Ravjaa ishiii, Okoshi & Takasaki & Chiba & Natori & Saneyoshi & Takahashi & Kodaira & Hayashi & Ishigaki & Mainbayar & Tsogtbaatar, 2025

Ravjaa ishiii sp. nov.

Figs. 2–4 View Fig View Fig 3 View Fig 4. 3 .

Zoobank LSID: urn:lsid:zoobank.org:act:.

Etymology: In honor of Ken-ichi Ishii, the director of Hayashibara Museum of Natural Sciences, Okayama, Japan, for his leading role in the Japanese team of the Mongolia-Japan joint expedition, which tremendously contributes to Mongolian paleontology.

Holotype: MPC-M 100s/001, partial right dentary with a distal portion of the ultimate premolar, damaged m1, and well-preserved m2–3.

Type locality: Bayan Shiree, located approximately 25 km southwest of Dzüünbayan, Dornogovi, Mongolia.

Type horizon: Cenomanian–Santonian, Upper Cretaceous, Bayanshiree Formation.

Diagnosis.—A zhelestid eutherian that differs from all other zhelestids in: the faint coronoid crest of the dentary with the shallow masseteric fossa, the concave dorsal alveolar margin of the dentary, the complete absence of the labial mandibular foramen, and the significantly high m2 crown that reaches approximately 70% of the height of the horizontal ramus directly below it on labial view.

Ravjaa ishiii sp. nov. differs from Borisodon kara Archibald & Averianov, 2012 , in the rounded mesiolingual vertical crest of the paraconid, the coronoid process tilting 110–115°, and the lack of Meckelian groove; differs from Gallolestes spp. ( Gallolestes pachymandibularis Lillegraven, 1976 , and Gallol estes agujaensis Cifelli, 1994) in the less bulbous, more anteroposteriorly compressed protoconid; differs from Azilestes ragei in the lack of hypolophid; differs from Parazhelestes spp. ( Parazhelestes robustus Nessov, 1993 , and Parazhelestes mynbulakensis Nessov, 1985b ) in the absence of a marked tuberosity or concavity in the medial side of the area of the coronoid facet between the m3 and base of the coronoid process; differs from Borisodon kara and Eoungulatum kudukensis Nessov et al., 1998 , by the absence of coronoid facet; differs from Eozhelestes in the complete labial attachment of the cristid oblique to the notch of the protocristid; differs from Zhalmouzia bazhanovi Averianov et al., 2014 , in the lack of cingulid of the lingual and labial side of molars; differs from Borisodon kara and Eozhelestes mangit Nessov, 1997 , by the trigonid height is less than twice the talonid height; differs from Avitotherium utahensis Cifelli, 1990 , and Lainodon orueetxebarriai Gheerbrant & Astibia, 1994 , by the slight labial shift of the paraconid; differs from Sheikhdzheilia rezvyii Averianov & Archibald, 2005 , and Lainodon orueetxebarriai by presence of the labial postcingulid; differs from Mistralestes arcensis , the entoconid subequal to the hypoconulid; differs from Mistralestes arcensis in the precingulid with a distinct posteroventral cingular shelf; differs from Eozhelestes mangit , Sheikhdzheilia rezvyii , Lainodon orueetxebarriai , Mistralestes arcensis in the transversed protocristid; differs from Zhalmouzia bazhanovi , Mistralestes arcensis , and Valentinella vitrollense in the position of the posterior mental foramen below the ultimate premolar; differs from Eozhelestes mangit , Parazhelestes spp. , Aspanlestes aptap Nessov, 1985a , Zhelestes termirkazyk Nessov, 1985a , Eoungulatum kudukensis , and Zhalmouzia bazhanovi by the ultimate molar hypoconulid being tall and recurved; differs from Eozhelestes mangit , Borisodon kara , Parazhelestes spp. , Aspanlestes aptap , Zhelestes termirkazyk , Eoungulatum kudukensis , and Zhalmouzia bazhanovi by the absence of the labial mandibular foramen; differs from Eozhelestes mangit , Shikhdzheilia rezvyii , Borisodon kara , Lainodon orueetxebarriai , Aspanlestes aptap , Zhelestes termirkazyk , Eoungulatum kudukensis , and Zhalmouzia bazhanovi by the compressed trigonid angle being less than 35°.

Description.—The new specimen, MPC-M 100s/001, is a partial right mandible retaining roughly the posterior half of the horizontal ramus, the base of the coronoid process, the anteroventral part of the ascending ramus, the talonid of the ultimate premolar, and m1–3 ( Fig. 2 View Fig ). MPC-M 100s/001 represents an adult individual based on the presence of the m3 and the absence of replacement tooth, observed in micro CT images ( Fig. 3 View Fig 3 ). Despite the adult status, this specimen represents one of the smallest zhelestids reported to date, together with Sheikhdzheilia rezvyii ( Averianov and Archibald 2005) and Aspanlestes aptap ( Archibald and Averianov 2012) . The dorsoventral depth of the horizontal ramus is consistent throughout its preserved region. The alveolar margin of the ramus is slightly concave in the labial and lingual view, unlike the other zhelestids in which the margin is straight throughout its length ( Fig. 2A, C View Fig ). The dorsal margin of the ramus is higher lingually than labially ( Fig. 2A 2 View Fig ). The ventral margin of the horizontal ramus is convex in labial and lingual views ( Fig. 2A, C View Fig ). Ventral to the ultimate premolar, the labial surface of the mandible bears a shallow sulcus that runs anteroposteriorly ( Fig. 2A View Fig ). The sulcus likely connects to a mental foramen anteriorly, as in Borisodon kara , Parazhelestes spp. , and Eoungulatum kudukensis ( Nessov et al. 1998; Archibald and Averianov 2012). The coronoid crest emerges as a low eminence, ventrally to the m2 talonid, at the level of the middle of the horizontal ramus ( Fig. 2A 2 View Fig ). Musculus buccinator is likely to insert anteromedial to the eminence. The crest extends posterodorsally and becomes prominent but is much less developed than the other zhelestids (e.g., Borisodon kara , Aspanlestes aptap , Eoungulatum kudukensis ; Archibald and Averianov 2012), which may be suggestive of a reduced M. temporalis on this individual. The posterior surface of the coronoid crest is rugose and lacks the labial mandibular foramen ( Fig. 2A 2 View Fig ). The coronoid crest defines the dorsal border of the masseteric fossa. The masseteric fossa is shallow, and its dorsal region likely provides the insertion for M. zygomaticomandibularis, whereas its ventral region is likely to be the insertion for M. masseter (Turnbull 1970). Ventral to the masseteric fossa, the ventral margin of the mandible, anterior to the angular process, is gently concave ( Fig. 2A View Fig ). Lingually, the ascending process lacks the coronoid facet, which can be observed in Eoungulatum kudukensis and Parazhelestes spp. ( Archibald and Averianov 2012). The Meckelian sulcus is absent, as in all members of the Zhelestinae ( Archibald and Averianov 2012). The CT images demonstrate the presence of the mandibular canal that runs anteroposteriorly throughout the mandible and the absence of replacement teeth ( Fig. 3 View Fig 3 ). Since no replacement teeth are present, all teeth observed in this specimen are considered to be permanent teeth.

MPC-M 100s/001 retains a partial ultimate premolar and all three molars ( Fig. 3 View Fig 3 ). While the anterior half of the ultimate premolar is severely damaged, the talonid and the posterior root are well preserved. Despite the damage, the anterior portion of the premolar width is clearly wider than the talonid. The talonid has a narrow and shallow basin anteriorly and has a single cusp posteriorly. All three molars are nearly complete, including two roots per molar, except for the apex of the m1 metaconid. The lower molars m1–3 are tribosphenic molars, and their shapes generally resemble each other. The overall sizes of the molars are subequal to each other, although the m3 is slightly smaller than the other two. The height of the m2 crown is 71% of the height of the horizontal ramus directly below it on labial view, which is markedly larger than in the other zhelestids (50–63%, with 56% on average).

The molars have moderately compressed trigonid with trigonid angles of 33° in m2 and m3 ( Fig. 4C View Fig 4. 3 1 and D 1). The maximum height of the trigonid is higher than that of the associated talonid but does not reach twice the talonid height ( Fig. 4B View Fig 4. 3 3 –D 3 View Fig 3 ). Their cusps are rounded as in Parazhelestes spp. and Eoungulatum kudukensis but unlike Borisodon kara . Among the trigonid cusps, paraconid is the smallest and is placed mesiolabially ( Fig. 4B View Fig 4. 3 1 –D 1). This is also the only trigonid cusp demonstrating a clear sign of attrition, especially pronounced in the m1. The other two trigonid cusps, the protoconid and the metaconid, are subequal in height ( Fig. 4B View Fig 4. 3 2 –D 2 View Fig and B 4 –D 4 View Fig 4. 3 ). The protoconid is labially expanded at its base, making it the most robust cusp in the trigonid ( Fig. 4B View Fig 4. 3 2 –D 2 View Fig and B 5 –D 5). The anteroposterior length of the protoconid is the longest in the m1 and becomes successively thinner in the posterior molars ( Fig. 4B View Fig 4. 3 1 –D 1). Compared to the protoconid, the metaconid is slightly less developed and lacks lingual expansion at its base, though it is still more robust than the paraconid ( Fig. 4B View Fig 4. 3 3 –D 3 View Fig 3 and B 4 –D 4 View Fig 4. 3 ). The paraconid and the metaconid are separated by a prominent gap ( Fig. 4B View Fig 4. 3 1 –D 1), which tends to be more prominent in the posterior molars. The trigonid basin is V-shaped in anterior and posterior views ( Fig. 4B 4 –D 4 View Fig 4. 3 ). The protocristid is oriented transversely in the m2 and the m3, while it is slightly oblique in the m1 ( Fig. 4B View Fig 4. 3 1 –D 1). The talonid width is subequal to the trigonid, and the talonid length accounts for more than half of the total length of the molar. Among the talonid cusps, the hypoconid is the most robust and the shortest cusp ( Fig. 4B View Fig 4. 3 1 –D 1 and B 5 –D 5). The attrition surface of the hypoconid is sloped lingually toward the talonid basin ( Fig. 4B 4 –D 4 View Fig 4. 3 ). The talonid basin of the m3 bears a fait spherical depression ( Fig. 4D View Fig 4. 3 1). The hypoconulid and the entoconid are in close approximation to each other ( Fig. 4B View Fig 4. 3 1 –D 1), as in all other zhelestids ( Archibald and Averianov 2012; Averianov et al. 2014). These two cusps are subequal in size, although the hypoconulids of the m1 and m2 are slightly worn mesiolabially ( Fig. 4B View Fig 4. 3 1 –D 1). The hypoconulid of the m3 is more massive than that of the m1 and the m2 and markedly procurved ( Fig. 4D View Fig 4. 3 3 View Fig 3 and D 5). The cristid obliqua emerges from the labial margin of the hypoconid and attaches slightly labially to the notch of the protocristid ( Fig. 4B View Fig 4. 3 1 –D 1). The postcristid is distinctly present between the hypoconid and the hypoconulid, while it is absent between the hypoconulid and the entoconid. The precingulid is present anterior to the trigonid in all molars and is more prominent in the posterior molars. The labial and lingual margins of the molars are smooth and lack a defined cingulid as in the other zhelestids except Zhalmouzia bazhanovi , in which the molar exhibits faint labial and lingual cingulids ( Averianov et al. 2014). A prominent labial postcingulid is present posterolabial to

Nanolestes drescherae 100 Paranyctoides sternbergi Vincelestes neuquenianus Peramus tenuirostris 91 Paranyctoides quadrans Kielantherium gobiense 87 Eoungulatum kudukensis Deltatheridium pretrituberculare Pucadelphys andinus 24 Zhalmouzia bazhanovi Acristatherium Eomaia Mayulestes Prokennalestes scansoria ferox yanensis spp. 87 87 3 Eozhelestes mangit Zhelestes temirkazyk Zhelestinae 5 Aspanlestes aptap Murtoilestes abramovi 62 Bobolestes zenge 0 Parazhelestes spp. Bulaklestes kezbe 62 75 Ravjaa ishiii Gallolestes spp. Uchkudukodon nessovi 1 Daulestes spp. 87 20 Borisodon kara Kennalestes gobiensis 11 Valentinella vitrollense Ukhaatherium nessovi Asioryctes nemegtensis 87 Avitotherium utahensis Cimolestes spp. 3 Lainodon orueetxebarriai Batodon tenuis Maelestes gobiensis Sheikhdzheilia rezvyii Mistralestes arcensis Asia Kulbeckia Zhangolestes Barunlestes kulbecke butleri jiliensis Albian Cenomanian Turonian Coniacian Santonian Campanian trichtian Maas- North Europe America Zalambdalestes lechei Alymlestes kielanae Early Late Placentalia Cretaceous (incl. Azilestes ragei ) 110 100 90 80 70 (Ma)

the talonid and bears a cusp-like tubercle protruding from the hypoconulid base; this structure might be comparable to the hypocingulid, the term coined in Van Valen (1993).

Measurements (in mm).—m1: length 2.05; trigonid length 0.96, width 1.53; talonid length 1.09, width 1.55. m2: length 2.17; trigonid length 0.84, width 1.58; talonid length 1.33, width 1.59; distance between paraconid and metaconid 0.190. m3: length 2.09; trigonid length 0.76, width 1.43; talonid length 1.33, width 1.37; distance between paraconid and metaconid 0.19.

Stratigraphic and geographic range.— Type horizon and locality only.