Subfamilial attribution of the Burmese mandible

Among chalicotheres, the lower jaws of the Chalicotheriinae are distinguishable from those of the Schizotheriinae by a more robust corpus, and shorter and lower crowned molars (e.g., Coombs 1989; Bonis et al. 1995; Anquetin et al. 2007; Coombs 2009).

The corpus of MFI-105 displays only a slight increase in depth between the p2 and the ml. The Chalicotheriinae, contrary to the Schizotheriinae, show frequently an increase in mandibular depth towards the distal side (Anquetin et al. 2007). Nevertheless, this feature cannot distinguish unambiguously these subfamilies so that the uniform depth of the corpus of MFI-105 does not necessarily imply schizotheriine affinities. For instance, a chalicotheriine discovered at Titov Veles (Macedonia) has a constant mandibular depth (Garevski and Zapfe 1983) while some Schizotheriinae (e.g., Metaschizotherium bavaricum von Koenigswald, 1932 from the middle Miocene of Germany) exhibit deeper corpora distally (Coombs 2009). Moreover, the increase in jaw depth is generally mostly concentrated posterior to the first molar in chalicotheriines (see Zapfe 1979; Bonis et al. 1995; Wang and Wang 2001; Anquetin et al. 2007), a region not preserved on MFI-105.

The corpus of MFI-105 possesses a thickness-depth ratio of 0.61 at the level of the m1. In comparison, a mandible of the schizotheriine Schizotherium cf. S. avitum Matthew and Granger, 1923, AMNH 103336, shows a ratio of 0.44 (estimated from Coombs 1978: fig. 3) while a mandible of the chalicotheriine Chalicotherium salinum, GSP 6006, displays a value of 0.51 for a similar ratio. The corpora of Metashizotherium bavaricum (Coombs 2009: figs. 1, 3) appear also notably more slender than that of MFI-05 at only slightly younger ages.

Table 1.

Mandibular and dental measurements and indices of MFI-105 and other Eurasian chalicotheriines. Distances are expressed in millimeters. Trigonid breadth index = (trigonid breadth)/(talonid breadth)* 100. Breadth index = (maximum breadth)/length* 100. Data: Anisodon grande, range of Neudorf an der March (Devínska Nová Ves, Slovakia) specimens from Zapfe (1979) and Zapfe (1989); Anisodon macedonicus, average values of left and right hemimandibles DKO 234 (Dytiko, Greece) from Bonis et al. (1995); Chalicotherium goldfussi, range of specimens of Rheinhessen and Höwenegg (Germany) from Zapfe (1989); Chalicotherium salinum, Colbert (1935) and direct measurements at Harvard Peabody Museum. GSP 23046 and 9665 come from the Chinji Formation (14–11.2 Ma according to Barry et al. 2002). *measured from roots.

Table 2.

Morphological comparison of MFI-105 with other Eurasian Miocene chalicotheriines. Data from Anquetin et al. (2007) (Chalicotherium goldfussi and Anisodon grande), Zapfe (1979) and Zapfe (1989) (Chalicotherium goldfussi and Anisodon grande), Bonis et al. (1995) (A. macedonicus), Wang and Wang (2001) (“Chalicotherium” cf. C. brevirostris), Colbert (1935) and personal observations in the collections of the Harvard-Geological Survey of Pakistan project (C. salinum). The choice of the characters mostly relies on the work of Bonis et al. (1995) and Anquetin et al. (2007). * the diastema is short in the juvenile individual AMNH 19577 (Colbert 1935: figs. 73, 74).

With a width-length index of 60 (Table 1), the m1 of MFI-105 is proportionally less elongated than those of Metaschizotherium bavaricum, M. fraasi von Koenigswald, 1932 and the schizotheriine material from the French locality of La Grive [respectively 50–56 (N = 8), 56 (N = 1) and 52–53 (N = 3). Data from Coombs 2009: table 2] but falls within the range showed by the species of Schizotherium (55–65; calculated from Coombs 1978: table 1) and is close to m1s of Phyllotillon naricus Pilgrim, 1910 and Ancylotherium pentelicum Gaudry and Lartet, 1856 (56 and 64 for the first, and 60 for the second; data from Coombs 2009: table 2). On the other hand, the width-length ratio of the ml of MFI-105 falls as well within the bracket of various chalicotheriines (Table 1). Thus, the measurements of the first molar of MFI-105 cannot be used to allocate the Burmese specimen either to the Schizotheriinae or to the Chalicotheriinae, the characteristic molar elongation of the Schizotheriinae seeming more accentuated on m2 and m3 than on m1.

Finally, the combination of low crowns, ml not elongated mesio-distally relative to its breadth and robust corpus displayed by MFI-105 exclude it from the Schizotheriinae and justify its allocation to the subfamily Chalicotheriinae.

Comparisons with other known chalicotheriines

Anquetin et al. (2007) recently published a discussion on chalicotheriine systematics and phylogeny. This work, which combines a new study of the material of the classical European species and a cladistic analysis based on a large set of cranial and dental features, will serve here as a reference for the systematics of the subfamily. The most important results of these authors from the perspective of our study can be summarized as follows:

Anisodon Lartet, 1851 is a valid genus name and applies to several taxa of the Old World formerly attributed to Chalicotherium, Macrotherium, and Nestoritherium.

Macrotherium is not a valid name. These two points had already been proposed by Geraads et al. (2001).

The chalicotheriines are divided into two principal clades, the Anisodon clade and the Chalicotherium clade. The Anisodon clade is formed of Anisodon grande Blainville, 1849, type-species of Anisodon, A. macedonicus Bonis, Bouvrain, Koufos, and Tassy, 1995, A sivalense Falconer and Cautley, 1843, A. wuduensis Xue and Coombs, 1985, and the material from Titov Veles (Macedonia) and Vathylakkos (Greece). The Chalicotherium clade comprises Chalicotherium goldfussi Kaup, 1833, the type-species of Chalicotherium, and C. brevirostris Colbert, 1934.

The next sections propose comparisons between MFI-105 and the main Eurasian chalicotheriines, beginning with the taxa analyzed by Anquetin et al. (2007) and subsequently extending to Asian chalicotheriine material unconsidered by these authors. Table 2 displays a summary of the main features of MFI-105 as compared with other Eurasian chalicotheriines.

Comparison with Butleria. —This genus is known by a single species, Butleria rusingensis Butler, 1965, from the early Miocene of Kenya (Butler 1965). B. rusingensis is considered as a basal chalicotheriine and the sister-group of all known post-early Miocene representatives of this subfamily (Bonis et al. 1995; Anquetin et al. 2007). This African species differs from MFI-105 by an increase in corpus depth distally. The strong “metastylid” on the molars of B. rusingensis cannot be compared with MFI-105 on which this trait is not accessible. B. rusingensis and MFI-105 share the retention of a p2, the posterior extent of the symphysis at the level of the p2, a distinct entoconid on the p4 and V-shaped trigonids on molars. However, these features are all primitive in the present taxonomical context. Hence, there are no exclusively shared synapomorphies between the Burmese mandible and B. rusingensis, which are presumably not closely related.

Comparison with the Anisodon clade.—According to Anquetin et al. (2007), the Anisodon clade members are characterized on the mandible by a strong ventral expansion of the angular area and a reduction of the “metastylid” on lower molars. Both features are unfortunately inaccessible on the Burmese mandible. Nevertheless, MFI-105 can easily be distinguished from all the taxa of the Anisodon clade with the exception of Anisodon grande by retention of the primitive state regarding the distal extension of the symphysis (ending at the level of the p2). In addition, MFI-105 clearly differs from these species by its smaller size, a clearly greater decrease in depth of the mandibular corpus in front of the p2, and a greater development of the entoconid on the p4. Hence, MFI-105 does not match with any of the Anisodon clade taxa.

As the Burmese mandible shares with A. grande a primitive symphyseal character, it also displays two other common features with this species. First, MFI-105 possesses a double rooted p2 like the A. grande specimen of Sansan MNHN Sa 15671 (Anquetin et al. 2007). However, intraspecific variation in the number of roots is common among mammals (Kovacs 1971). Indeed, Zapfe (1979: fig. 14) figured a single-rooted p2 from Devínska Nová Ves, implying a possible intraspecific variation of this feature in A. grande. Moreover, the double-rooted p2 is most probably another primitive trait, since that a similar character state is observable in the primitive chalicotheroid Eomoropus (Obsorn 1913) and in the schizotheriines Schizotherium and Moropus (Coombs 1978; Coombs et al. 2001). Second, MFI-105 exhibits narrow trigonids on its p4 and ml and resembles in this respect the A. grande sample from Devínska Nová Ves (Table 1; Zapfe 1979, 1989) and the Anisodon from Titov Veles (Garevski and Zapfe 1983). While the tendency of front to rear breadth enlargement is common on lower cheek teeth of chalicotheriines, the p4 and ml trigonids of MFI-105, A. grande and the Anisodon from Titov Veles are the narrowest (relative to the talonid) among the chalicotheriines (Table 2; Garevski and Zapfe 1983). Note that the narrowness of the first lobe is greater on the m1 of MFI-105 than on the corresponding tooth of A. grande at Devinska Nova Ves, the trigonid breadth index of the Burmese mandible being lower than the minimum value of the European sample. This character state is difficult to interpret from an evolutionary perspective. If derived, it might only illustrate a lineage-restricted allometry rather than a synapomorphy. Hence, no derived feature on MFI-105 clearly testifies for affinities with A. grande and the Anisodon from Titov Veles and more generally with the Anisodon clade.

Comparison with the Chalicotherium clade.—None of the two unambiguous synapomorphies of the Chalicotherium clade of Anquetin et al. (2007) can be checked on MFI-105 since these features concern the upper dentition and skull morphology.

Chalicotherium brevirostris.—This species is known from the middle Miocene Tunggur Formation in northern China (Colbert 1934). Unfortunately, it is now impossible to perform a direct comparison between MFI-105 and this species because no mandibular specimens are undoubtedly referable to it (but see further comments below).

Chalicotherium goldfussi.—This European species is much larger than MFI-105 (Zapfe 1989). Its molars have U-shaped trigonids (Anquetin et al. 2007) that are generally as broad as the talonid (Zapfe 1989; Table 2), as opposed to the narrow and V-shaped trigonid of the ml for the Burmese mandible. Moreover, the p4 of C. goldfussi, unlike that of MFI-105, is quadrate in shape with a trigonid as broad as the talonid (Zapfe 1989; Table 2). This tooth also possesses a less oblique metalophid and a thicker and more complete buccal cingulum in C. goldfussi (Zapfe 1989). Thus, MFI-105 is distinct from the type-species of Chalicotherium. Chalicotherium goldfussi and MFI-105 share a long diastema mesial to the premolars but this feature is presumably primitive for chalicotheriines since it is displayed by the primitive Eocene chalicotheroid Eomoropus (Osborn 1913), in the schizotheriine Moropus (Coombs et al. 2001) and by at least four chalicotheriine lineages (Table 2).

Other Asian chalicotheriines.—Although the analysis of Anquetin et al. (2007) is comprehensive regarding the European chalicotheriine fossil record, its sampling is not exhaustive for the Miocene Asian chalicotheriines. The geographical provenience of MFI-105 invites comparisons with these taxa.

Chalicotherium cf. C. brevirostris.—In northern China, two mandibular specimens have been considered as putatively corresponding to C. brevirostris. The first is a virtually complete mandible from Hebei province reported by Hu (1959) and assigned to Chalicotherium cf. C. brevirostris on the basis of the short anterior region of the symphysis displayed by the fossil. Interesting features of this specimen, accessible through the illustrations of Wang and Wang (2001), are: posterior extent of the symphysis at the level of the p4, ventrally expanded angular region, V-shaped trigonid, sub-rectangular outline, lack of distinct “metastylid” and low paralophid on lower molars, probably single rooted p2, p3–p4 with short lengths, great posterior breadth, very reduced talonid on the p3, distinct entoconid on the p4, retromolar gap, extremely shortened anterior region of the mandible.

The second is a jaw fragment with m2–m3 discovered in the late middle Miocene deposits of the Lower Youshashan Fm. in Qinghai province and assigned to the same taxon as the Hebei province mandible because of their similar size and molar morphology (Wang and Wang 2001).

MFI-105, with its narrow trigonid on the ml, its symphysis reaching the level of the p2, its double- rooted p2, its elongated p3 talonid and its much smaller size is obviously different from these Chinese fossils. Note that the latter fossils display a combination of features that match the diagnosis of Anisodon sensu Anquetin et al. (2007): all known species of Anisodon have reduced “metastylids” on lower molars and a ventrally expanded angular area; all species of Anisodon except A. grande have the posterior extent of the symphysis at the level of p3–p4. Consequently, unless a parallelism, the lower jaws described by Hu (1959) and Wang and Wang (2001) are unlikely to represent Chalicotherium following the criteria of Anquetin et al. (2007).

Chalicotherium salinum.—In the Siwaliks of Pakistan, chalicotheriine specimens discovered in the middle Miocene Chinji Formation and early late Miocene Nagri Formation were allocated to the species Chalicotherium salinum Forster-Cooper, 1922 (Forster-Cooper 1922; Colbert 1935; Pickford 1982). The Siwalik material consists mostly of isolated dental and postcranial elements. The holotype of the species is a left M3 collected in the Chinji Formation (Forster-Cooper 1922). The most complete specimen attributed to C. salinum, and consequently the key fossil for comparisons, is GSP 6006. This fossil was collected in the Nagri Formation and has an age of 9.6 Ma (John Barry, personal communication 2008). It consists of a left hemi-mandible with m2–m3, the angular region and a part of the ascending ramus and a right hemi-mandible with p2, m1, m3 erupting and the coronoid process. The dental material of C. salinum, while scarce, presents an important range of size between the smaller middle Miocene specimens and the larger late Miocene ones (Table 1). Morphological variation on the “metastylid” is also noticeable. This cusp is distinct in some of the specimens from the Chinji Formation and absent from the younger GSP 6006. These observations bring into question the monospecific status of the chalicotheriine material from the Chinji and Nagri formations proposed by Pickford (1982), but the lack of material impedes further analysis (Coombs 1989).

The teeth of MFI-105 teeth are intermediate in size between the middle Miocene specimens and GSP 6006 (Table 1). The Burmese specimen shares with GSP 6006 a posterior extent of the symphysis at the level of the p2 and a very modest increase in depth of the mandible between the p2 and the ml. Nevertheless, several morphological discrepancies indicate that MFI-105 most probably does not belong to the same species as GSP 6006. The p2 of GSP 6006 is large and built of a single large root of circular section whereas the p2 has two roots on MFI-105. The molars of GSP 6006 display wider trigonids (Table 1), a greater lingual extension of the paralophid, and more complete and thicker buccal and distal cingula than the m1 of MFI-105. The ml of the juvenile mandible AMNH 19577 (Colbert 1935: figs. 73, 74) seems close to that of GSP 6006 in terms of extension of the paralophid, cingula development and breadth of the trigonid while the m2 AMNH 19437 (Colbert 1935: fig. 75) also displays a lingually extended paralophid and a trigonid close in breadth to the talonid. Finally, the specimens from the Chinji Fm., which possess weaker cingula, have also broader trigonids (Table 1) and a more lingually extended paralophid than the m1 of MFI-105. Hence, conspecific or not, the fossils regrouped under the binomen C. salinum do not match satisfyingly MFI-105.

Chalicotherium from Thailand.—In South Asia, Chalicotherium was reported from the Thai middle Miocene locality of Huai Siew (Thomas et al. 1990) and identified as Chalicotherium cf. brevirostris (Ducrocq et al. 1994). Unfortunately, this material has never been described or figured and cannot be compared with the Burmese mandible.

Chalicotheriines from southern China.—Some of the Lufengpithecus-bearing localities of Yunnan province have yielded putative chalicotheriine remains. In the Yuanmou basin, a chalicothere baptized “Macrotherium” yuanmouensis is listed in the associated fauna of Lufengpithecus hudienensis (e.g., Qi et al. 2006). Another chalicothere has been identified in the hominoid locality of Lufeng as the chalicotheriine from the Miocene of the Siwaliks of Pakistan (“Macrotherium” salinum; e.g., Li et al. 1984). Unfortunately, no formal descriptions of these chalicotheres have been published to our knowledge.