See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/236617870 The Earliest Hapalodectes (Mesonychia, Mammalia) from the Paleocene of Mongolia Article in Paleontological Journal · July 2001 CITATIONS READS 10 212 1 author: Alexey V. Lopatin Russian Academy of Sciences 369 PUBLICATIONS 1,647 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: New genera of baleen whales (Cetacea, Mammalia) from the Miocene of the northern Caucasus and Ciscaucasia View project Early evolution of mammals View project All content following this page was uploaded by Alexey V. Lopatin on 05 August 2014. The user has requested enhancement of the downloaded file. Paleontological Journal, Vol. 35, No. 4, 2001, pp. 426–432. Translated from Paleontologicheskii Zhurnal, No. 4, 2001, pp. 90–96. Original Russian Text Copyright © 2001 by Lopatin. English Translation Copyright © 2001 by åÄIä “Nauka /Interperiodica” (Russia). The Earliest Hapalodectes (Mesonychia, Mammalia) from the Paleocene of Mongolia A. V. Lopatin Paleontological Institute, Russian Academy of Sciences, ul. Profsoyuznaya 123, Moscow, 117868 Russia Received October 20, 1999 Abstract—A lower jaw containing complete tooth rows of the earliest hapalodectid mesonychian, Hapalo- dectes dux sp. nov., is described from the Upper Paleocene of Tsagan-Khushu, Mongolia (Naran Bulak Forma- tion, Zhigden Member). The new species is smaller than the Middle Eocene H. serus and larger than the Early Eocene H. hetangensis. The lower molars of H. dux have distinct metaconid, protocristid, rudimentary hypo- conulid, and entoconid; M2 and M3 are equal in size. These characters suggest that H. dux is the most primitive species of the genus Hapalodectes. INTRODUCTION arate subfamily of Mesonychidae. Ting and Li (1987) Hapalodectes Matthew, 1909 is the type genus of examined the skull structure of H. hetangensis and con- the Hapalodectidae, an Early Paleogene family of small cluded that Hapalodectes should be regarded as a mem- specialized mesonychians (Mesonychia, or Acreodi) ber of a separate family that is strongly distinct from characterized by well-pronounced predatory adapta- Mesonychidae. tions in dental structure. The following four species The type species H. leptognathus from the Middle– have been assigned to the genus: H. leptognathus Late Wasatchian (zones Wa-4–Wa-7) is characterized (Osborn et Wortman, 1892) and H. anthracinus Zhou et by its relatively large size and the presence of rudimen- Gingerich, 1991 from the Lower Eocene of North tary metaconids on the lower molars. The teeth of America; H. hetangensis Ting et Li, 1987 from the H. serus lack the metaconid. The most primitive struc- Lower Eocene of China; and H. serus Matthew et ture of the lower molars occurs in the smallest species, Granger, 1925 from the Middle Eocene of China (Mat- the Asiatic H. hetangensis (Early Bumbanian, Orien- thew and Granger, 1925; Szalay and Gould, 1966; Sza- talophus Zone), which is characterized by a distinct lay, 1969; Ting and Li, 1987; Zhou and Gingerich, metaconid possessing a detached apex (Ting and Li, 1991; O’Leary and Rose, 1995). In addition, Hapalo- 1987, p. 166, fig. 4). The molars of H. anthracinus of dectes sp. was discovered in the Lower Eocene of Mon- comparable geological age (Early Wasatchian, Wa-1 golia (Dashzeveg, 1982) and H.? serus was found in the Zone) had already lost the metaconid (Zhou and Gin- Middle Eocene of China (Qi, 1987). ?Hapalodectes gerich, 1991; O’Leary and Rose, 1995). Consequently, auctus Matthew et Granger, 1925 from the Middle the first adaptive radiation of the genus probably Eocene of China was referred to the genus Mongolo- occurred as early as the Paleocene. ryctes Van Valen, 1966 (Didymoconidae) and H. lush- A remarkable confirmation of this assumption is the iensis Chow, 1965 from the Late Eocene of China was finding of Late Paleocene Hapalodectes from Mongo- referred to the genus Lohoodon Chow, Li, et Chang, lia described below. The specimen came from the 1978 (Mesonychidae). Late Paleocene ?Hapalodectes Zhigden Member of the Naran Bulak Formation of the sp. from China (Zhang et al., 1979) is currently treated Tsagan-Khushu locality (collected by V.Yu. Reshetov as Mesonychidae (Ting and Li, 1987). in 1987). The Zhigden Member is dated at the end of The second hapalodectid genus, Hapalorestes Gun- the Late Paleocene, and the fauna is assigned to the nell et Gingerich, 1996, was discovered at the bottom of Gashatan Asian Land Mammal Age (Badamgarav and the Middle Eocene of North America. The only mem- Reshetov, 1985; Ting, 1998). ber of this genus, H. lovei, is more than twice as large as the largest species of Hapalodectes, H. leptognathus and H. serus (Gunnell and Gingerich, 1996). The Early SYSTEMATIC PALEONTOLOGY Paleogene Asiatic genera Plagiocristodon Chow et Qi, 1979, Metahapalodectes Dashzeveg, 1976, and Lohoodon Order Mesonychia Matthew, 1909 Chow et al., 1978 were originally placed in Hapalodec- Family Hapalodectidae Szalay et Gould, 1966 tinae, but are currently assigned to Mesonychidae (Ting Genus Hapalodectes Matthew, 1909 and Li, 1987; Ting, 1998; Meng et al., 1998). Based on its unique dental structure, Szalay and Hapalodectes dux Lopatin, sp. nov. Gould (1966) proposed ranking Hapalodectes as a sep- Etymology. From Latin dux (leader). 426 THE EARLIEST HAPALODECTES (MESONYCHIA, MAMMALIA) 427 (a) mm 5 (b) 0 10 mm 0 (c) Fig. 1. Hapalodectes dux sp. nov., holotype, PIN, no. 3104/371, lower jaw: (a) right lateral view; (b) occlusal view; and (c) right P4–M3, occlusal view. Holotype. PIN, no. 3104/371, lower jaw with zontal ramus, there is a wide longitudinal groove (prob- complete tooth rows; South Mongolia, Tsagan-Khushu; ably for the attachment of the mylohyoideus muscle), Upper Paleocene, Naran Bulak Formation, Zhigden extending from the P3 level to the level of the posterior Member. edge of M2. The symphysis is long and weak and reaches the level of the posterior edge of P2. There are Description (Figs. 1 and 2). The lower jaw is three mental foramina; the anterior foramen is located deformed, the rami are isolated at the symphysis and under the interval between P and P , the middle fora- slightly displaced from each other in the vertical and 1 2 men is under the interval between P2 and P3, and the horizontal planes. The ascending ramus is broken off. posterior foramen is under the middle of P or under the The tooth rows are well preserved; on the left side, the 3 interval between P3 and P4. An additional small fora- I3 crown and the trigonid of M2 are broken off. men is on the anterior side of each jaw ramus under I2. The species is a medium-sized member of the The dental formula of the lower jaw is I3C1P4M3. genus, and the reconstructed skull is approximately The incisors are located near the anterior base of the 65 mm long. The mandibular body is narrow. The angle canines and form an arch projecting weakly anteriorly. between the jaw rami is approximately 25°. The I1 < I2 > I3. The crowns are small, chisel-shaped, and ascending ramus only gently slopes at the base. The lacking heels; the anterior surface is convex; the poste- masseteric fossa is superficial. The horizontal ramus is rior surface is flattened. low and narrow (the maximum depth is under M3), and relatively strongly curved lingually at the interval The canine is relatively large, narrow, and long. between P2 and P3. On the lingual surface of the hori- It curves strongly and is transversely slightly com- PALEONTOLOGICAL JOURNAL Vol. 35 No. 4 2001 428 LOPATIN (a) (b) (d) (e) 0 10 mm (f) (g) (c) (h) (j) (i) mm 5 (k) 0 (l) Fig. 2. Hapalodectes dux sp. nov., holotype, PIN, no. 3104/371, lower jaw: (a) right and (b) left lateral views, (c) occlusal view; (d) incisors and canines, front view; (e) right P1, labial view; (f) right P2, labial view; (g) left P2, lingual view; (h, i) left P3: (h) lingual and (i) labial views; (j, k) right P4–M3: (j) labial and (k) occlusal views; and (l) left P4–M3, lingual view. pressed. The posterior crest is weak. The canine is the anterior slope is very abrupt. The heel is short and weakly inclined anteriorly at the base, as are the inci- bears a small cuspule at the posterior edge. P3 is similar sors. Behind the canine, the upper part of its alveolus to P2 in shape and size but differs by a better developed extends posteriorly and looks like a small, narrow posterior cusp, the position of the protoconid apex above superficial fossa. The canine and premolars (except for the middle of the tooth, and, consequently, by a more P4, adjoining M1) are separated from each other by gentle anterior slope. It is only slightly lower than P2. intervals, the longest of which is between P2 and P3. P4 is semimolariform, relatively large and high, and P1 is small and single-rooted; the crown is strongly substantially higher than the molars. It bears an compressed transversely and weakly inclined anteri- extremely small anterior basal cusp, a rudimentary orly. P2 is substantially larger than P1 and double- paraconid; a stout protoconid that is strongly inclined rooted. The protoconid is inclined somewhat anteri- posteriorly; and a narrow trenchant talonid possessing orly; the apex is on a level with the anterior root, and two cusps, i.e., the hypoconid located in the middle of PALEONTOLOGICAL JOURNAL Vol. 35 No. 4 2001 THE EARLIEST HAPALODECTES (MESONYCHIA, MAMMALIA) 429 Table 1. Cheek teeth measurements P1 P2 P3 P4 M1 M2 M3 Tooth sin dex sin dex sin sin dex sin dex dex sin dex Length 1.7 1.7 2.55 2.5 2.6 4.2 4.2 4.1 4.0 4.5 4.4 4.5 Width 0.9 0.95 1.05 1.0 1.05 1.5 1.5 1.4 1.5 1.65 1.65 1.65 Height 2.0 2.1 2.8 3.0 2.5 4.2 4.4 3.2 3.3 3.5 3.2 3.4 the shearing crest and a very small hypoconulid located and under M3, 6.0.