Type Species: Shartegodon altai, sp. nov.

Etymology: Sharteg-, Mongolian, after the type locality.

Age: As for the type and only species.

Diagnosis: As for the type and only species.

Holotype: PSS-MAE 633, a badly damaged skull with cheek teeth and partial right and left dentarles with three postcanine teeth each (figs. 3–10).

Type Locality: Shar Teg locality, Gobi-Altai Aimag, southwestern Mongolia (fig. 1).

Stratigraphic Horizon: From a maroon bed of the lower part of the Ulan Malgait Sequence (just above the boundary with the Shar Teg Sequence); Upper Jurassic (Gereltsetseg, 1992; Ponomarenko et al., 2014).

Etymology: The species name altai, derived from Mongolian, refers to the aimag (“province”), Gobi-Altai, in which the Shar Teg beds are located.

Diagnosis: Differing from other tritylodontids in having a palatine that contacts the alveoli of the last upper postcanine teeth (PC4); a lack of interdigitations on the maxillo-palatine and premaxillo-palatine sutures; three foramina (two of which are probably just small round pockets) present on the jugal above PC2; upper postcanine teeth with four roots; and upper postcanine teeth with a cusp formula of 2-4-4. Differs from the species, Bienotheroides shartegensis, previously recorded at the Shar Teg locality in having a maxilla that extends laterally to contribute to the zygoma (zygomatic process of the maxilla), and in having M0 and L0 cusps on the upper postcanine (PC) teeth (Watabe et al., 2007).

Description: The skull of Shartegodon altai (PSS-MAE 633) is extensively crushed (figs. 3A–B, 4A–B), and the anterior region of the skull is badly damaged, but the nasals, maxilla, premaxilla, palatine, jugal, and lacrimal are partly preserved (figs. 3A–B, 4A–B). The nasals are shattered, making it difficult to observe their contact with other bones (figs. 3B, 4B). The palate is smooth. In the palate the maxilla (preserved only on the left side) is very small and is confined to form the alveoli of PC1-PC3, with a narrow band forming the lingual margins of PC1-PC3 (fig. 3A). The maxilla extends laterally to form the zygomatic process. This process constitutes the ventral surface of the anterior root of the zygomatic arch (fig. 3A). There is no contribution of the maxilla to the orbital wall. Most of the anterior and dorsal aspects of the premaxilla are missing. Ventrally, the premaxilla is well developed and it contacts both the palatine and maxilla (fig. 3A). The premaxilla-palatine contact is a transverse suture that aligns with the posterior portion of PC1. The premaxilla-maxillary contact follows the mesiolingual shape of PC1 (fig. 3A). In lateral view the anterior premaxilla is distinctly downturned (fig. 4A–B). On left side of the skull, damage to the dorsal premaxilla reveals the presence of two incisors (figs. 3B, 4B). Root remnants of two small mesial incisors (left I1 and I2), and a large partial alveolus for the 13 root are preserved (figs. 3B, 4B). The palatine is the bestpreserved bone in the skull. Posteriorly, the palatine presents extensive horizontal processes. The suture between palatine and pterygoid cannot be observed. On the right horizontal process the lesser palatine foramen is located posteriomedial to the last postcanine tooth (fig. 3A). The posterior margin of the horizontal process of the palatine is damaged and, therefore, the shape of its posterior margin or internal narial opening cannot be determined. Along the median palatine suture, there is a posteriorly pronounced ridge. Additionally, a caudal nasal spine is present. The posterior termination of the median palatine suture ends at the level of the posterior margin of PC3 (fig. 3A). The palatal process of the palatine is bordered anteriorly and laterally by the maxilla and anteriorly by the premaxilla. The palatine contacts the premaxilla along a transverse suture at level to the posterior margin of PC1 (fig. 3A). Laterally the palatal process of the palatine directly contacts the alveolus of PC4 and the palatal process of the maxilla (fig. 3A). The latter is a very thin process situated between PC1–PC3 and the palatine. The anterior portion of the palatine has two small foramina (greater palatine foramina) (fig. 3A). The suture between the palatal processes of the maxillary and palatine is broadly curvilinear and lacks interdigitations. Posteriorly, the lateral walls of the nasopharyngeal canal converge, nearly contacting each other.

The anterior margin of the orbit is located directly dorsal to the distal margin of PC1 (fig. 4A). The lacrimal is exposed in lateral view, and it ventrally contacts the jugal and anteriorly contacts the premaxilla (fig. 4A). These sutures are not interdigitated. The suture between the lacrimal and the nasal cannot be observed due to the shattered condition of the nasals (figs. 3B, 4B). The lacrimal forms the medial orbital wall (fig. 3B). The jugal is partly preserved on the left side of the skull and its zygomatic process gently curves outward and forms the dorsal surface of the anterior root of the zygomatic arch. The jugal also forms part of the medial orbital wall and the inferior orbital wall. A large infraorbital foramen lies just below the suture between the jugal and the lacrimal (figs. 3B, 4A). There is a foramen, and two other small round depressions, possibly for muscle attachments, on the left jugal above PC2 (fig. 4A).

The basicranium is badly damaged with only a partial left region attached to the skull and an isolated portion of the right region preserved (figs. 3A–B, 4A, 5). The only identifiable structures are the left fenestra ovalis and left jugular foramen (fig. 3A). The stapedial ratio, calculated by proxy as length (1.56 mm) divided by width (0.87 mm) of the fenestra ovalis, is approximately 1.8.

Partial right masseteric fossae and left coronoid processes are preserved on both the left and right jaws (figs. 3B, 4A). The right partial dentary is preserved as a posterior fragment containing three postcanines (fig. 6A–B). The left partial dentary is also preserved only as a posterior fragment containing three mature postcanines (anterior most detached) and a fourth erupting postcanine (fig. 7A–B). The depths of the right and left dentaries below the penultimate postcanine are 6.7 mm and 7.1 mm respectively with the difference in depth likely due to distortion. The proximal portions of the coronoid crests are present on both fragments (figs. 6A, 7A) and both fragments show a well-defined Meckelian groove with a small portion of the splenial present in the groove of the right dentary (fig. 6B).

In the left premaxilla, there is evidence of the presences of three incisors. I1 and I2 are represented by remnants of their roots (figs. 3B, 4B). Lateral to the I1 and I2 roots, the left I3 partial alveolus suggests that I3 was at least five times the width of I1 or I2 (fig. 3B). No lower incisors are preserved (figs. 6A–B, 7A–B).

The upper postcanine alveolar tooth rows diverge posteriorly. Four upper postcanine teeth, almost identical in size, are preserved on both sides of the skull (table 1; figs. 3A, 8). On both sides, fifth postcanine teeth are erupting (fig. 3A). There are no diastemata between them. Each upper postcanine has four long and cylindrical roots, two anterior and two posterior, arranged in two transverse rows. The four are independent and the lingual roots are larger than the buccal ones. In occlusal view the teeth are rectangular with rounded corners and a convex mesial edge (fig. 8). There are three cusp rows that run mesiodistally across each tooth with two deep valleys running between them (figs. 3A, 8). Most of the upper postcanine teeth have two buccal, four medial, and four lingual cusps; the only exception is the right PC3, which has six lingual cusps instead of the four present in all the other teeth (fig. 8). Therefore, the generalized cusp formula is 2-4-4 (fig. 8). The cusp heights in each row are as follows (with 0 or 1 representing the most mesial cusp and 2 or 3 representing the most distal cusp): B1 < B2; M0 < M1 < M2 ≊ M3; and L0 < L1 < L3 < L2. In the case of the right PC3 the lingual cusps heights are: L0 < L1 ≊ L2 < L4 ≊ L5 < L3. There are six broken cusp apices on the left (PC1 [B2, M2]; PC2 [L2]; and PC3 [B1, B2, M3]) side and four broken apices on the right (PC1 [B1, B2]; PC2 [B2]; and PC3 [M3]) side (fig. 8). The largest cusps on all upper postcanines, B2, M2, M3, and L2, are subequal in height. The smallest cusps on all upper postcanines are M0 and L0. Five cusps exhibit different degrees of crescentic shape: M2, M3, and L2 are strongly crescentic and present sharp lingual and buccal cristae, whereas B1 and B2 are somewhat crescentic and have a sharp lingual and blunt buccal cristae (fig. 8).

The three posterior lower postcanines are preserved on both dentaries (figs. 6A–B, 7A–B, 9, 10), the left dentary also includes a fourth erupting postcanine. Each lower postcanine has only one root (fig. 10). The root is long and square throughout its length. The upper section of the root descends vertically from the base of the crown, while the lower two-thirds of the root forms a C-shaped curve (fig. 10). The lower postcanine tooth cusps occlude in the deep valleys of the upper postcanine (figs. 3A, 8, 9). The lower postcanine teeth are rectangular in occlusal view, being longer in length than in width. There are two mesiodistal cusp rows with one valley running between them. All the lower postcanine teeth have two buccal and two lingual cusps (fig. 9). The cusp formula for the lower postcanines is 2–2 (b1, b2, l1, and l2). All cusps are subequal in height. There are several broken cusp apices on both the left (pc2 [b1]; and pc4 [b2]) and right (pc2 [b1, b2, and l1]; pc3 [b1, b2, I1, and l2]; and pc4 [b1]) sides (fig. 9). The cusps that are unbroken show a crescentic pattern. All lower postcanine cusps (lingual and buccal) present sharp lingual and buccal cristids (fig. 9). The cusp rows are slightly offset; the lingual cusps (l1 and l2) are located more mesially than the buccal cusps (b1 and b2) (fig. 9). The eruption sequence of the lower postcanines was not synchronous between both sides of the jaw: the left dentary shows the two proximal cusps (b1 and l1) and the tip b2 of pc5 in eruption, whereas the right pc5 has not erupted.

Type Species: Nuurtherium baruunensis, sp. nov.

Etymology: Nuur-, Mongolian, “lake,” fossils at Shar Teg were preserved in a cyclic sequence of fluvial and lake beds (see Ponomarenko et al., 2014).

Age: As for the type and only species.

Diagnosis: As for the type and only species.

Holotype: PSS-MAE 632, a damaged skull with cheek teeth, partial edentulous left dentary, partial right dentary with three postcanine teeth, left quadrate, and partial skeleton including two dorsal, and two caudal vertebrae; three ribs; right clavicle; right scapula; left ulna; left femur; right tibia; left astragalus; two metacarpals; and one metatarsal (figs. 11–14).

Type Locality: Shar Teg locality, Gobi-Altai Aimag, southwestern Mongolia (fig. 1).

Stratigraphic Horizon: From the lower part of the Ulan Malgait Sequence (just above the boundary with the Shar Teg Sequence); Upper Jurassic (Gereltsetseg, 1992; Ponomarenko et al., 2014).

Etymology: The species name, baruun-, from the Mongolian, “west,” refers to location of the Shar Teg beds in far western Mongolia.

Diagnosis: Differing from other tritylodontids in having all the upper postcanine teeth with a cusp formula of 2-3-4 (fig. 12). Nuurtherium baruunensis differs from the species, Bienotheroides shartegensis, previously recorded at the Shar Teg locality in having a different cusp formula (B. shartegensis = 2-3-3) and in having MO and L0 cusps, and a much more robust Ml cusp on the upper postcanine teeth.

Description: The exposed portion of this crushed specimen shows only part of the palate with cheek teeth, left zygomatic arch, partial dentaries, and some postcranial elements preserved (fig. 11). The skull of Nuurtherium (PSS-MAE 632) is badly damaged (fig. 11), and the rostrum and basicranium are not preserved. The only partially preserved cranial elements are the palatine, maxilla, and jugal. The palatine is crushed extensively. The posterior terminus of the median palatine suture ends at the level of the fourth from the last postcanine teeth (fig. 11). The only portion of the maxilla preserved is a narrow band forming the lingual margins of the upper right postcanines (fig. 11). The upper postcanine alveolar line diverges posteriorly. Only a portion of the left jugal is preserved (fig. 11). It is long and curved outward; at the level of the fourth from the last postcanine tooth it bends to become parallel to the right jugal. Posteriorly, the jugal increases in size to 15.7 mm at the level of the last postcanine and forms a thin sheet of bone.

The right dentary is better preserved than the left dentary (fig. 11). Three postcanines are preserved in the right dentary while none remains in the left. The anterior portion of the right dentary is not preserved (fig. 11). The depth of the right dentary below the last preserved postcanine is 16 mm. The coronoid process is very tall (>45 mm), with a gently curved anterior margin (fig. 11). The tip of the coronoid process has been lost. The posterior margin of the coronoid process is concave. The tips of the articular and angular processes are broken off. The coronoid is missing and the coronoid boss is partly cracked (fig. 11). The alveolar line and the anterior margin of the coronoid process form an angle <90°. At the anterior base of the coronoid there is a crypt for the replacement teeth (fig. 11). Both fragments show a well-defined Meckelian groove, without signs of the presence of the splenial (fig. 11). The left quadrate is teeth; ePC, erupting upper postcanine preserved (fig. 11). The dorsal angle of the quadrate is absent. The lateral trochlear condyle is preserved, but the medial trochlear condyle and the stapedial process are embedded in the matrix (fig. 11).

No upper or lower incisors are preserved (fig. 11). The upper postcanine alveolar tooth rows diverge posteriorly. The more posterior six right and four left upper postcanines are preserved (table 1; fig. 11). On both sides, the last postcanine teeth are in the process of erupting (fig. 11). There are no diastemata between them (fig. 12). The upper postcanine teeth have five long and cylindrical roots, two anterior and three posterior, arranged in two transverse rows. The anterior pair are united at their bases, whereas the posterior roots are independent throughout their length. In occlusal view, the upper postcanines are rectangular and buccolingually wider, with rounded corners. There are three cusp rows that run mesiodistally along each tooth, with two deep valleys running between them (fig. 12). The upper postcanine teeth have two buccal, three medial, and four lingual cusps (fig. 12). The generalized cusp formula is therefore 2-3-4. Most of the cusps have broken tips, but the generalized cusp heights in each row are as follows: B1 < B2; MO < M1 < M2; and L0 < L1 < L3 < L2. The largest cusps on all upper postcanine teeth are B2 and M2. The smallest cusp on every upper postcanine is L0. Five cusps (B1, B2, M1, M2, and L2) exhibit a small degree of crescentic shape. Ml and M2 present sharp lingual and buccal cristae; B1 and B2 present sharp lingual and blunt buccal cristae; and L2 presents a sharp buccal and a blunt lingual cristae.

Five lower right postcanines are preserved, three are still emplaced in the dentary while two are detached and isolated (figs. 11, 13, 14). Each lower postcanine has only one root (fig. 13B). The root is long and square throughout its length. The upper third section of the root descends vertically from the base of the crown, while the lower two-thirds of the root form a C-shaped curve (fig. 13B). The lower postcanine tooth cusp rows occlude in the deep valleys of the upper postcanines. The lower postcanine teeth are rectangular in occlusal view, i.e., greater in length than in width (table 1). There are two mesiodistal cusp rows with one valley running in between them. All the lower postcanine teeth have two buccal and two lingual cusps (fig. 13A). The cusp formula for the lower postcanines is 2–2 (b1, b2, l1, and l2). Most of the cusp apices are broken off, but it can be inferred from the unbroken cusps that all the cusps on a tooth are subequal in height (fig. 13A). The cusps that are unbroken show a crescentic pattern (fig. 13A). All lower postcanine cusps (lingual and buccal) present sharp lingual and buccal cristids. l1 is located more mesially than b1, whereas l2 and b2 are mesiodistally aligned (fig. 13A).

Several postcranial elements are preserved: four dorsal, and two caudal vertebrae; five ribs; right clavicle; right scapula; left ulna; left femur; right tibia; left astragalus; two metacarpals; and one metatarsal (fig. 14).

Only the cranial and lateral aspects of the isolated dorsal vertebrae are visible (fig. 14). The neural spine is tall, narrow, and distinctively inclined posterodorsally. The centrum is wider than it is tall (12.3 × 7.9 mm). The neural canal is triangular. The prezygapophyses project beyond the anterior margin of the centrum. The articular processes of the prezygapophyses are oval and are inclined at about 45° from the vertical plane. The postzygapophyses are located posterodorsal to the prezygapophyses. The prezygapophyses are centric (roughly perpendicular to the plane of the lamina). The parapophyses are not preserved. Each robust transverse process is almost half the size of the centrum and does not project beyond the anterior margin of the centrum. The transverse processes are directed ventrolaterally. The diapophyses show a roughly triangular outline and are ventrolaterally oriented and flat.

The two isolated proximal caudal vertebrae are present (fig. 14). Even though the neural spines are broken off, it can be inferred from the remaining recesses that the spines are distinctively inclined posterodorsally. The centra are hourglass shaped in lateral view. The distal centrum is wider than tall (10.2 × 8.1 mm). The neural canal is triangular but narrower than it is in the dorsal vertebrae. The pre- and postzygapophyses are broken off. The transverse processes are broken off, but it is clear that they originate near the neurocentral junction and are positioned near the anterior part of the body. These vertebrae lack chevrons.

The rib material is poorly preserved; only five ribs, four anterior and one posterior, with the distal ends broken off are preserved (fig. 14). The anterior ribs are long and curved, (defining the surface of the thorax), and flattened anteroposteriorly. The capitular and tubercular facets are proximal to each other, with the tuberculum situated above the capitulum. The tubercular facet is circular and flat, while the capitular is round. The posterior rib is short and straight, and has only one articular facet.

Only the right clavicle is preserved (fig. 14). The medial end is spatulated and shows striations for contact with the interclavicle, which is not preserved. The lateral end presents two recesses and articular contacts for the acromion.

Only the medial surface of the right scapula is preserved, it but presents extensive damage, where both ends are missing (fig. 14). The scapular blade is slightly widened dorsally. The acromion and scapular glenoid facet are badly damaged.

The badly damaged left ulna has a long olecranon process (fig. 14). Damage obscures the facets for the ulnar condyle of the humerus, the ulnar condyle, and the distal articular. The ulnar shaft is shattered throughout its length.

Only the anterior view of the left femur is visible: it is broken in half and both ends are abraded (fig. 14). Both ends are greatly expanded (∼20 mm) and the femur is extremely constricted toward the middle of the shaft (6.4 mm). The bulbous femoral head is oriented dorsomedially relative to the long axis of the shaft. The edges of the greater trochanter are abraded, but it is apparent that the femoral head and the greater trochanter are of the same height. The lateral flaring edge of the greater trochanter is oriented dorsally. The intertrochanteric fossa is slightly concave. Both the lateral and medial condyles are abraded.

The anterior view of the right tibia is visible; it is complete, but its proximal end is abraded (fig. 14). The maximum length of the tibia is 42.9 mm. Proximally, the tibia is lateromedially expanded. Its proximal and distal thirds are triangular in transverse section, with medial, lateral and anterior sides. The distal articular surface is oval.

The plantar view of the left astragalus is preserved (fig. 14). The sustentacular facet is long and narrow, whereas the ectal facet is oval. The facets are separated by a slightly curved deep groove, the sulcus tali.

Only three isolated metapodials (two metacarpals and one metatarsal) are preserved (fig. 14). All the metapodials recovered are broadly expanded proximally and distally.