Introduction

Neogene freshwater sediments yielding abundant mammalian fossils are widely distributed in central Myanmar. Most of the mammalian fossils recovered have been from the upper Miocene to lower Pleistocene Irrawaddy Group (Colbert, 1938; Bender, 1983). To date, four mammalian orders (Carnivora, Proboscidea, Perissodactyla, and Artiodactyla) have been described from the Irrawaddy sediments, and the fossil record of the artiodactyls is relatively abundant, comprising representatives of seven families (Takai et al., 2006). Among the Artiodactyla, the genus Tetraconodon (Suidae) is distinguished by its extremely large P^3–4/_3–4 and it is presumed to have originated in Myanmar during the late middle Miocene (Thaung-Htike et al., 2005). In Tetraconodon, dental size variation has been used to distinguish species, and two types, large and small, are recognized in Myanmar (Chit-Sein et al., 2006).

Recently, three new large dentognathic specimens were described from central Myanmar (Chit-Sein et al., 2006). These materials were correlated with T. magnus of the Middle Siwalik Group (India/Pakistan) and described under the name T. cf. magnus. That discovery prompted us to reevaluate all Tetraconodon species, and we have reclassified the large Tetraconodon specimens from Myanmar, resulting in the description of two new species.

Abbreviations

NMM, National Museum, Yangon, Myanmar; NMMP–KU–IR, National Museum, Myanmar, Paleontology–Kyoto University–Irrawaddy (stored at the National Museum, Yangon); GSI, Geological Survey of India, Kolkata, India; AMNH, American Museum of Natural History; YUDG–Mge, University of Yangon, Department of Geology–Migyaungye (name of the township, Magway Division, central Myanmar); Kpg, Gyatpyegyi (= Kyatpyegyi) fossil locality (southwest of Male village, Sagaing Division, central Myanmar).

Materials and methods

All materials were collected in central Myanmar (Figure 1). They are now stored at the National Museum (Yangon, Myanmar) and the Department of Geology, University of Yangon (Yangon, Myanmar). Dental terminology and measurement method used are according to Thaung-Htike et al. (2005). Dental measurements of the Tetraconodon specimens discussed here and other correlated Tetraconodon specimens are listed in Tables 1, 2, and 3.

Figure 1.

Map showing the fossil localities of Tetraconodon in central Myanmar.

Table 1.

Upper dental measurements (mm) of Tetraconodon irramedius sp. nov. and T. irramagnus sp. nov. Abbreviations: L = mesiodistal length; W = buccolingual width; W1 = first lobe width; W2 = second lobe width; W3 = third lobe width of M³; * = estimate.

Table 2.

Measurements of the P³, P⁴, M¹, P₄ and M₁ of Tetraconodon which are used in Figure 4, and measurement of the M3 of T. magnus which is used in the text. The measurement of the M³ of T. magnus was modified and adopted from Falconer (1868), and other dental measurements were adopted from Colbert (1935), Pickford (1988), Thaung-Htike et al. (2005) and Chit-Sein et al. (2006). Abbreviations: L = mesiodistal length; W = greatest buccolingual width.

Table 3.

Lower dental measurements (mm) of Tetraconodon irramedius sp. nov. and T. irramagnus sp. nov. Abbreviations: L = mesiodistal length; W1 = first lobe width; W2 = second lobe width; W3 = third lobe width of M₃; * = estimate.

Systematic paleontology

Order Artiodactyla Owen, 1848

Family Suidae Gray, 1821

Subfamily Tetraconodontinae Lydekker, 1876

Genus Tetraconodon Falconer, 1868

Tetraconodon irramagnus sp. nov. Figure 2

Figure 2.

Tetraconodon irramagnus sp. nov. A–C. YUDG-Mge 089, a right maxillary fragment with P³–M² and mesial half of M³: A. Occlusal view (stereo pair); B. Lingual view; C. Buccal view. D–F. YUDG-Mge 091, a left maxillary fragment with M^2–3: D. Occlusal view (stereo pair); E. Lingual view; F. Buccal view. G–I. YUDG-Mge 090, a left mandibular fragment with P₄: G. Occlusal view; H. Lingual view; I. Buccal view.

Holotype.—YUDG–Mge 089, a right maxillary fragment with P³–M² and mesial half of M³ (Figure 2A–2C) (Chit-Sein et al., 2006, fig. 4).

Paratype.—YUDG–Mge 090, a left mandibular fragment with P₄, and YUDG–Mge 091, a left maxillary fragment with M^2–3 (Figure 2D–2I) (Chit-Sein et al., 2006, figs. 4 and 5).

Type locality.—West of Tebingan Village (19° 57′ 51.1″N; 95° 08′ 37.8″E), Migyaungye Township, Mag-way Division, central Myanmar.

Type horizon and age.—Basal part of the ‘Irra-waddy Group’, early late Miocene.

Etymology.—Irra-, the first four letters of the ‘Irra-waddy Group’, where the type specimen was discovered; -magnus, signifying the largest tooth size among Tetraconodon species of Myanmar.

Diagnosis.—A large species of Tetraconodon. The occlusal dimensions of the first molar are congruent with those of T. magnus, however, the dimensions of the last two premolars and third molar are distinctly smaller than those of T. magnus. The enamel wall and root at the distobuccal end of P⁴ are more flared and larger than at the mesiobuccal end of P⁴. The paracone is located distally to the mesial margin of the distobuccal root of P⁴. The mesiodistal length of M₁ is about 32 mm.

Description.—A detailed description for the dental morphology has been given by Chit-Sein et al. (2006), and the term T. cf. magnus was used in their description. We here redescribe only the distinct characters, and add some new morphology which was not described in Chit-Sein et al. (2006).

Three dentognathic specimens comprising maxillary and mandibular fragments with P³–M³ and P₄, are known. The cheek teeth show typical bunodont and brachyodont suid dental morphologies. The tooth enamel is very thick and highly wrinkled, which is distinct in P^3–4 and P₄. The roots of the last two premolars are much larger and more flared than those of the molars. In the maxillary fragment, the mesial border of the zygomatic process is estimated to be above the mesial surface of P⁴ (Figure 2C); other cranial features cannot be deduced from the present materials. The highly robust mandible is broken at the base, so it is impossible to measure its depth (Figure 2H–2I).

P³ and P⁴ are rugose and much larger than M¹ and M² (Figure 2A). They are characterized by the flared occlusal outline and the presence of three roots in each. Both in P³ and P⁴, the paracone and metacone are worn and connected as a single confluent large cusp. The distolingual heel in P³ is large and distinct. P³ is mesiodistally longer and buccolingually narrower than P⁴. In P⁴, the enamel wall and root at distobuccal are more flared and larger than those at mesiobuccal (Figure 2C) and the paracone is located distally to the mesial margin of the distobuccal root. The protocone of P⁴ is very distinct. The beaded anterior and posterior cingula are well developed in both P³ and P⁴.

M¹, M², and M³ show the typical morphology of teeth of tetraconodontine suids: very thick enamel; four major cusps (paracone, protocone, metacone, and hypocone) with rounded outline and shallow furchen; a well developed anterior cingulum. The minor cusps (protopreconule, hypopreconule and pentapreconule) are clearly outlined. The pentacone is tiny but distinct in all upper molars. A simple and minute talon is present on M³. The buccal cingulum is poorly developed on M¹ and M². The distolingual root of M² is slightly bifurcate. The buccolingual width of the first lobe is greater than that of the second lobe in upper molars. The occlusal dimensions of M² are nearly the same as those of M³. M¹ < M².

The large and rugose P₄ is moderately worn, and the morphology of the protoconid and metaconid cannot be investigated. The anterior precristid and prestylid are isolated. The enamel is thick and slightly wrinkled distally. The hypoconid and posterior cingulum are distinct.

Although no lower molars have been discovered, the alveolus of M₁ remains in one of the specimens, and the approximate length and width were traced (Chit-Sein et al., 2006).

Comparison.—The specimens show typical tetraconodontine suid features, and their morphological characters (extremely large last two premolars and small M³ with a minute talon) are congruent with the definition of the genus Tetraconodon. The occlusal dimensions of the M1_/1 are similar to those of T. magnus, however, the dimensions of P³, P⁴, and P₄ are distinctly smaller than those of the corresponding teeth of T. magnus (see Figure 3). Moreover, the size of M³ (length × width = 39 mm × 34.3 mm) is distinctly smaller than the size of M³ in the lectotype of T. magnus (52 mm × 37.6 mm).

Figure 3.

Bivariate plots for the dental measurements of Tetraconodon specimens from Siwalik and Myanmar. A. Occlusal area of P₄ on that of M₁. B, C. Occlusal areas of upper premolars on that of M1: B. P3 on M1; C. P4 on M1. Comparative measurements are listed in Table 2.

Figure 4.

Tetraconodon irramedius sp. nov. A–C. NMMP-KU-IR 0225, a right maxillary fragment with P³–M³: A. Occlusal view (stereo pair); B. Lingual view; C. Buccal view. D–F.NMM 839/80, a left mandibular fragment with P₄–M₃: D. Occlusal view (stereo pair); E. Lingual view; F. Buccal view.

T. irramagnus sp. nov. differs from T. irramedius sp. nov. in the following characters: larger size; more flared and larger distobuccal wall and root of P⁴; distally located paracone of P⁴. T. irramagnus sp. nov. differs from T. intermedius in its distinctly larger first molar and the smaller last two premolars, and from T. minor and T. malensis in its much larger size.

Tetraconodon irramedius sp. nov. Figure 4

Holotype.—NMMP–KU–IR 0225, a right maxillary fragment with P³–M³ (Figure 3A–3C) (Thaung-Htike et al., 2005, fig. 5).

Paratype.—NMM 839/80, a left mandibular fragment with P₄–M₃ (Figure 3D–3E) (Thaung-Htike et al., 2005, fig. 4).

Type locality.—Holotype from near Chaungsong Village, about 25 km south of Pauk City, Magway Division, paratype from near Male Village, Sagaing Division (Figure 1). However, the exact locations are unknown (Thaung-Htike et al., 2005).

Type horizon and age.—Basal part of the ‘Irra-waddy Group’, early late Miocene.

Etymology.—Irra-, first four letters of the Irrawaddy Group, from where the type specimen was recovered; -medius, medium, a species intermediate in size between T. minor and T. irramagnus sp. nov.

Diagnosis.—A large species of Tetraconodon. The occlusal dimensions of M¹ are congruent with those of T. intermedius, however, the dimensions of P^3–4 and M³ are distinctly smaller than those of T. intermedius. In P⁴, the mesiobuccal root and distobuccal root are nearly the same size. The paracone of P⁴ is distinctly separated from the metacone, and located above the mesiobuccal root. The mesiodistal length of M₁ is 26.5 mm.

Description.—We described the detailed dental morphology of specimens in our previous paper (Thaung-Htike et al., 2005), and identified them as T. cf. intermedius and T. intermedius. Here, we redescribe only the distinct dental characters and add some new characters which were not described in the previous work.

The maxillary and mandibular fragments are known, with well preserved P³–M³ and P₄–M₃. Both the upper and lower cheek teeth are bunodont and brachyodont. The tooth enamel is very thick and highly wrinkled, especially on P^3–4 and P₄.

The broken edge of the zygomatic process remains on the left maxilla. The mesial border of the zygomatic process can be seen above P³, and the distal border can be estimated above the mesial end of M³, indicating an elongated junction of the zygomatic process and the maxilla. This character is comparable to those of other tetraconodontine suids, such as Conohyus sindiensis (see Colbert, 1933) (In Sus, the junction is shrunken and appears dorsally between M² and M³).

P³ is nearly triangular in occlusal view and wider distally. The paracone is clearly separated from the metacone. The distolingual heel is well developed.

P⁴ is wider buccally than lingually. The buccolingual width is greater than the mesiodistal length. The paracone is clearly separated from the metacone. The protocone is distinct, and separated from the paracone and metacone by a protofossa. The paracone and metacone are located above the mesiobuccal and distobuccal roots, respectively. The sizes of the mesiobuccal and distobuccal roots are nearly the same.

M¹ and M² are rectangular in occlusal view. M³ is nearly triangular in occlusal view and narrower distally. All molars have four major cusps (paracone, protocone, metacone and hypocone) with rounded outline and shallow furchen. The minor cusps (protopreconule, hypopreconule and pentapreconule) are distinct. The pentacone is small but well defined in each molar. The talon of M³ is quite small. The buccal cingula are well developed in M¹ and M². The occlusal dimension of M² is larger than that of M³. M¹ < M².

The robustness of the mandible suggests that it belonged to a male. It is broken at the base and impossible to measure the depth of the mandible. The anterior ridge of the ascending ramus is broken at the buccal side of P₄ and the distal side of M3 (Figure 4F). The outline of the canine cannot be traced.

P₄ is large and rugose. The protoconid and metaconid are worn, and cannot be observed. The hypoconid is distinct. Three clearly separated roots are present.

M₁ and M₂ are nearly rectangular in occlusal view, and smaller than the P₄. M₃ is elongated and narrows distally. The four major cusps (protoconid, metaconid, hypoconid and entoconid) and three minor cusps (protopreconulid, hypopreconulid and pentapreconulid) are distinct. The pentaconid is well defined in all molars, especially in M₃. The talonid of M₃ is simple. The anterior cingulum is well developed in all molars, and protrudes at the mesiobuccal corner of the crown. The mesiodistal length of M₂ is shorter than that of M₃ but its buccolingual width is greater. M₁ < M₂.

Comparison.—The characters of the extremely large last two premolars and small M3_/3 indicate that the specimens belong to the genus Tetraconodon. The occlusal dimensions of M1_/1 are congruent with that of T. intermedius, however, the Siwalik species T. intermedius can be differentiated from T. irramedius sp. nov. by its distinctly larger P^3–4 and M³ (see Figure 3). The other Myanmar species; T. irramagnus sp. nov. is clearly distinguished from T. irramedius sp. nov. (a detailed comparison is made in the above text); T. minor and T. malensis are smaller than T. irramedius sp. nov. (Figure 3A), and their size differences exceed the limit of individual variation of the sympatric artiodactyl species of the same genus (Made, 1991). T. magnus from the Middle Siwalik Group differs from T. irramedius sp. nov. by its much larger size.

Discussion

Tetraconodon was first described by Falconer (1868). The type species Tetraconodon magnus (= T. magnum) is distinct for its large size (M₁ length is about 33 mm). Pilgrim (1926) grouped Tetraconodon with the Asian Conohyus (C. sindiensis Lydekker, 1878, and C. indicus Lydekker, 1884) which is a small tetraconodont, and suggested that the latter genus might be the ancestor of the former. He also described the important distinctions between these two genera such as the much greater relative enlargement of the premolars in Tetraconodon. The later-described T. minor (Pilgrim, 1910) and T. intermedius (Made, 1999) are also large (M₁ length is about 23 mm in T. minor and about 27 mm in T. intermedius) and large size had been used as a diagnostic character of Tetraconodon until we discovered a small species, Tetraconodon malensis (M₁ length is about 14 mm) from the middle Miocene of Myanmar (Thaung-Htike et al., 2005).

Among Tetraconodon species, T. intermedius from the upper Miocene of the Middle Siwalik Group has been known only from its upper dentition. We previously described the discovery of T. intermedius and T. cf. intermedius from the late Miocene of Myanmar (Thaung-Htike et al., 2005). We classified the mandibular fragment (NMM 839/80) as T. intermedius based on similarity between the occlusal dimensions of its M₁ and M1 of T. intermedius, and classified the maxillary fragment (NMMP–KU–IR 0225) as T. cf. intermedius because of its distinctly smaller P^3–4 and M³ compared to those of T. intermedius. We mentioned that the smaller last two premolars and third molar in the Myanmar specimens compared to the specimens of T. intermedius from Siwalik could be due to individual variability, and it was difficult to classify the Myanmar specimens as a new species rather than as T. intermedius and T. cf. intermedius.

Recently, Chit-Sein et al. (2006) described three new large Tetraconodon specimens from the late Miocene of Myanmar. The occlusal dimensions of M1_/1 in their materials resemble those of T. magnus of the Middle Siwalik Group; however, the last two premolars and M³ in their specimens are smaller than those of T. magnus. Therefore, they attributed their specimens to T. cf. magnus.

When reevaluating these three large Tetraconodon from Myanmar, T. cf. magnus, T. intermedius, and T. cf. intermedius (Chit-Sein et al., 2006; Thaung-Htike et al., 2005) together with other Myanmar species (T. minor and T. malensis) all of the Myanmar species are similar in the following character: the relative sizes of the last two premolars with respect to the first molar. It can be seen that the relative dental sizes of the Myanmar species are situated on the same trend of size enlargement (Figure 3). However, the Siwalik species (T. magnus and T. intermedius) are distinct for their relatively larger last two premolars and third molars compared to those of Myanmar species (Figure 3).

Finally, we reclassify the large Tetraconodon species of Myanmar, redefining T. cf. magnus (Chit-Sein et al., 2006) as T. irramagnus sp. nov., and T. intermedius and T. cf. intermedius (Thaung-Htike et al., 2005) as T. irramedius sp. nov. Among the Myanmar species (T. irramagnus, T. irramedius, T. minor and T. malensis) the most obvious interspecies distinctions are their dental size differences, which exceed the individual variability and sexually dimorphic differences documented in extant suids (Made, 1991).

Tetraconodon specimens are rare and discovered only from the Middle Siwalik Group of India/Pakistan and the Miocene sediments of Myanmar. Myanmar species Tetraconodon irramagnus, T. irramedius, and T. minor have been discovered from the basal part of the Irrawaddy Group (early late Miocene), and T. malensis has been discovered from the uppermost part of the Pegu Group (late middle Miocene). The known materials of Myanmar species were recovered from different localities, and it is difficult to correlate their stratigraphic levels. However, the larger teeth of the tetraconodontine suids have come from the younger sedimentary strata, which suggests that the large species of Myanmar Tetraconodon are chronologically younger than the small species.

Relatively smaller premolars are common in all Myanmar species. In contrast, the Siwalik species (T. magnus and T. intermedius) have relatively larger premolars with respect to the first molar, and this enlargement could be regarded as a derived character. It suggests that the Siwalik species and Myanmar large species (T. irramagnus and T. irramedius) evolved from the smaller T. minor of Myanmar, and the derived character of the Siwalik species probably occurred after they entered the Indian Subcontinent during the late Miocene. Moreover, the chronological gap between the possible youngest Myanmar species (T. irramagnus, early late Miocene) and the oldest species (T. malensis, late middle Miocene) is relatively short, which suggests that a rapid evolution of Tetraconodon occurred during the middle and late Miocene.

The gradual enlargement in the overall dental size of Tetraconodon has been accepted as an evolutionary trend. In addition, larger sizes of the last two premolars and third molar with respect to the first molar in Siwalik species are the most obvious character to distinguish this species from the Myanmar species. In conclusion, the enlargement of the last two premolars and third molar can also be accepted as an evolutionary change that occurred in the Siwalik lineage.