bony shell of the inflated vestibule of the petrosal. By its absolute size, it is larger than the vestibule of Lambdopsalis ( Miao 1988; Meng and Wyss 1995), but relative to the skull size as inferred from the teeth and preserved cranial parts of Sphenopsalis , it is propotionally less inflated than that of Lambdopsalis . Although an inflated vestibule is a common figure in multituberculates ( Miao 1988; Meng and Wyss 1995; Fox and Meng 1997), the hypertrophic condition in Lambdopsalis and in Sphenopsalis , if our identification of the structure here is correct, would be unique among multituberculates.

Scapulocoracoid: The proximal right scapulocoracoid (IVPP V19030, Fig. 11A ) shows a distinct scapular spine and infraspinous fossa, but it is difficult to determine whether the supraspinous fossa is present. The acromion is broken, but the base of the coracoid is present, and is not completely fused to the scapula, with the suture between the two elements remaining discernible. The distal tip of the coracoid is broken so that its original size is unknown, but the basal portion of the coracoid is broad and contributes to the anterior part of the glenoid fossa. The glenoid fossa is shallowly concave and has an oval outline with the long axis parallel to the scapular blade; it measures 16.8 mm long and 11.3 mm wide. With the scapula orientated in anatomical position, the glenoid fossa would face primarily ventrally. In general, the preserved portion of the scapulocoracoid is similar to that of Lambdopsalis (JM personal observations) and other multituberculates where the structure is known ( Simpson 1928; McKenna 1961; Clemens and Kielan-Jaworowska 1979; Jenkins and Weijs 1979; Krause and Jenkins 1983; Kielan-Jaworowska 1989; Kielan-Jaworowska and Gambaryan 1994; Sereno and McKenna 1995).

Humerus: The present collection includes an incomplete left humerus preserving parts of both the proximal and distal ends (IVPP V19031; Fig. 11B ). Although the two segments probably belong to the same individual (they were collected from the same pit), a reconstruction of the entire humerus is not possible, as the diaphysis is largely missing, and an estimate of the degree of humeral torsion accordingly cannot be made. The images in Fig. 11 display the general, not the precise, relationship of the humerus fragments. It is certain, however, that the humerus is robust, with the proximal and distal ends being transversely broad, suggesting fossorial adaptations. The posterior crest is well developed and the intertubercular groove is broad. In anterior (anteroventral) view ( Fig. 11B 1 ), the crest leading to the lesser tubercle is narrow, whereas the crest leading to the greater tubercle is broad. Distally, the deltopectoral crest is not preserved, but judging from the massive breakage at the base of the crest, it must be strong. The cross section of the mid-portion of the shaft has a triangular outline, which is considered characteristic of multituberculates ( Kielan-Jaworowska and Qi 1990). Although incomplete, it is apparent that the distal end of the humerus is transversely wide. In anterior view ( Fig. 11B 1 ), the ulnar condyle is oval with the long axis parallel to the diaphysis; the condyle has a measurable maximum diameter of 10.4 mm. Although the distal part of the radial condyle is broken, it nonetheless appears globular and is slightly more proximally positioned than the ulnar condyle. The two condyles are separated by the intercondylar groove, which is more distinctive anteriorly than posteriorly. There is no trace of a trochlea. The entepicondyle is broken, but apparently projected strongly medially as reflected by its massive broken base. The entepicondylar foramen is small, with its posterior aperture at the dorsomedial edge of the thick entepicondyle, rather than at the posterior side of the bone Fig. 11B 2 ), similar to that in the recently described multituberculate humerus from the Late Cretaceous of Uzbekistan ( Chester et al. 2010). Unlike many other multituberculates where the ectepicondyle is usually small, that in Sphenopsalis , although broken, is large. The ectepicondyle continues proximally as a wide crest on the lateral side of the distal humerus. In posterior (posterodorsal) view, the posterior crest is strong ( Fig. 11B 2 ). The shape of the broken radial condyle cannot be determined, but the ulnar condyle is prominent and is orientated diagonal to the long axis of the humerus. The maximum length and width of the ulnar condyle in posterior view are 13.3 mm and 8.4 mm, respectively.

Femur: Two incomplete femora are included in the present collection: a proximal part of a left femur, with the growth line of the epiphysis still visible, and a right femur with the proximal end missing. The incomplete femora were collected from the same pit and are here interpreted to be from the same individual (IVPP V19030; Fig. 12 ). Using the position of the post-trochanteric fossa and related landmarks on the two femora, the complete morphology and length of the femur can be reconstructed. The femur is stout, but in contrast to the humerus, the diaphysis is cylindrical, rather than triangular ( Fig. 12 ). As in other multituberculates ( Kielan-Jaworowska and Gambaryan 1994) and many therians, the femur has a nearly spherical head (maximum diameter of 14.3 mm) with an extensive articular surface, implying significant rotational capability at the hip. On the posteromedial side of the head is a small concavity, the fovea capitis, for the ligamentum teres. The neck is long and forms an angle of 50–60° with the greater trochanter, similar to many other multituberculates ( Kielan-Jaworowska and Gambaryan 1994). The neck is rounded ventrally but is ridge-like on its dorsal border. The greater trochanter is robust and extends above the level of the head as in other multituberculates. In posterolateral view, the surface of the greater trochanter is pear-shaped, with a broad dorsal end that tapers ventrally. This area is commonly described as the rugose area of the greater trochanter in multituberculates ( Kielan-Jaworowska and Gambaryan 1994; Kielan-Jaworowska et al. 2004), but in Sphenopsalis , it is smooth. The lesser trochanter is broken, but the breakage indicates that its long axis was parallel to the neck. A sharp ridge arising from near the breakage, presumably from the base of the lesser trochanter, runs distally for a short distance along the diaphysis. Lateral to the breakage is a shallow depression, the post-trochanteric fossa ( Kielan-Jaworowska and Gambaryan 1994). The third trochanter is absent. A subtrochanteric tubercle, present in some multituberculates on the anterior side at the divergent point of the neck and the greater trochanter ( Kielan-Jaworowska and Gambaryan 1994), is absent in Sphenopsalis .

The proximal end of the right femur is broken, but the post-trochanteric fossa and the ridge from the base of the lesser trochanter are preserved and mirror the structures on the proximal part of the left femur; as such, a nearly complete femur was reconstructed to have an estimated length of 86 mm. The diaphysis measures 11 × 8.7 mm in diameter at its proximodistal mid-point, but flares out toward the distal end, and measures 26.2 mm wide (transversely) and 15 mm long (anteroposteriorly). The epiphyseal line is distinct, and the distal condyles are massive. In anterior view, the lateral condyle is smaller and extends slightly more distally than the medial condyle. Unlike therians where the patellar trochlea (groove) is well defined, long and symmetrical, the trochlea of Sphenopsalis is shallow and does not have a definitive shape. In posterior view, the lateral condyle is larger than the medial condyle. In distal view of the distal end of the femur, the medial condyle has an elongated oval shape, and the lateral condyle is more nearly spherical.

The right tibia (IVPP V19030) is nearly complete, with only the anterior side of the distal end being slightly crushed ( Fig. 13 ). The diaphysis is somewhat distorted, but the length is 58.2 mm, and the diameter at the midpoint measures 8.8 mm mediolaterally and 7.5 mm anteroposteriorly. The mediolateral width of the proximal end is 26.6 mm, over twice the anteroposterior width, which measures 13 mm. In anterior or posterior view, the proximal end is strongly asymmetrical, owing to a large lateral process that extends laterally as a hook-like projection, with its tip pointing distally. In the proximal view, the articular surface of the tibial head consists of three areas, the first two consisting of an oval and smoothly concave articular facet for the medial condyle of the femur, and a transversely elongate articular facet for the lateral condyle of the femur; the former is more proximal (higher) than the latter, and the two facets are separated by the intercondylar eminence. In posterior view, the popliteal notch delimits the two facets. Anterior to the facet for the lateral condyle, there is a transverse groove, probably the extensor groove, which extends towards the lateral extremity of the head. Anterior to the extensor groove is the third area that has an irregular shape and does not seem to be part of the articular facet for the lateral condyle. In lateral view, the extensor groove makes a notch on the lateral edge of the proximal end of the tibia; the notch marks the route of the long digital extensor muscle in life. In posterior view, the edges of the two articular facets project posteriorly so that there is a large pocket on the posterior side of the proximal end of the tibia. On the posterolateral end of the lateral process is the facet for the fibula, which shows that the tibia and fibula are not fused and that the latter does not have a contact with the femur.

Remarks.—As mentioned above, “ Sphenopsalis noumugenensis ” was noted from the Nomogen Formation of Inner Mongolia ( Jiang 1983), but this species has never been established through publication and should be considered a nomen nudum. Given the limited information from the type series of S. nobilis , as well as the variation we report on here (see below), there is no compelling evidence that the specimens from Urtyn Obo represent a different species. At present, we consider that the specimens collected from the Gashato Formation at Shabarakh Usu, Mongolia, and those reported here belong to the same species, and the morphological differences observed in these specimens represent individual variation within the species.

Comparisons.— Lambdopsalis : Among known multituberculates, Sphenopsalis is most similar to Lambdopsalis in overall dental morphology (Miao 1986, 1988; Fig. 14 ). In both taxa, the enamel of M2 and m2 bear reddish pigmentation and the cusps are strongly crescentic, particularly those of the buccal row of M2 and the lingual row of m2. The I2 is not greatly enlarged in either genus, the crown is conical, tapering toward the tip, and the lower incisors are not transversely compressed. Furthermore, the central furrow of the upper molars is closed by a ridge that connects the mesial lingual and medial cusps, and in a reversed manner, a ridge connecting the distal lingual and buccal cusps closes the central furrow of lower molars distally. When Matthew et al. (1928: 3) described the partial m1 of Sphenopsalis , they recognized its smaller size in comparison with the M2; thus the authors noted: “it is not certain that this tooth belongs in this species”. In light of the new specimens described in this study, it is clear that m1 is significantly narrower than m2 and M2, which is a feature shared by Sphenopsalis and Lambdopsalis ( Figs. 7 , 14 ) and reflects a occlusal pattern that differs from that of other taeniolabidoids (see Discussion).

As Miao (1986) noted, I3 of Lambdopsalis at the labial edge of the premaxilla is usually regarded to be plesiomorphic when compared with the general condition of mammals and early plagiaulacoid multituberculates, such as Kuehneodon simpsoni ( Hahn, 1969) and Psalodon fortis Simpson, 1929 ). Because in all the other Asian multituberculates where I3s are preserved in situ, they are situated on the palatal process close to the midline of the skull ( Kielan-Jaworowska 1971, 1974; Kielan-Jaworowska and Dashzeveg 1978), Miao (1986) considered that the I3 position in Lambdopsalis is an apomorphic feature. As we show in this study, I3 of Sphenopsalis is not so reduced and is also marginally or buccally positioned, similar to that of Lambdopsalis . It seems that the marginal position of I3

is shared by Lambdopsalis , Sphenopsalis , and Prionessus (Meng et al. 1998; see below).

In describing the dentition of Lambdopsalis, Miao (1986) indicatedthatthebuccalcusprowofm2isonlyhalfaswideas the lingual row, contrasting with the nearly equivalent width of both cusp rows of m 2 in Prionessus and Sphenopsalis . It is clear that both cusp rows of m 2 in Prionessus are subequal, but the new specimens reported here show that Sphenopsalis has the same m2 condition of Lambdopsalis . When compared with the ultimate molars in other multituberculates ( Kielan-Jaworowska et al. 2004), the m2 condition in which the lingual cusps are twice as wide as the buccal ones can be considered as another derived character shared by Sphenopsalis and Lambdopsalis . The M2s of both genera share a condition similar to that of m2, but in a reversed direction where the medial cusps are twice as wide as the lingual ones.

However, Sphenopsalis differs from Lambdopsalis in being significantly larger. Although I3 position is similar to that of Lambdopsalis , but minor differences exist. For instance the I3 is much smaller than I2, compared to those of Sphenopsalis , and slightly distolingual to I 2 in Lambdopsalis (Miao 1986, 1988; F-YM and JM personal observation). Miao (1986) recognized that Lambdopsalis differs from Prionessus and Sphenopsalis in having its sharper posterior end of m1 wedged between first cusp apices of buccal and lingual rows of m2. The m1 of Sphenopsalis does not seem to have such a sharp distal projection, although its distal end does project distally and indent the mesial surface of m2 ( Fig. 7 ).

Cusps of M1 and m1 of Lambdopsalis are all crescentic at a greater degree than those of Sphenopsalis ( Fig. 14 ). In Sphenopsalis the lingual side of the medial cusps of M1 has only a short ridge and the cusps show a rectangular outline after wear instead of being distinctively crescent. Cusps on both rows of m1 have long ridges that extend distally in Lambdopsalis , in contrasting the relatively short ridges in Sphenopsalis ( Fig. 7 ). The M2 and m 2 in Sphenopsalis are more crescent than M1 and m2, respectively, but their ridges are not so sharp as those of Lambdopsalis ( Fig. 14 ).

Cranial features of Sphenopsalis are still poorly known compared to Lambdopsalis ( Miao 1988) . The incisive foramen of Sphenopsalis is proportionally much larger than that of Lambdopsalis , which is certainly a unique and peculiar feature of Sphenopsalis . Again, if the structure we interpreted as the inflated vestibule for Sphenopsalis is correct, it will be a cranial feature shared by Sphenopsalis and Lambdopsalis . Other aspects of the middle ear cavity of Sphenopsalis are significantly different from that of Lambdopsalis . For instance, Lambdopsalis does not have a broad tensor tympani fossa ( Miao 1988; Meng and Wyss 1995) and the promontorium of Sphenopsalis is different from that of Lambdopsalis ; the latter is finger-like shaped ( Miao 1988; Meng and Wyss 1995; F-YM and JM personal observation).

One cranial character that is unclear is the contribution of the frontal to the orbital rim. It is one of the diagnostic features for Taeniolabidoidea that the frontal is small, pointed posteriorly, and almost or completely excluded from the orbital rim ( Kielan-Jaworowska and Hurum 2001; Kielan-Jaworowska et al. 2004). This is the condition of Sphenopsalis as we interpreted from the fragmentary skull. However, Lambdopsalis , a member of Taeniolabidoidea , has a narrow frontal contribution to the orbital rim, on which there is a postorbital process ( Miao 1988). The narrow part of the frontal is separated from the main portion of the frontal on the skull roof by the nasal-parietal connection. This configuration indicates that the nasal-parietal must have overlapped a significant part of the frontal in Lambdopsalis . In this regard, Sphenopsalis differs from Lambdopsalis .

When Kielan-Jaworowska and Qi (1990) described some postcranial elements from Lambdopsalis , they were not sure whether those are from Lambdopsalis , Sphenopsalis , or even Prionessus . With the new specimens reported here, it is clear that those described by Kielan-Jaworowska and Qi (1990) were not from Sphenopsalis or Prionessus . It is most likely from Lambdopsalis , which is supported by our observations of numerous postcranial specimens of Lambdopsalis collected in recent years. All these postcranial elements cannot be from Prionessus because they are too large for Prionessus . Kielan-Jaworowska and Qi (1990) considered the humeri they described were possibly the stoutest ones within multituberculates at the time. The new materials of Sphenopsalis are certainly much more robust, both in absolute and relative size, than that of Lambdopsalis . Although the forelimbs are robust in both taxa, which perhaps suggest fossorial life, their incisors are relatively small, much weaker than those of the North American taeniolabidoids and unlike typical fossorial mammals that use teeth for burrowing.

To sum up, the dental, cranial and postcranial show that Sphenopsalis and Lambdopsalis are two distinctive taxa and yet, they share many features that strongly indicate a close relationship. The similar morphological features suggest similar adaptation and life styles. However, because they co-existed in the same fauna, one may assume that they had differentiated ecological niches when they existed sympatrically. Their phylogenetic relationship will be further discussed below.

Prionessus : Prionessus lucifer ( Matthew and Granger, 1925) was named on the basis of an edentulous mandible AMNH 20423). As noted by Meng et al. (1998), several additional specimens that were collected in 1925 were not described, including a lower jaw with p4, m1 and erupting m2 (AMNH 21731), which was designated as the topotype. In reporting additional specimens of Prionessus from the Bayan Ulan locality, Inner Mongolia, Meng et al. (1998) revised the diagnosis of Prionessus . Prionessus is similar to Sphenopsalis in having a lower incisor that is oval in cross section, with enamel covering the labial side of the tooth; in having a conical I2 with the enamel restricted to the labial side; in having a relatively simple I3 that is positioned at the labial margin of the palate, lateral to the posterior end of the elongate, medium sized incisive foramen; and in P4 being single-cusped and having one or two roots.

The main features that distinguish Prionessus from Sphenopsalis include: much smaller in body size; cusp formula for m1–2 and M1–2 being 5:4, 3:2 or 4:2, 6–7:7–8:4–5 and 1:2:3, respectively; I3 proportionally much more reduced than I2; p4 at least in some specimens having two roots; molar cusps conical and with weak crests; the lingual and buccal cusp rows of m2 subequal in width. Simmons 1993) recognized several features of Prionessus that she considered more primitive than those of Lambdopsalis and Sphenopsalis , including fewer cusps on M1, the posterior border of the palate occurring medial to the midpoint of M2, and the nasal not contacting theparietal. The last two features are presently unknown in Sphenopsalis .

In general, the molar morphology of Prionessus is more similar to those of the Late Cretaceous multituberculates from Mongolia ( Kielan-Jaworowska et al. 2004) and represented a relatively primitive form of taeniolabidoids. The most distinctive features shared by these three taeniolabidoids are the simple incisors with I3 positioned at the labial margin of the palate and the small, simplified and only P4/p4.

Taeniolabis : Sphenopsalis nobilis from the late Paleocene is by far the largest known multituberculate in Eurasia, whereas Taeniolabis taoensis from the early Paleocene is the largest known multituberculate ( Cope 1882; Granger and Simpson 1929; Simmons 1987; Wilson et al. 2012). When the teeth of the two species are compared, T. taoensis is considerably larger than S. nobilis and has a distinctively different cusp arrangement and morphology ( Fig. 15 ). In revising the diagnosis for Taeniolabis, Simmons (1987) emphasized the greater dimensions of i1, I2, and M2/m2, the cusp formula and the ratio of P4/M1. The new specimens of Sphenopsalis show that cusp formula and P4/M1 length ratio may not adequately distinguish Taeniolabis and Sphenopsalis , but incisor size and shape differ significantly between the two genera. The upper incisors, particularly I2, of Sphenopsalis are proportionally much smaller than the beaver-like incisors of Taeniolabis , which has a well-defined and thick enamel band on the labial (anterior) side of I2. Other major differences between these two taxa are the molar size proportions and cusp shapes. Compared to Taeniolabis , the M1 crown of Sphenopsalis is low and cusps are slim (not so inflated), but both taxa show deepest wear at the distal part of M1. The major dental differences for Taeniolabis and Sphenopsalis are between their M2/m2s. The M2/m2 of Sphenopsalis are highly crested, forming strong and sharp cutting edges appropriate for shearing, whereas those of Taeniolabis are heavy and quadrangular, suggesting the use in grinding ( Fig. 15 ).

Taeniolabis and Sphenopsalis were two of at least five genera previously placed in the family Taeniolabididae (Taeniolabidoidea) ( Fox 1999, 2005; Kielan-Jaworowska and Hurum 2001; Kielan-Jaworowska et al. 2004). The main dental features that shared by them include the follows: only one upper premolar, a long diastema between I3 and the premolar, the premolar strongly reduced in proportion to enlarged upper and lower multicusped molars, M1 with three rows of cusps with lingual one extending for the entire length of the tooth ( Kielan-Jaworowska and Hurum 2001). Some of these features, such as the size of P4 and morphologies of M1 and m2, are unknown for Sphenopsalis until this study. One of the diagnostic features for Taeniolabidoidea , sharing with most Djadochtatherioidea and Eucosmodontidae , is a strong, self-sharpening incisor with limited enamel band ( Kielan-Jaworowska and Hurum 2001; Kielan-Jaworowska et al. 2004). This feature was originally used to define Taeniolabidoidea ( Sloan and Van Valen 1965) . However, the self-sharpening incisors are not developed in Sphenopsalis , nor are in Lambdopsalis and Prionessus , as discussed above. In addition to the limited enamel band on the incisor, incisor self-sharpening requires several other features, as best shown in Glires , such as Rhombomylus ( Meng et al. 2003) . The enlarged upper and lower incisors are strong and generally chisel-like and have a cutting edge formed by the enamel layer at the tooth tip, as in Taeniolabis ( Granger and Simpson 1929) and Catopsalis ( Middleton 1982) . To create and maintain the cutting edge, the lower jaw must be able to move forward and backward, and the upper and lower incisors can constantly hone each other on the distal surface (the dentine part) of the incisor tip. This further implies that the incisors are evergrowing, although the enamel deposition ceased at a stage of ontogeny during the life of the animal, as in Taeniolabis ( Granger and Simpson 1929) . As we show above ( Fig. 3 ), except for the enamel band, the upper incisor of Sphenopsalis has a circular cross-section and does not have the cutting edge at the tip; the closely packed I2 and I3 actually prevent such a cutting function. The similar condition is present in Lambdopsalis (Miao 1986, 1988) and Prionessus (Meng et al. 1998; F-YM and JM personal observations).

The similar comparison can be made between Sphenopsalis and Catopsalis , which is smaller than Taeniolabis but has a similar tooth pattern ( Granger and Simpson 1929; Middleton 1982; Lucas et al. 1997). Its relationship to Taeniolabis is perhaps analogous to that of Lambdopsalis to Sphenopsalis .

Stratigraphic and geographic range.— The late Paleocene Gashato Formation at Shabarakh Usu, Mongolia (the type horizon and locality). The Nomogen Formation at the Erden (Urtyn) Obo locality (also named as Aliwusu on the topographic map) (this study) and possibly the Haliut locality ( Chow and Qi 1978) in the Nomogen district , Inner Mongolia; the late Paleocene Gashatan land mammal age .

Figures

Fig. 2. Multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 from the late Paleocene Gashato Formation at Shabarakh Usu, Mongolia. A. Holotype (AMNH 21736), the upper left M2 in occlusal, lingual and buccal views (A1–A3). B. Paratype (AMNH 21713), anterior part of a left m2 in occlusal view. C. Paratype (AMNH 21715), anterior parts of a left m1 in occlusal view. D. AMNH 21719.001, broken?M2 in occlusal ( D1) and lateral (D2) views. E. AMNH 21719.002, anterior parts of a left m1 in occlusal view.

Fig. 3. Multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 (IVPP V19025) from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. Partial right rostrum with I2–3 in lateral (A) and ventral (B) views.A2, B2, explanatory drawings; A3, B3 details of A1, B1.

Fig. 4. Multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 (IVPP V19025) from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. Partial left maxilla with M1 and M2 in occlusal (A), medial (B), and lateral (C) views. The arrow indicates the broken anterior root of the zygomatic arch; wear facets: 1, on buccal sides of the lingual cusps of M2; 2, on lingual sides of the medial cusps of M2; 3, on buccal sides of the medial cusps of M2; 4, on the cusp of the buccal (external) row, which is aligned with the buccal wear facets of medial cusps of M2. Photographs (A1–C1), SEM images (A2–C2).

Fig. 5. Multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 (IVPP V19032) from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. Left upper molars (M1–2) from the same individual as the lower ones in Fig. 7 in occlusal (A), medial (B), and lateral (C) views. The arrow indicates the broken anterior root of the zygomatic arch. Photographs (A1–C1), SEM images (A2–C2).

Fig. 6. Lower incisors of multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. A. IVPP V19025, symphysis of mandibles with partial incisors in lateral right (A1) and left (A2), dorsal (A3), and ventral (A4) views. B. IVPP V19034, right incisor in medial (B1) and lateral (B2) views.

Fig. 7. Multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 (IVPP V19032) from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. Left lower cheek teeth (p4–m2) from the same individual as in Fig. 5 in occlusal (A), medial (B), and lateral (C) views. Photographs (A1–C1), SEM images (A2–C2).

Fig. 8. Occlusal views of molars of multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. A. IVPP V19037, right m2. B. IVPP V19030, left m2. C. IVPP V19028, right m2. D. IVPP V19029, right m2 (D1), right M1–2 (D2), left M1–2 (D3). E. IVPP V19025, right M1, which belongs to the same individual as in Fig. 4. F. IVPP V19027, a developing right m2? G. IVPP V19033, a deeply worn left m2. H. IVPP V19026, right M2. I. IVPP V19036, right m2.

Fig. 9. Skull fragments of multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. A. IVPP V19025, skull fragment showing the orbital crest in lateral (A1) and dorsal (A2, A3) views. B. IVPP V19029, skull fragment showing the temporal crest in lateral (B1) and dorsal (B2, B3) views. The anterior end of each element is toward upside. Photographs (A1, A2, B1, B2), explanatory drawings (A3, B3).

Fig. 10. Fragmentary pterygoid and middle ear cavity of multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. A. IVPP V19025, middle ear with part of the promontorium in ventral view; photograph (A1), explanatory drawing (A2). B. IVPP V19029, possible bony shell of the inflated vestibule in lateral (B1) and inside (B2) views. The anterior end of each element is toward upside.

Fig. 11. Fragmentary scapula and humerus of multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. A. IVPP V19030, proximal portion of the right scapula in lateral (A1), medial (A2), and ventral (glenoid fossa) (A3) views. B. IVPP V19031, left humerus in anterior (B1), posterior (B2), lateral (B3), and medial (B4) views. Due to the breakage, the proximal (top) and distal (bottom) portions may not be displayed in their precise anatomical positions.

Fig. 12. The femur of multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 (IVPP V19030) from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. A. Proximal portion of the left femur in anterior (A1), posterior (A2), and proximal end (A3) views. B. Right femur (with proximal end broken) in anterior (B1), posterior (B2), and distal end (B3) views.

Fig. 13. The tibia of multituberculate mammal Sphenopsalis nobilis Matthew, Granger, and Simpson, 1928 (IVPP V19030) from the upper Paleocene Nomogen beds at the Erden (Urtyn) Obo locality, Inner Mongolia, China. A. Right tibia in anterior (A1) and posterior (A2) views, proximal end in lateral (A3) and proximal (A4) views. B. Femur-tibia articulation in posterior (B1) and anterior (B2) views.

Fig. 1. Late Paleocene Gashatan localities for multituberculates in the Mongolian Plateau. A. Shabarakh Usu, Mongolia, where Sphenopsalis nobilis and Prionessus lucifer were first discov- ered. B. Erlian Basin, Inner Mongolia, China. Lambdopsalis bulla, P. lucifer, and Mesodmops tenuis were reported from Subeng (Missiaen and Smith 2008). L. bulla and P. lucifer were present in Nuhetingboerhe (Wang et al. 2010) and Bayan Ulan (Meng et al. 1998). L. bulla, S. nobilis, and P. lucifer were present in Haliut (Chow and Qi 1978) and Erden (Urtyn) Obo area (this study).

Fig. 14. The upper and lower left molars of multituberculate mammal Lambdopsalis bulla Matthew, Granger, and Simpson, 1928 (IVPP V20101) from the upper Paleocene Nomogen beds at the Bayan Ulan locality, Inner Mongolia, China. Crown view of m1–2 (A) and M1–2 (B). Wear facets: 1, on lingual sides of the buccal cusps of m2; 2, on buccal sides of the lingual cusps of m2; 3, on lingual sides of the lingual cusps of m2; 4, on buccal sides of the lingual cusps of M2; 5, on lingual sides of medial cusps of M2; 6, on the buccal sides of medial cusps of M2; 7, on buccal sides of the buccal cusps of m1; 8, on lingual sides of the buccal cusps of m1; 9, on buccal sides of the lingual cusps of m1; 10, on lingual sides of the lingual cusps of m1; no wear facet was developed yet on the cusp of the buccal (external) cusp row of M2 in this specimen.

Fig. 15. Teeth of multituberculate mammal from the lower Paleocene Puercan in the San Juan Basin, New Mexico. A. Taeniolabis sp., AMNH 117415, cast, occlusal view of right p4–m2. B, C. Taeniolabis taoensis Cope, 1882. B. AMNH 16310, occlusal view of the left p4–m2. C. AMNH 16321, occlusal view of the right P4–M2.