A giant bamboo rat from the latest Miocene of Yunnan

Lawrence J.FLYNN LI Qiang Jay KELLEY, Nina G.JABLONSKI JI Xue-Ping Denise F.SU WANG Xiao-Ming,6

(1 Department of Human Evolutionary Biology,Harvard University Cambridge MA 02138,USA)

(2 Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology,Chinese Academy of Sciences Beijing 100044,China)

(3 Institute of Human Origins and School of Human Evolution and Social Change,Arizona State University Tempe,AZ 85287,USA)

(4 Department of Anthropology,The Pennsylvania State University University Park,PA 16802,USA)

(5 Kunming Natural History Museum of Zoology,Kunming Institute of Zoology,Chinese Academy of Sciences Kunming 650201,China)

(6 Department of Vertebrate Paleontology,Natural History Museum of Los Angeles County 900 Exposition Blvd.,Los Angeles,CA 90007,USA)

Abstract The Shuitangba subbasin lignite deposits of the Zhaotong Basin in northern Yunnan Province have produced vertebrate fossils of terminal Miocene age.We conducted test wet screening of fossiliferous sediment in 2014 to increase representation of small mammals.This effort produced four teeth of a very large bamboo rat,much larger than the previously known bamboo rat present at Shuitangba,and representing a new species.This new species is characterized by its molars being remarkably larger than those of other known species of Miorhizomys,and being hypsodont with cementum,and less anterorposteriorly compressed.The age of this new species from Shuitangba is in the range of 6.2 to 6.7 Ma.It appears that diverse bamboo rats of the extinct genus Miorhizomys were present in the Late Miocene of Yunnan,somewhat before the 6 Ma appearance of extant Rhizomys to the north in the vicinity of Shanxi Province.

Key words Shuitangba,Yunnan,Late Miocene,small mammals,bamboo rat

1 Introduction

Shuitangba is an open lignitic pit mine site in a small subbasin of the expansive Zhaotong Basin of northern Yunnan Province.Zhaotong Basin is well known for its rich deposits of Late Miocene terrestrial sediments with multiple lignite layers quarried for fuel.In addition to plant material the deposits are known to produce fossil vertebrates.The pit at Shuitangba has been exploited periodically for lignite,during which abundant vertebrate fossils,especially mammals,have been exposed.The Zhaotong Formation at Shuitangba is thinner than in the main body of Zhaotong Basin.

Fossil mammals at Shuitangba are moderately diverse (for example,Dong et al.,2014,Ji et al.,2015).Hand excavation has produced exquisite fossils,but slow and careful digging leads to small collections,especially of the microfauna.Representation is also affected by taphonomic and collecting bias against retrieval of very small vertebrates.We have attempted to reduce such bias by systematic screening of large volumes of sediment for small mammals with some success (Flynn et al.,2019).

Additional screening initiated in 2014 in productive fossiliferous layers within the Shuitangba sequence,added to the list of small mammals reported in Jablonski et al.(2014).The results of that work will be developed elsewhere,but we report here the recovery of a new,surprisingly large species of bamboo rat that coexisted with a more common,smaller form at Shuitangba.While the presence of bamboo rats in the Late Miocene of South China has been documented (Flynn and Qi,1982),and while two species co-occur today in parts of Southeast Asia,it was unexpected to find a species of bamboo rat at Shuitangba of such large size.

2 Material and methods

The specimens reported here are currently housed at the Zhaotong Institute of Cultural Relics Protection and Archaeology in the Zhaotong (ZT) catalogue system.They were retrieved by wet screening test samples from layers 9 to 11 of the Shuitangba section.The normally magnetized sediment correlates in age with chron C3An.At this time,it cannot be resolved whether the normal sediment represents C3An.1n or C3An.2n,so the age of the specimens falls in the broad window of 6.2 to 6.7 Ma on the Gradstein et al.(2012) time scale.Comparative materials include fossils from Lufeng,Yunnan Province,and from Yushe Basin,Shanxi Province,as well as casts of extant species from the zoological collection of the American Museum of Natural History,New York,USA (AMNH).Measurements in mm.

3 Systematic paleontology

Mammalia Linnaeus,1758

Rodentia Bowdich,1821

Spalacidae Gray,1821

Rhizomyinae Winge,1887

Miorhizomys Flynn,2009

Miorhizomys gigas sp.nov.

Etymologygigas,Greek for giant.

HolotypeZT 2014-0266,right M1 (Fig.1A).

Fig.1 Isolated molars of the large bamboo rat Miorhizomys gigas sp.nov.,in occlusal view

Referred specimensZT 2014-0170,left m3;ZT 2014-0392,left m2;ZT 2014-0470,right m1 (Fig.1B-D).

Locality and ageShuitangba hominoid exposure,shell horizon in normally magnetized sediment of layers 9 to 11 of the local sequence and correlated with chron C3An,6.2 to 6.7 Ma.

DiagnosisLarge rhizomyine rodent showing the molar crown features ofMiorhizomys(molars not compressed anteroposteriorly as they are in livingRhizomysandCannomys);molars hypsodont and with cementum;largest known species of the genus.

DescriptionFour isolated teeth are attributed to this new species.The upper dentition is represented by the holotype ZT 2014-0266,right M1.This large tooth widens toward the roots,where its length × width=6.3 ×7.1 mm.Maximum length (see Table 1) is greater,6.8 mm,because the high crown is inclined posteriorly.The tooth is hypsodont (Fig.2A),measuring 12.0 mm in height;the hypostria as preserved is 9.0 mm long.There are four major transverse lophs.The anteroloph and protoloph join the protocone;the mesoloph and metalophposteroloph complex join the hypocone.The anterior part of M1 is not as wide as the posterior moiety.M1 is convexly rounded without an anterolingual flexus.There is a narrow protoconehypocone connection.The thin enamel of the posteroloph is chipped.This molar has three small roots.

Table 1 Dimensions (maxima) of isolated molars of Miorhizomys gigas sp.nov.in occlusal view (mm)

Fig.2 Line drawing of molars of Miorhizomys gigas sp.nov.and Rhizomys sinensis

Of the lower dentition,the first molar,ZT 2014-0470,is worn and broken anteriorly(posterior width,4.75 mm).The metaconid joins the anterolophid,but not directly buccally toward the protoconid.The mesolophid,hypolophid and posterolophid are complete and parallel,and extend posterolingually.The left m2,ZT 2014-0392,is in early wear with height=9.27 mm,hypostriid=7.02 mm.Its crown widens toward the base.From the protoconid,the anterolophid-metalophid complex and the mesolophid extend lingually.The anterolophid-metalophid complex includes two small enamel lakes.Confluent with the base of the hypolophid,the hypoconid joins the protoconid narrowly.In this early wear stage,the posterior enamel lake is incomplete.The hypostriid is filled with cementum.There are two small posterior roots and a larger anterior root.The left m3,ZT 2014-0170 (l × w=7.8 × 7.6 mm) expands posteriorly toward its base.Preserved crown height is 9.6 mm,its hypostriid about 8 mm long and bearing cementum.In early wear,the anterolophid-metalophid complex is proportionally larger than that of m2.The mesolophid is incomplete.The hypolophid is broadly connected to the protoconid,but has a narrow junction with the hypoconid.The oblique posterolophid is short and directed posterolingually.

Occlusal surfaces of molars are flattened by wear that approximates planes.Wear of the first lower molar was the most advanced,however,and was dominantly antero-posterior,with the occlusal surface worn as a shallow trough,deepest along the midline.

4 Discussion

Large size and hypsodonty distinguishMiorhizomysgigassp.nov.,from other species of the genus.Full hypsodonty (Fig.2A,molar height being the greatest dimension of unworn teeth) is an unusual condition forMiorhizomys(Flynn,2009).Molars retain relatively small roots and major molar reentrants are buttressed by cementum,which is also unusual for the genus.Miorhizomysgigasis the largest species of its genus,larger than the Late Miocene Siwalik speciesM.pilgrimi(Table 1).Miorhizomysgigasis among the largest of known rhizomyines,even rivaling the Pliocene ageAnepsirhizomys opdykeiin size (Table 2 and Flynn 1982).The genusAnepsirhizomyshad different,derived molar morphology: a deepened lingual reentrant nearly bisecting m2-3.

Table 2 Size comparisons for select lower second molars of fossil bamboo rats,Tribe Rhizomyini (mm)

Miorhizomysgigassp.nov.,is the second of two bamboo rat species recognized at Shuitangba.The first,cf.Miorhizonys tetracharax,is smaller and is represented by several specimens.Its single known m2 shows greater width (when worn) than length: 4.1 × 4.6 mm(see Table 2).Cf.Miorhizomystetracharaxappears to be the same species as the common bamboo rat at the somewhat older Lufeng locality of Yunnan.The body mass of the latter was probably 1-2 kg.

The moist habitat of the Miocene Lufeng community of Yunnan supported a diverse mammalian fauna including multiple bamboo rat species.Flynn and Qi (1982) distinguished three rhizomyines at Lufeng largely by size,and at that time considered them indistinguishable from Late Miocene species of the Siwaliks of Pakistan.These were the smallMiorhizomys nagrii,M.tetracharax,and cf.M.pilgrimi.The latter was largest,a worn m2 measuring 4.7 ×5.0 mm (see Table 2).Presently the species level identification of all three Lufeng bamboo rats must be re-evaluated through comparison with other fossil rhizomyines.It seems reasonable to note that,regarding large size,the similar “cf.M.pilgrimi” of Lufeng represents a suitable predecessor ofMiorhizomysgigasat Shuitangba.

Body sizeThe body mass of the living bamboo rat of China,Rhizomyssinensis,averages about 2 kg,but individuals vary considerably.For fossils,relative dental size may be used to estimate body size,assuming a broadly constant relationship between molar size and body proportions.Correlations of molar size to body mass have relatively large prediction errors,so body mass estimates usually incorporate a measure of the wear surface area of the entire tooth row,or (for large rodents) the dimensions of the incisor (Millien and Bovy,2010).

Given thatMiorhizomysgigassp.nov.is represented by only four cheek teeth (one broken),comparison of its m2 with a jaw of a living relative of known mass provides at least an approximation of body size.We compared theM.gigasm2 with the skull and mandible ofRhizomyssinensis(AMNH 115527),a living species of about 2 kg mass (Fig.2B).Miorhizomysgigaswas larger: occlusal width of the second lower molar ofM.gigaswhen worn was somewhat greater,but its length was nearly 50% greater than that ofR.sinensis.The mass ofM.gigaswould have been well over 2 kg.The only other Shuitangba rodent larger than the uncommonM.gigas,was the phylogenetically distinct beaver,so frequently encountered during excavation.The adult semi-aquatic beaverSteneofiber zhaotungensishad wider molars plus an expanded p4 as in all members of its family.

Comments on early RhizomysThe oldest known fossilRhizomysis from the latest Miocene of the Yushe Basin,Shanxi Province (the origin of the sister genusCannomysremains unknown).Yushe bamboo rats show features that are incompletely transformed toward modernRhizomys,so while classified asRhizomys,are subsumed by the subgenusBrachyrhizomysTeilhard de Chardin,1942 (Flynn,2009).The earliest Yushe record isRhizomys(Brachyrhizomys)shajiusfrom a Late Miocene site projected to fall high in chron C3An.1n,so dated as 6.0 to 6.1 Ma (Flynn and Wu,2017).It is small for a bamboo rat,but shows the reduced lower molar mesolophid that accompanies shortening of the tooth row inRhizomys.While peripheral to the South China biogeographic region,the Yushe population was contemporary with or slightly younger than the Shuitangba bamboo rats.Conceivably,a species ofMiorhizomyscould be ancestral toRhizomys,and whileMiorhizomysdiversity persisted in South China,the peripheral bamboo rats of Yushe in North China may have differentiated asRhizomys(Brachyrhizomys).This scenario rests on few data,but is testable with more fossils.

5 Conclusion

The Late Miocene mammalian assemblage from Shuitangba in northeastern Yunnan had two species of bamboo rats,including the very large new species described here,Miorhizomys gigas.The large body size lineage to whichM.gigasbelongs may also include the large bamboo rat species recorded at the somewhat older Yunnan site of Lufeng.LivingRhizomyslikely originated at about this time (6-7 Ma) and in a fauna roughly contemporary with Shuitangba but biogeographically separate.Concerning body mass,M.gigaswas second in size in the Shuitangba rodent community only to the beaverSteneofiber zhaotungensis.

AcknowledgementsWe wish to thank the family of Mao Gudong for accommodating our work,and the people of Zhaotong for their assistance.Many thanks are given to the reviewers and editress Ms.Zhou Shuang for their constructive suggestions.