Saxatilomys

Musser, Guy G., Lunde, Darrin P. & Son, Nguyen Truong, 2006, Description of a New Genus and Species of Rodent (Murinae, Muridae, Rodentia) from the Tower Karst Region of Northeastern Vietnam, American Museum Novitates 3517 (1), pp. 1-41 : 31-34

Saxatilomys .

Qianomys wui is certainly a different species than Tonkinomys daovantieni . Whether the gross similarities between the two in molar occlusal patterns signal close phylogenetic relationship or convergence cannot be determined until we can compare the fossils directly with our extant samples, and until cranial material representing Qianomys is recovered and described. Resolving the ambiguous association between the present sample of upper molars and lowers allocated to Qianomys is also critical. That genus would resemble only Saxatilomys among members of the Dacnomys Division if the five-rooted first upper molars with their Niviventer -like occlusal patterns actually belong to Qianomys with its two-rooted lower molars.

A recent report by Zheng (2004) documents identities of rodents, represented mostly by isolated molars, collected from sedimentary layers in Longgudong Cave (30 ° 39914.90N/ 110 ° 04929.10E), the ‘‘Jianshi Homind Site’’ in western Hubei Province of central China. Geological age was given as Early Pleistocene, but correlation between the 11 sedimentary layers sampled and the geomagnetic polarity timescale indicated a range extending from the Olduvai Event of the Matuyama Chron (the Pleistocene/Pliocene boundary) back to earlier than 2.15 million years before the present, reflecting initial deposition during the Late Pliocene (see Flynn et al., 1997, and Qiu, 1989, for illuminating expositions of Chinese geochronology at the end of the Neogene). Among the murines recovered were three species in the Dacnomys Division. One is the extinct Niviventer preconfucianus , which Zheng himself had named and described in 1993, and another is represented by isolated molars identified only as an undetermined species of Leopoldamys . The third is a new species of the extinct Wushanomys , W. ultimus , described from molars recovered from all but the top two layers of the cave sediments, most from layers dated between 1.95 to earlier than 2.15 million years before the present (Late Pliocene). Molars from W. ultimus are intermediate in size between the earlier described W. brachyodus and W. hypsodontus ( Zheng, 1993) , express greater hypsodonty, and differ slightly in coronal patterns. Ranges in length and breadth values for first upper (3.90– 4.92 mm by 2.20–2.65 mm, 22 examples) and lower (2.90–4.30 mm by 1.98–2.30 mm, 18 specimens) molars of W. ultimus (see Zheng, 2004: 173) overlap with those of Tonkinomys daovantieni (table 4), but average slightly longer and wider. Each first lower molar of W. ultimus has two roots, the pattern common to the other two species of Wushanomys , but first upper molars have either four (anterior, divided lingual, posterior) or five roots (a small labial holdfast or nubbin in addition to the four primary roots). The pattern of the four primary roots beneath the first upper molars and two anchoring first lower molars also characterizes Tonkinomys daovantieni , but molars of that Vietnamese rat are not hypsodont, and although the extinct and extant species share molars similar in size, their occlusal patterns are dissimilar. The occlusal cusp patterns characteristic of W. ultimus resemble, except for minor details, those in W. brachyodus and differ from T. daovantieni in the configurations we previously described contrasting W. brachyodus and the Vietnamese species.

These new samples of the Dacnomys Division recovered from the Jianshi Homind Site provide additional information on Late Pliocene geographic distributions of Niviventer and Leopoldamys . The geographic range of Wushanomys is also expanded as is the diversity of species in that genus. Judged from Zheng’s (2004) descriptions, measurements, and illustrations, none of the identified fossil molars represent the Vietnamese Tonkinomys .

Our final survey of fossils assesses the identification of murines recovered from Pleistocene sediments in Ma U’Oi cave in northern Vietnam, south of Hanoi ( Bacon et al., 2004). One mandible containing a complete molar row was identified as Niviventer fulvescens ; an upper first molar and a lower first molar were attributed to N. andersoni ; and a dentary containing all molars, and isolated first and second molars, were determined as Leopoldamys sabanus . The molars were identified by comparing them with Chaimanee’s (1998) treatise on fossil Plio-Pleistocene Thai rodents. Unfortunately, we are not treated to drawings of molar occlusal cusp patterns, or to measurement values for the sample of N. fulvescens .

We cannot verify the identification of N. fulvescens . That species is common in northern Vietnam and in many places occurs together with N. tenaster (results of our trapping surveys in several different regions of northern Vietnam), which is somewhat larger in dimensions of body and skull, as well as length of molar row and breadths of molars (Musser and Lunde, ms.). Niviventer tenaster also occurs in northern Thailand ( Musser and Carleton, 2005), but Chaimanee (1998) did not include it in her report. The material reported by Bacon et al. (2004) should be reexamined to determine whether it is really N. fulvescens or possibly N. tenaster .

Measurements listed by Bacon et al. (2004) for two first lower molars of Leopoldamys (4.0 by 2.6 mm and 4.7 by 2.8 mm) fall within the range of variation for the larger species of that genus ( Chaimanee, 1998; Zheng, 1993), and well outside the range for our sample of Tonkinomys (table 4). The lower second and third molars that were identified as Leopoldamys are also large and likely represent that genus. But these fossil molars may not belong to L. sabanus . Leopoldamys requires taxonomic revision because more species exist than are usually recognized (four were listed by Musser and Carleton, 1993, and six by Musser and Carleton, 2005, which still underestimates the number of species), and the situation in northern Vietnam is especially complex. At least two species occur there. One may be L. edwardsi , the other is usually identified as L. sabanus , but is actually a different species ( Gorog et al., 2004; Musser and Carleton, 2005).

An upper first molar and lower first molar were identified as Niviventer andersoni in Bacon et al. (2004). Ma U’Oi cave is far south of the present geographic distribution of N. andersoni , which extends from eastern Xizang (Tibet) through northern Yunnan, western Sichuan, and northern Ghizhou to southern Shaanxi, and over an altitudinal range from 6000 to 10,000 ft ( Musser and Chiu, 1979; Musser and Carleton, 1993, 2005). We sought to determine whether these two fossil molars belong to Tonkinomys . Dimensions of the first

TABLE 4 Summary Statistics for Lengths and Breadths (mm) of First Upper and Lower Molars of Niviventer andersoni ( China, Yunnan and Sichuan), N. excelsior ( China, Sichuan), N. tenaster ( Vietnam) , Pleistocene First Molars Identified as N. andersoni by Bacon et al. (2004), and Tonkinomys daovantieni ( Vietnam) (Mean, plus or minus one SD, and observed range in parentheses are listed for each measurement. Samples consist entirely of AMNH specimens except those reported by Bacon et al., 2004.)

upper molar (3.7 by 2.0 mm) do fall within the range of variation of our values (table 4) and those tabulated by Zheng (1993: 181) for large sample sizes of N. andersoni . Breadth of the fossil molar fits within our observed range for first upper molars of Tonkinomys , but the length is much too short—based on these data, the molar did not come from the Vietnamese limestone rat. However, it may not represent N. andersoni . The width, for example, matches our range of values for N. excelsior , and the length is only 0.1 mm shorter (table 4). Niviventer excelsior is a close relative of N. andersoni that presently occurs only in high mountains north of the Yangtze River ( Musser and Chiu, 1979). For N. excelsior to have once occurred in northern Vietnam, however, seems as implausible to us as the presence of N. andersoni there, especially because all the fossils of other mammals found in situ in the Ma U’Oi cave define ‘‘… a relatively modern fauna …’’ ( Bacon et al., 2004: 312). Niviventer tenaster currently lives in northern Vietnam in forest habitats. Lengths and breadths of first upper molars from this species in our sample from that country nearly embrace dimensions of the fossil molar (table 4), and that tooth just might be from a large individual of N. tenaster . Both N. andersoni and N. excelsior can easily be distinguished from N. tenaster by occlusal patterns. In the former pair, the anterior lamina on the first upper molar consists of a nearly straight segment, formed by the coalesence of cusps t2 and t3, and a posteriorly displaced lingual cusp t1 (see figs. in Musser, 1981, and Zheng, 1993). The comparable lamina has an arcuate outline on the upper molar in N. tenaster , similar to that found in N. confucianus ( Musser, 1981) . The fossil molar needs to be reexamined; it is not an example of Tonkinomys , and we question its identity as N. andersoni .

The first lower molar determined to be N. andersoni in Bacon et al. (2004) is large (4.1 by 2.5 mm), and its dimensions fall far outside the range of variation in our samples of Tonkinomys , Niviventer andersoni , N. excelsi- or, and N. tenaster (table 4). The tooth is actually about the same size as the two first lower molars (4.0 by 2.6 mm, 4.7 by 2.8 mm) they identified as Leopoldamys . The values are either misprints, or the molar does belong to Leopoldamys ; it, and the first upper molar assigned to N. andersoni , warrants restudy.

Summary: There would have been an elegant symmetry in discovering ancient, fossilized remnants hidden in limestone fissures that represented the same murine species living today in creviscular habitats within the tower karst landscape forming northern Vietnam. Unfortunately, we cannot unambiguously tie any published identifications of fossilized fragments representing Indochinese murines to the extant Tonkinomys daovantieni . Its past distribution and phylogenetic alliance to extinct taxa, as might have been illuminated by fossils, remain a mystery.

Unresolved also is the phylogenetic position of Tonkinomys with respect to Leopoldamys , Niviventer , and Saxatilomys , the three extant genera with morphologies most like that of Tonkinomys . The differences enumerated above between Tonkinomys daovantieni on one hand and species of Leopoldamys and Niviventer on the other reflect a combination of primitive and derived features. Relative to the pelage coloration, shape of tail, and its length relative to length of head and body in Leopoldamys and Niviventer , the contrasting condition in Tonkinomys is derived. Also representing specializations in Tonkinomys are the position of the squamosal zygomatic roots high on the sides of the braincase, slender or no alisphenoid struts, its long incisive foramina, a very long bony palate that projects beyond the molar rows to form a broad shelf, posteriorly diverging maxillary molar rows, simple molar occlusal patterns, and a divided lingual root beneath the first upper molar. Presence of three roots beneath each second and third molar, and two roots anchoring each lower molar represent the primitive condition in Tonkinomys compared with the derived state in Leopoldamys and Niviventer . Tonkinomys daovantieni resembles the Indochinese Leopoldamys neilli in body size, and it along with the other Indomalayan species of Leopoldamys in general cranial conformation and a few cranial particulars (position of the squamosal zygomatic roots, for example); with Niviventer , Tonkinomys shares long incisive foramina, comparable inflation of the auditory bulla relative to the underlying periotic bone, and similar molar occlusal patterns. Other traits exclude T. daovantieni from each of those genera as they are currently diagnosed ( Musser, 1981). No species of Leopoldamys or Niviventer (or Chiromyscus ), whether occurring in Indochina or on the islands on the Sunda Shelf, has Tonkinomys ’s combination of dark grayish black upperparts and dark gray underparts; thick tail with a stubby tip that is much shorter than length of head and body; large, bulbous, and contiguous plantar pads; bony palate projecting well beyond the third molars to form a shelf with a smooth posterior margin; strongly diverging (along the anteroposterior axis) maxillary molar rows; derived root pattern for the first upper molar coupled with the primitive root patterns underlying the other uppers and all the lower molars.

Both Tonkinomys daovantieni and Saxatilomys paulinae are associated with forested karstic habitats. Both genera display dark gray pelage, large and swollen plantar tubercles, a bony palate that projects beyond the molar rows to form a broad platform, maxillary molar rows that appreciably diverge posteriorly (along the anterior-posterior axis) and are long relative to length of skull, and two-rooted mandibular molars. All but the root pattern are specialized traits. This combination of attributes is not combined in any species of either Leopoldamys or Niviventer (or Chiromyscus ). We do not know whether the morphological specializations shared by T. daovantieni and S. paulinae phylogenetically link these species closer to each other than to species of Leopoldamys and Niviventer , or rather reflect independent evolutionary adaptations to forested karstic environments in the Indochinese tropics. In opposition to the shared specializations, external morphology of Tonkinomys (except for relative length of tail) recalls a small-bodied Leopoldamys , while that of Saxatilomys resembles most species of Niviventer . Overall cranial form in Tonkinomys also resembles Leopoldamys , and except for those cranial traits that distinguish S. paulinae from species of Niviventer (comparisons are enumerated by Musser et al., 2005), the general cranial conformation of Saxatilomys is not unlike most species of Niviventer . Inferring the phylogenetic relationship of Tonkinomys among species now allocated to the Dacnomys Division in general, and especially to species of Leopldamys, Niviventer (including Chiromyscus ), and Saxatilomys will rest on results derived from future phylogenetic analyses of information derived from both morphological and molecular sources.