Nuurtherium baruunensis, Velazco & Buczek & Novacek, 2017

Velazco, PaúL M., Buczek, Alexandra J. & Novacek, Michael J., 2017, Two new tritylodontids (Synapsida, Cynodontia, Mammaliamorpha) from the Upper Jurassic, southwestern Mongolia, American Museum Novitates 2017 (3874), pp. 1-36 : 15-28

publication ID	https://doi.org/ 10.1206/3874.1
persistent identifier	https://treatment.plazi.org/id/065087F6-FFB5-0C73-689E-FDCE2BA42CE7
treatment provided by	Carolina
scientific name	Nuurtherium baruunensis
status	sp. nov.

Treatment

Nuurtherium baruunensis , sp. nov.

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 M0 and L0 cusps, and a much more robust M1 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 teeth; J, jugal; MG, meckelian groove; mc, metacarpal; Mx, maxilla; lD, left dentary; pc, right lower postcanine; Pl, palatine; ptMPS, posterior terminus of median palatine suture; Q, left quadrate; rD, right dentary. Scale bar = 10 mm.

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; M0 <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. M1 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 con- stricted 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.

COMPARISONS Of SHARTEGODON AND NUURTHERIUM WITH OTHER TRITYLODONTS

We compared Shartegodon altai and Nuurtherium baruunensis with all the known genera within the family ( Simpson, 1928; Young, 1947; Kühne, 1956; Chow and Hu, 1959; Chow, 1962; Waldman and Savage, 1972; Cui, 1981; Kermack, 1982; He and Cai, 1984; Clark and Hopson, 1985; Sues, 1985, 1986a, 1986b, 1986c; Cui and Sun, 1987; Sun and Cui, 1989; Setoguchi et al., 1999; Maisch et al., 2004; Sues and Jenkins, 2006; Watabe et al., 2007; Lopatin and Agadjanian, 2008; Hu et al., 2009; Jasinoski and Chinsamy, 2012; Fedak et al., 2015).

The snout is short and lacks a postincisive constriction in Shartegodon , Bienotherium , Bienotheroides , Bocatherium , Dianzhongia , Dinnebitodon , Kayentatherium , Lufengia , Polistodon , Stereognathus , and Yunnanodon , whereas it is longer and has a postincisive constriction in Oligokyphus and Tritylodon . In Shartegodon and Nuurtherium the palatal process of the maxilla is markedly reduced; it is preserved as a narrow band forming the lingual margins of the postcanine teeth, and the palatine is in contact with the premaxilla. This is similar to the condition in Bienotheroides , Bocatherium , Dinnebitodon , Stereognathus , and Yuanotherium . In contrast, the maxilla is large and occupies most of the area of the palate and the palatine contacts the maxilla in Bienotherium , Kayentatherium , Oligokyphus , and Tritylodon . The zygomatic process of the maxilla constitutes the ventral aspect of the anterior root of the zygomatic arch in Shartegodon , Bienotherium , Kayentatherium , Oligokyphus , Polistodon , and Tritylodon , whereas it constitutes the dorsal aspect of the anterior root of the zygomatic arch in Yuanotherium . The premaxilla-maxillary contact follows the mesiolingual outline of PC 1 in Shartegodon , Kayentatherium , Bienotheroides , Bocatherium , and Yuanotherium , whereas premaxilla-maxillary contact occurs in the snout in Bienotherium , Oligokyphus , and Tritylodon . Anteriorly and laterally, the palatine is bordered by the maxilla and premaxilla in Shartegodon , Bienotheroides , Bocatherium , whereas it is bordered only by the maxilla in Bienotherium , Oligokyphus , and Tritylodon . In Shartegodon the palatine laterally contacts the alveolus of PC4 and the palatal process of the maxilla is merely a thin splint of bone located between the lingual margins of PC1–PC3 and the palatine. The palatine does not laterally contact any PC alveoli in Bienotherium , Bienotheroides , Bocatherium , Kayentatherium , Oligokyphus , and Tritylodon . There are no interdigitations on the maxillo-palatine or premaxillopalatine sutures in Shartegodon . There are no interdigitations on the maxillo-palatine suture, but there are interdigitations on the premaxillo-palatine suture in Bienotheroides , Bocatherium , and Yuanotherium . Interdigitations are observed only on the proximal aspect of the maxillo-palatine suture in Bienotherium , Kayentatherium , Oligokyphus , and Tritylodon . The anterior portion of the palatine has two greater palatine foramina in Shartegodon , Bocatherium , and Yuanotherium , but these foramina are lacking in Oligokyphus . Only one foramen is present in Bienotherium , Kayentatherium , and Tritylodon , and between one and three are present in species of Bienotheroides . The anterior margin of the orbit is above the distal edge of PC 1 in Shartegodon , Bienotherium , Bocatherium , Kayentatherium , and Oligokyphus , whereas it is above the anteroposterior midpoint of PC 2 in Bienotheroides . The lacrimal is large and forms the medial orbital wall in Shartegodon , Bienotherium , Bienotheroides , Bocatherium , Kayentatherium , and Oligokyphus , whereas it is much smaller in Polistodon . The lacrimal contacts the jugal ventrally and the premaxilla anteriorly in Shartegodon , Bienotheroides , and Bocatherium . In Bienotherium , Kayentatherium , Oligokyphus , Polistodon , and Tritylodon , it contacts the jugal ventrally and the maxilla anteriorly. The lacrimal lacks lacrimal foramina in Polistodon , whereas there is one lacrimal foramen in Bienotherium , Bienotheroides , Bocatherium , and Kayentatherium , and two lacrimal foramina in Oligokyphus . The jugal contributes to the medial and inferior orbital wall in Shartegodon and Polistodon , whereas it does not contribute to either wall in Kayentatherium and Oligokyphus . There is one foramen and possibly two small, round depressions for muscle attachments on the left jugal above PC 2 in Shartegodon , whereas these foramina are absent in Kayentatherium and Oligokyphus .

The coronoid process is very tall with gently curved anterior margin in Nuurtherium , Bocatherium , Kayentatherium , and Tritylodon . The coronoid process is shorter, with gently curved anterior margin in Polistodon , and is shorter, with a straight anterior margin in Oligokyphus . The alveolar line and the anterior margin of the coronoid process form an angle of less than 90° in Nuurtherium and Kayentatherium , whereas the angle is greater than 90° in Oligokyphus and Polistodon , and the angle is roughly equal to 90° in Bocatherium and Tritylodon .

Three upper incisors are present in Shartegodon , Bienotherium , Bienotheroides , Bocatherium , Dinnebitodon , and Oligokyphus , whereas only one is present in Kayentatherium and Polistodon , and two in Dianzhongia . Upper postcanine alveolar tooth rows diverge posteriorly in Shartegodon , Nuurtherium , Bienotherium , Bienotheroides , Dinnebitodon , Kayentatherium , and Oligokyphus ; whereas they are parallel in Bocatherium , Dianzhongia , and Tritylodon .

In upper postcanine dimensions, Shartegodon clusters with the smaller tritylodont species and especially with Bienotherium minor , Oligokyphus minor , and Yunnanodon brevirostre (table 2; fig. 15); however, Shartegodon differs from these latter taxa in crown morphology, including upper postcanine cusp formula. In the same dimensions, Nuurtherium clusters with mediumsized species, with Bienotherium elegans and Montirictus kuwajimaensis (larger type) the closest (table 2; fig. 15). Nuurtherium is, however, easily distinguished from those two species by the upper postcanine cusp formula (fig. 12).

Each upper postcanine has four roots in Shartegodon , whereas five are present in Nuurtherium , Bienotheroides , Lufengia , Stereognathus , Tritylodon , and Yunnanodon , six are present in Bienotherium , Montirictus , and Oligokyphus , and seven in Dianzhongia . The upper postcanine teeth lack an anterior median root in Shartegodon , Nuurtherium , Bienotheroides , Lufengia , Montirictus , Stereognathus , and Yunnanodon , whereas an anterior median root is present in Bienotherium , Oligokyphus , and Tritylodon . The upper postcanine generalized cusp formula varies among the different tritylodontid genera: Shartegodon (2-4-4); Nuurtherium (2-3-4),

2 4 6 8 10 12 LENGTH (��)

Bienotherium (2-3-3), Bienotheroides (2-3-3 [3-3- 3 in B. wansienensis ]), Bocatherium (2-2-2), Dianzhongia (2-3-2), Dinnebitodon (2-3-2), Kayentatherium (2-3-3), Lufengia (2-3-3), Montirictus (2-2-2), Oligokyphus (3-4-4), Polistodon (2-2-2), Stereognathus (2-2-2), Tritylodon (2-3-3), Xenocretosuchus (2-2-2), Yuanotherium (2-4-3), and Yunnanodon (2-3-2).

The B0 cusp on the upper PC teeth is absent in Shartegodon , Nuurtherium , Bienotherium , Bocatherium , Dianzhongia , Dinnebitodon , Kayentatherium , Lufengia , Montirictus , Polistodon , Stereognathus , Tritylodon , Xenocretosuchus , and Yunnanodon , whereas it is present in Bienotheroides (absent only in B. wansiensis ), Oligokyphus , and Yuanotherium . M0 and L0 cusps on the upper PC teeth are present in Shartegodon , Nuurtherium , Oligokyphus , and Yuanotherium , whereas they are absent in Bienotherium , Bienotheroides , Bocatherium , Dianzhongia , Dinnebitodon , Kayentatherium , Lufengia , Montirictus , Polistodon , Stereognathus , Tritylodon , Xenocretosuchus , and Yunnanodon . M1 cusps on the upper PC teeth are large in Nuurtherium , Bienotherium , Dianzhongia , Dinnebitodon , Kayentatherium , Lufengia , Oligokyphus , Tritylodon , Yuanotherium , and Yunnanodon , whereas they are small in Shartegodon and Bienotheroides , and absent in Bocatherium , Montirictus , Polistodon , Stereognathus , and Xenocretosuchus . L1 cusps on the upper PC teeth are small in Shartegodon , Nuurtherium , Bienotherium , Bienotheroides , Kayentatherium , and Yuanotherium , whereas they are large in Dianzhongia , Dinnebitodon , Lufengia , Oligokyphus , Tritylodon , and Yunnanodon , and absent in Bocatherium , Montirictus , Polistodon , Stereognathus , and Xenocretosuchus . L3 cusps on the upper PC teeth are large in Shartegodon , Nuurtherium , Bienotherium , Bienotheroides , Bocatherium , Kayentatherium , Lufengia , Montirictus , Oligokyphus , Polistodon , Stereognathus , Tritylodon , and Xenocretosuchus , whereas they are small in Dianzhongia and Yuanotherium , and absent in Dinnebitodon and Yunnanodon .

Each lower postcanine has one root in Shartegodon , Nuurtherium , and Bienotheroides , whereas two are present in Bienotherium , Lufengia , Montirictus , Oligokyphus , and Yunnanodon . The roots are long with the proximal section (30%) straight and the distal section (70%) curved in Shartegodon , Nuurtherium , and Bienotheroides zigongensis ( Cui and Sun, 1987: fig. 4C), whereas the roots are long and curved throughout their entire length in Bienotherium , Montirictus , and Oligokyphus , and short and slightly curved in Lufengia and Yunnanodon . Most genera have a lower postcanine generalized cusp formula of 2-2 ( Shartegodon , Nuurtherium , Bienotheroides , Kayentatherium , Montirictus , Polistodon , and Xenocretosuchus ); the only genus that presents a different generalized cusp formula is Oligokyphus (3-3).

In addition to Nuurtherium , there are postcranial elements known from three genera: Bienotheroides , Kayentatherium , and Oligokyphus ( Kühne, 1956; Maisch et al., 2004; Sues and Jenkins, 2006). The preserved postcranial elements of Nuurtherium do not depart from the known postcranial morphology exhibited by the other tritylodontid genera.