The Upper Paleocene Zhigden Member of the Naran Bulak Formation of the Tsagan-Khushu locality in southern Mongolia contains a rich assemblage of mammalian fossils [1, 2]. The finds of some groups are rare. These groups include hapalodectids, small mesonychians with well-pronounced carnivorous dental adaptations. Hapalodectes dux Lopatin, 2001 described from Tsagan-Khushu is based only on the holotype lower jaw with both tooth rows [3, 4]. H. dux was described as the earliest member of the genus (the faunal assemblage from the Zhigden Member is dated to the Late Paleocene, Gashatan) [1, 2]; at present, two more Paleocene species of Hapalodectes are known from China: H. lopatini Solé et al., 2017 (Middle Paleocene, Nongshanian, Bothriostylops Interval Zone) and H. paleocenus Beard et al., 2010 (Late Paleocene, Gashatan) [5, 6]. H. hetangensis Ting et Li, 1987 (Orientolophus Interval Zone) and H. huanghaiensis Tong et Wang, 2006 (Homogalax Interval Zone) inhabited China in the Early Eocene (Bumbanian), and H. serus Matthew et Granger, 1925 found there in the Middle Eocene (Irdinmanhan) [7–11]. H. anthracinus Zhou et Gingerich, 1991 (Wa-1 Zone) and H. leptognathus (Osborn et Wortman, 1892) (= H. compressus Matthew, 1909; zones Wa-4–Wa-7) are described from the Lower Eocene (Wasatchian) of the United States of America [12–16].

The description of a new find of Hapalodectes from the Zhigden Member of the Naran Bulak Formation of the Tsagan-Khushu locality (collections of the South Gobi party of the Joint Soviet-Mongolian paleontological expedition led by V.Yu. Reshetov, 1987) is presented below. The specimen is assigned to a new species, which is characterized by small size and a combination of primitive and advanced morphological features of the lower molars.

The studied material is stored in the Borissiak Paleontological Institute of the Russian Academy of Sciences (PIN) in Moscow. The illustrations were prepared using the Nikon D800 digital photo camera with an AF-S Micro NIKKOR 60mm f/2.8G ED lens, and the Neoscan N80 micro-CT-scanner at PIN. The scan parameters for the specimen PIN, no. 3104/775 were set at 84 kV, 48 μA, 5.5 μm pixel size, 180° rotation with 0.2° rotation step, and 0.1 mm Cu filter. The scan parameters for the specimen PIN, no. 3104/371 were 101 kV, 159 μA, 20 μm pixel size, 180° rotation with 0.2° rotation step, and 0.5 mm Cu filter. Acquired X-ray images (2800 × 2400 pixels) were processed with Neoscan software; 3D models were visualized in CTvox (Bruker microCT).

Order Mesonychia Matthew, 1937

Family Hapalodectidae Szalay et Gould, 1966

Genus Hapalodectes Matthew, 1909

Hapalodectes paradux Lopatin, sp. nov.

Etymology. From Ancient Greek para, beside, and the species name Hapalodectes dux.

Holotype. PIN, no. 3104/775, right dentary fragment with M2–M3; Mongolia, South Gobi, Nemegt Basin, Tsagan-Khushu; Upper Paleocene, Naran Bulak Formation, Zhigden Member.

Description (Figs. 1, 2, 3b, 4b). The sizes are small for the genus. The lower molars are sectorial, strongly compressed transversely. The M2 and M3 have approximately the same size and similar structure (Fig. 1). The paraconid is half as high as the protoconid. At the base of the paraconid, there are two distinct additional cusps (anterolabial and anterolingual), delimiting the anterior reentrant groove and, together with it, providing strong tooth interlocking to the talonid of the more anterior tooth. The protoconid–metaconid region is separated from the paraconid and talonid by deep notches; the wide and very deep posterior notch is especially strongly developed. The protoconid is large and high, strongly compressed transversely, characterized by peculiar lanceolate outlines of the labial side. The protoconid trenchant blades (the preprotocristid and postprotocristid) are strong and form almost right angle (about 95° on M2, 100° on M3) in lateral view. The metaconid is mostly fused with the protoconid, reduced but distinct. The posterolingual side of the protoconid–metaconid regon lacks a clear emargination, emphasizing the metaconid distally. The proto-cristid directed anterolingually. The talonid is relatively narrow, significantly less in width than the trigonid. The strong  shearing crest of the talonid is divided in the middle by the protruding hypoconid apex. The reduced hypoconulid is located markedly below the hypoconid at the posterior end of the shearing crest of the talonid. The rudimentary entoconid appears as a tiny basal cuspule in the posterolingual part of the crown; on M2, it is located closer to the base of the crown and slightly larger than on M3. In lateral view, the posterior margin of the talonid of the M2 is oblique posterodorsally while that of the M3 is vertical.

Fig. 1.
figure 1

Hapalodectes paradux Lopatin, sp. nov., holotype PIN, no. 3104/775, right dentary fragment with M2–M3: (a–c) photographs; (d–f) CT model; (a, d) labial view; (b, e) occlusal view; (c, f) lingual view; Mongolia, Tsagan-Khushu; Upper Paleocene, Naran Bulak Formation, Zhigden Member.

Full size image
Fig. 2.
figure 2

Hapalodectes paradux Lopatin, sp. nov., holotype PIN, no. 3104/775, right dentary fragment with M2–M3, CT model: (a) sagittal section, labial view; (b) frontal section ventral to the paraconid apices, occlusal view; (c) sagittal section, lingual view; Mongolia, Tsagan-Khushu; Upper Paleocene, Naran Bulak Formation, Zhigden Member.

Full size image
Fig. 3.
figure 3

Comparison of two species of Hapalodectes from the Tsagan-Khushu locality in Mongolia: (a) H. dux Lopatin, 2001, holotype PIN, no. 3104/371, lower jaw, right M1–M3 region, dorsolabial view; (b) H. paradux Lopatin, sp. nov., holotype PIN, no. 3104/775, right dentary fragment with M2–M3, dorsolabial view; Upper Paleocene, Naran Bulak Formation, Zhigden Member.

Full size image
Fig. 4.
figure 4

Comparison of the lower molar structure of two species of Hapalodectes from the Tsagan-Khushu locality in Mongolia, CT models, occlusal view: (a) H. dux Lopatin, 2001, holotype PIN, no. 3104/371, lower jaw, right M1–M3 region; (b) H. paradux Lopatin, sp. nov., holotype PIN, no. 3104/775, right dentary fragment with M2–M3; Upper Paleocene, Naran Bulak Formation, Zhigden Member.

Full size image

On M2, the reduced metaconid has a distinct separate apex, which is strongly displaced anteriorly in relation to the protoconid apex and therefore clearly visible even from the labial side (Figs. 1a, 1d, 3b). The apices of the protoconid and metaconid are connected by the distinct elongated protocristid, which is strongly oblique anteriorly. In the horizontal plane, the angle formed by the protocristid and preprotocristid (the protocristid angle) is about 40° (Figs. 1e, 4b). The paraconid is equal in massiveness to the metaconid. The talonid widens distally and has a straight posterior margin.

On M3, the rudimentary metaconid is apically ridge-shaped, without a distinct separate apex, but the lingual metaconid swelling and protocristid are well developed. The protocristid angle is about 30°. The M3 noticeably exceeds the M2 in width of paraconid and region of additional cusps. The talonid tapers substantially backward and becomes rounded distally; it is longitudinally elongated more than on M2. The posterior root of the M3 is strongly expanded longitudinally at the base.

The internal structure of the teeth is characterized by a significant thickening of the dentin in the apical part of the protoconid–metaconid region, at the bottom of the posterior notch, and in the hypoconid area (Fig. 2a). On M2, the strong reduction of the pulp cavity (the pulp recession) in the trigonid region is fixed due to a visually distinct inner dentin layer (Figs. 2b, 2c); it may be associated with abnormally enhanced formation of the secondary dentin.

Measurements of the holotype, in mm. M2: length, 3.85; trigonid width, 1.45; talonid width, 1.35; protoconid height, 2.7; M3: length, 3.95; trigonid width, 1.5; talonid width, 1.3; protoconid height, 2.8.

Comparison. Hapalodectes paradux sp. nov. is markedly larger than H. hetangensis, but significantly smaller than all other described species of the genus (Table 1).

Table 1. Comparisons of the length of M2 and M3 (average values in parentheses, in mm) in Hapalodectes (the data on previously described species are given after [3, 5, 6, 9, 11, 14–16])
Full size table

The new species differs from the contemporaneous H. dux in smaller size (Fig. 3) and a number of features of the M2–M3 (Fig. 4), including a reduced metaconid strongly shifted forward (with a ridge-like apex on M3), a long protocristid, and a smaller protocristid angle (40° on M2, 30° on M3; in H. dux this angle is approximately 75° on M1, 65° on M2, and 60° on M3), as well as a deeper posterior notch and narrow talonid (in H. dux, the talonid is as wide as the trigonid).

H. paradux sp. nov. differs from H. lopatini, H. paleocenus, H. anthracinus, H. leptognathus, and H. hetangensis in approximately equal lengths of M2 and M3 (in the first four species, M3 is greatly increased, while in the latter species, it is slightly reduced).

The presence of the metaconid on molars distinguishes the new species from H. anthracinus and H. serus, the longer protocristid on M2–M3 and the ridge-like shape of the metaconid apex on M3 differs it from H. paleocenus and H. leptognathus, the presence of the entoconid distinguishes it from H. huanghaiensis and H. serus, and the relatively slight reduction of the latter cusp differs it from H. hetangensis, H. anthracinus, and H. leptognathus.

H. paradux sp. nov. differs from species that retain a relatively well-developed metaconid on molars (see Table 2) by its greater reduction, strong anterior displacement and, accordingly, by the sharper protocristid angle, as well as the ridge-shaped apex of this cusp on M3.

Table 2. Comparisons of the structure of M2 and M3 in Hapalodectes (data on previously described species are given after [3, 5, 6, 9, 11, 14–16]; protocristid angle measured from images)
Full size table

Remarks. Findings of hapalodectids are quite rare, and their diversity in each locality is usually limited to one species. Currently, Tsagan-Khushu is the only locality where two species of Hapalodectes co-occur (larger H. dux and smaller H. paradux sp. nov.).

The holotypes of H. dux and H. paradux sp. nov. differ greatly in the size and dental structure and therefore cannot be sex- and age-related or other intraspecific variations. Sexual dimorphism in Hapalodectes is presumably reflected in the depth of the horizontal ramus of the mandible [6].

The calculation according to the formula proposed by Zhou (Y = 1.327 × X – 3.355, where X = ln(L × B) (L, length of M2; B, width of M2, mm) and Y = ln W (weight, kg)) [17] made it possible to estimate the body mass of H. paradux sp. nov. in 360 g. The body mass calculated using this formula is 700–1100 g (mean 900 g) for H. leptognathus, 870 g for H. serus, 740 g for H. lopatini, 670 g for H. huanghaiensis, 530 g for H. anthracinus, 500 g for H. dux, 460 g for H. paleocenus, and 190–200 g for H. hetangensis [3, 6, 17]. According to the size of the postcranial skeleton bones, the body mass of H. leptognathus is estimated at 1000–1500 g [18].

Material. Holotype.

The recent phylogenetic analysis of the mesonychians (Mesonychia, or Acreodi), archaic carnivorous ungulates from the Paleogene of the Northern Hemisphere, suggests the following divergence order of the clades of Hapalodectes ([6], fig. 3, majority-rule consensus tree): (H. lopatini + H. dux) ((H. hetangensis + H. huanghaiensis) (H. paleocenus (H. leptognathus (H. anthracinus + H. serus)))). This evolutionary scenario implies the basal position of the Middle Paleocene H. lopatini and the Late Paleocene H. dux, based on a large number of primitive features of these early Asian species. Their lower molars are characterized by a well-defined metaconid, relatively large additional cusps in front of the paraconid, a wide talonid, and distinct talonid cusps (the hypoconid, hypoconulid, and entoconid). The ultimate lower molar M3 is approximately equal in length to the M2 in H. dux and slightly longer than that in H. lopatini.

Evolutionary more advanced species are divided into two main clades [3, 5, 6]. The first of them includes the Early Eocene H. hetangensis and H. huanghaiensis from China, the second comprises the Late Paleocene H. paleocenus from China, the Early Eocene H. anthracinus and H. leptognathus from North America, and the Middle Eocene H. serus from China.

The close relationship between H. hetangensis and H. huanghaiensis is supported by the similar structure of the upper molars, as well as the presence of a well-defined metaconid apex and relatively large additional anterior cusps on the lower molars [6]. The entoconid is rudimentary in H. hetangensis and absent in H. huanghaiensis; the hypoconid and hypoconulid are distinct. The M3 is equal in length to the M2 in H. huanghaiensis and inferior to the latter in H. hetangensis.

The second clade is characterized by relatively large size, small crestiform additional anterior cusps, and progressive reduction of the metaconid and entoconid up to their complete disappearance in H. serus (see Table 2). In members of this lineage, the M3 is significantly longer than the M2 (this is not known for H. serus), and talonids are relatively narrow, as in H. paradux sp. nov. The distinct reduction of the metaconid (especially on M3), recorded in H. paradux sp. nov., suggests that the new species is close to this Hapalodectes lineage. This reduction was associated with a decrease in the width of the middle part of the lower molars (to enhance their shearing action) and occurred through the fusion of the metaconid base with the protoconid. The narrowing of the protoconid–metaconid region in H. paradux sp. nov. included the anterior displacement of the metaconid apex, decreasing the protocristid angle (also occurred in H. leptognathus, see [13, 14]). However, the new species has retained some features that are primitive for the genus, namely, relatively large additional anterior cusps, presence of the entoconid and approximately the same length of the ultimate and penultimate lower molars. Based on the combination of the dental features, H. paradux sp. nov. can be considered as a sister species to H. paleocenus.

Thus, four species of Hapalodectes are now known from the Paleocene of Asia (H. lopatini, H. dux, H. paleocenus, and H. paradux sp. nov.) that confirms the conclusion about a fairly bushy Paleocene radiation of this group [6]. The scenario of the evolution of the genus in the Eocene includes the following assumptions. The H. hetangensis and H. huanghaiensis lineage, endemic to southeastern China, may be related in origin to this region, from which the Middle Paleocene H. lopatini is also known [6]. At the beginning of the Eocene, members of another lineage dispersed to North America, producing local species radiation on the new continent (H. anthracinus and H. leptognathus). The relatively small-sized Late Paleocene H. paleocenus and H. paradux sp. nov., inhabited the area of the Mongolian Plateau, could be at the base of this lineage. The common ancestor of the latter two species, in its turn, apparently appeared as a result of the earlier Paleocene radiation, which also gave rise to H. dux. At an earlier stage of evolution, there was the split of the ancestor of all the discussed species and H. lopatini.

The special role of the Mongolian Plateau area in the Late Paleocene radiation of Hapalodectes [3, 6] should be noted in connection with the existence of three species of this genus here in the Gashatan, namely, H. paleocenus, H. dux, and H. paradux sp. nov., and the co-occurrence of the latter two in the Tsagan-Khushu locality. Apparently, the significant dissimilarities in the structure of molars and body size in coexisting species H. dux and H. paradux sp. nov. contributed to the minimization of trophic competition between them. This corresponds to the concept of morphological divergence in dentition of carnivorous mammals as a factor in sympatric speciation [19] and emphasizes the role of the Mongolian Plateau area as the probable center of the Late Paleocene diversification of hapalodectids.

It is assumed that at the beginning of the Middle Eocene, a member of the North American lineage of Hapalodectes migrated back to Asia, where it gave rise to H. serus [3, 5, 6]. However, it cannot be ruled out that H. serus arose as a result of the parallel development of an Asian lineage of Hapalodectes, which in the Middle Eocene independently reached the same level of specialization as the North American H. anthracinus did in the Early Eocene [3]. In this part, the evolutionary scenario can be significantly corrected and detailed when the remains of evolutionary more advanced members of the H. hetangensis and H. huanghaiensis lineage or other descendants of the Paleocene radiation of Hapalodectes are found in the Lower and Middle Eocene of Asia.