Naturwissenschaften

, Volume 94, Issue 3, pp 237–241

A new Early Oligocene peradectine marsupial (Mammalia)from the Burqin region of Xinjiang, China

Authors

    • Institute of Vertebrate Paleontology and PaleoanthropologyChinese Academy of Sciences
    • Division of PaleontologyAmerican Museum of Natural History
  • Jin Meng
    • Division of PaleontologyAmerican Museum of Natural History
  • Wenyu Wu
    • Institute of Vertebrate Paleontology and PaleoanthropologyChinese Academy of Sciences
  • Jie Ye
    • Institute of Vertebrate Paleontology and PaleoanthropologyChinese Academy of Sciences
Short Communication

DOI: 10.1007/s00114-006-0182-2

Cite this article as:
Ni, X., Meng, J., Wu, W. et al. Naturwissenschaften (2007) 94: 237. doi:10.1007/s00114-006-0182-2

Abstract

Tertiary marsupial records are very scarce in Asia. A new peradectine marsupial, Junggaroperadectes burqinensis gen. et sp. nov., is reported from the Early Oligocene Keziletuogayi Formation in the Burqin region, Xinjiang, China. This new species is based on a single right upper M2. The tooth possesses a straight centrocrista, a characteristic of peradectines. Its main cusps lean buccally, with the paracone being smaller and lower than the metacone. The conules and stylar cusps are weakly developed. These characters distinguish J. burqinensis from Euro-American Tertiary peradectines, but they also imply a close phylogenetic relationship to Siamoperadectes and Sinoperadectes, two Asian Early Miocene peradectines.

Introduction

Living marsupials are geographically limited to the Australian region and to the Americas, whereas fossil marsupials were more widely distributed. Tertiary marsupials have been recovered from all continents, including Antarctica (Woodburne and Zinsmeister 1984). Marsupials were present in most Early Tertiary micromammalian faunas of Europe and North America (Crochet 1979; Korth 1994; Krishtalka and Stucky 1983). In Asia, however, Tertiary marsupials are very scarce. Only four are named to date with four other unnamed taxa known (Ducrocq et al. 1992; Emry et al. 1995; Gabunia et al. 1984, 1990; Kappelman et al. 1996; Maas et al. 1998; Qi et al. 1996; Storch and Qiu 2002; Thewissen et al. 2001). With the exception of Asiadidelphis tjutkovae, all of these fossil marsupials are based on a single or a few isolated teeth.

In this paper, we report yet another new species of Early Oligocene marsupial from Burqin region, Xinjiang, China. Terrestrial beds on the northern bank of the Irtysh River near Burqin City (Fig. 1) consist of the lower Irtysh River Formation, a set of barren rusty-yellow, purplish red sediments, and the upper Keziletuogayi Formation, a set of yellowish green sandstones with variegated mudstones (Ye et al. 2005). Three mammalian fossil assemblages have been discovered from the Keziletuogayi Formation (Table 1), and all are correlated with the Ergilian–Shandgolian faunas in Kazakhstan and Mongolia (Wu et al. 2004; Ye et al. 2005). This new marsupial is among Assemblage 2, an assemblage that consists mainly of small mammals from sites XJ200303, XJ99031, and XJ99035 (Fig. 1). This assemblage correlates to the early Shandgolian faunas (Ye et al. 2005) and is probably earliest Oligocene in age. The specimen reported here was obtained by screen-washing matrix from site XJ99031.
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Fig. 1

Location map for Burqin mammalian fossil localities

Table 1

Mammalian fossil assemblages from the Keziletuogayi Formation, Burqin County, Xinjiang, China

 

Assemblage 1

Assemblage 2

Assemblage 3

Didelphimorphia

 Junggaroperadectes burqinensis gen. et sp. nov.

 

*

 

Lagomorpha

 Desmatolagus sp. nov.

 

*

*

Rodentia

 Prosciurus? sp.

 

*

*

 Sciuridae gen. et sp. indet.

  

*

 Karakoromys decessus

 

*

*

 Cyclomylus lohensis

 

*

*

 Cricetops dormitor

 

*

*

 Eucricetodon caducus

 

*

*

 Eucricetodonasiaticus

  

*

 Eucricetodon sp. nov.

 

*

 

 Ulaancricetodon cf. U. badamae

  

*

 Cricetidae gen. et sp. nov.

  

*

 Parasminthus aff. P. asiae-centralis

  

*

 Parasminthus tongingoli

 

*

*

 Tatalsminthus sp. nov.

 

*

*

 Propalaeocastor irtyshensis

  

*

Erinaceomorpha

 Tupaiodon cf. T. morrisi

 

*

*

 Palaeoscaptor cf. P. acridens

 

*

*

Creodonta

 Hyaenodon sp.

 

*

 

Perissodactyla

 Brontotheriidae gen. et sp. indet.

*

  

 Cadurcodon cf. C. ardynensis

*

  

 Amynodontinae gen. et sp. indet.

*

  

 Gigantamynodon giganteus

*

  

 Indricotheriinae gen. et sp. indet.

*

  

 Rhinocerotidae gen. et sp. indet.

*

  

Systematic paleontology

Taxonomy

  • Didelphimorphia Gill 1872

  • Didelphidae Gray 1821

  • Peradectinae Crochet 1979

  • Junggaroperadectes burqinensis, gen. et sp. nov.

Etymology

The genus name is derived from the Junggar Basin and in allusion to the fossil’s systematic affinities. The species name is derived from the Burqin City.

Holotype

A right upper M2 (IVPP V14410, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences).

Locality and horizon

Locality XJ99031 (47°57.501′N, 86°38.306′E, Fig. 1), north to Burqin City, Xinjiang Uygur Autonomous Region, China. Keziletuogayi Formation, earliest Oligocene.

Diagnosis

The marsupial is a small peradectine with a straight centrocrista on its upper molar, a similarity to Peradectes, Siamoperadectes, and Sinoperadectes. It differs from Peradectes in having more buccally inclined main cusps, less developed conules, the absence of the stylar cusp D, and equally developed stylar cusps B and C. It differs from Siamoperadectes and Sinoperadectes in being larger in size, having a higher crown with more developed conules and stylar cusps, and lacking a distal cingulum. It differs from herpetotheriines, such as Amphiperatherium, Herpetotherium, Peratherium, Copedelphys, and Asiadidelphis, in possessing a straight, rather than a V-shaped, centrocrista and in having a relatively broader stylar shelf with weakly developed stylar cusps.

Description

The holotype (Fig. 2) is identified as a right M2, because the M4 of marsupials is distobuccally reduced, M3 is normally more symmetrical, and M1 is more extended distobuccally with its more protruding mesiobuccal corner. In occlusal view, the M2 has a triangular outline. Its mesiodistal length measured along the buccal margin is 1.77 mm, buccolingual width along the mesial margin is 1.73 mm, and its buccolingual width along the distal margin is 2.47 mm. The mesiobuccal corner of the tooth forms a right angle, whereas the distobuccal corner extends distally at a sharper angle. The apices and crests of the three main cusps are sharp, with the apices being buccally deflected. The metacone is much higher than the paracone, whereas the protocone is slightly lower than the paracone. The protocone is robust, positioned somewhat mesiolingually, with its apex leaning towards the mesial margin of the tooth. Both paracone and metacone are slightly compressed mesiodistally. Two thick, nearly parallel crests are formed on the lingual extensions of the two cusps.
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Fig. 2

Right upper M2 of Junggaroperadectes burqinensis, gen. et sp. nov. holotype, IVPP V14410, occlusal (a), buccal (b), lingual (c), mesial (d), and distal (e) views

The preprotocrista is almost vertically oriented and extends buccally to join the paraconule. The paraconule is small and shaped as a triangular pyramid. The preparaconule crista is sharp but low. It stretches buccally and is confluent with the mesial cingulum. No postparaconule crista exists, and the paraconule is indeed not connected to the paracone. The postprotocrista is longer than the preprotocrista and stretches distobuccally, but is separated from the lingual base of the metacone by a small U-shaped notch. The buccal end of the postprotocrista is slightly swollen, but not enough to form a cusp. The preparacrista directs buccally to connect the mesial part of the stylar cusp B. The postparacrista extends distally, whereas the premetacrista extends mesially. The connection between these two crests is weak and low. Both crests constitute a mesiodistally straight centrocrista. The postmetacrista is long and high, being more distinct than the other crests, and extends obliquely to the distobuccal extremity of the tooth forming a sharp shearing blade. The preparacrista, centrocrista, and postmetacrista do not form a dilambdodont pattern, commonly seen in molars of Tertiary herpetotheriines.

The buccal stylar shelf is broad and flat, with all stylar cusps positioned on the buccal margin. The ectoflexus is narrow and shallow, and lateral to the posterior part of the paracone. Three stylar cusps exist: A, B, and C. Stylar cusp A is small, rounded, and nodule-like, with its base being merged with stylar cusp B. It is slightly higher than the mesial cingulum. Stylar cusp B is prominent and opposite the mesiobuccal part of the paracone on the mesial margin of the ectoflexus. Stylar cusp C defines the distal end of the ectoflexus opposite to the mesiobuccal part of the metacone. It is slightly larger than stylar cusp B and has an oval outline. Stylar cusps B and C are linked by a weak enamel crest on the buccal margin of the tooth. This crest continues mesially to meet stylar cusp A. The distobuccal extremity of the postmetacrista is lower than stylar cusps B and C and does not terminate as a stylar cusp. The stylar shelf between the stylar cusp C and the distobuccal extremity of the postmetacrista is open buccally. There is a small wear facet on the lingual edge of the stylar shelf, immediately buccal to the centrocrista. The buccal margin of the facet is slightly higher than the base of the stylar shelf, indicating a rudimentary mesostyle was probably present when the tooth was unworn.

The mesial cingulum is narrow and buccally connects stylar cusp A. It slightly expands mesially to the paracone where it bears a small wear facet. A shallow and narrow groove exists between the mesial cingulum and the mesial wall of the paracone. The distal cingulum is absent, so that the lower part of the distal wall of the protocone continues smoothly with the metacone.

Discussion

Except the Anatolian marsupials, all Asian Tertiary marsupials are allocated within the Peradectinae and Herpetotheriinae subfamilies. One Anatolian taxon reported by Kappelman et al. (1996), presents a very buccally oriented postparacrista and premetacrista that do not contact each other but do join at the base of the stylar cusp “C”. Its paracone and metacone are subequal in size and are separated by a narrow groove. On the buccal margin of the stylar shelf, it bears five significantly developed stylar cusps. A second Anatolian marsupial is represented by a single robust and bunodont upper molar (Maas et al. 1998). Its protocone, paracone, and metacone are all similar in height, with the labial cusps being subequal in size. The stylar cusps are conical and very large. These two Anatolian taxa display particular character combinations that differ significantly from other Asian Tertiary marsupials. They probably represent an endemic group and may have a distant evolutionary relationship with other Early Tertiary marsupials of northern continents.

A single lower molar of a marsupial from Pakistan was identified as a herpetotheriine (Thewissen et al. 2001). Unfortunately, it cannot be compared with any of the other Asian Tertiary marsupials because all, except one Anatolian marsupial, are represented only by upper molars.

Asiadidelphis from Kazakhstan is the best-known Asian Tertiary marsupial. The Late Eocene A. zaissanense and the Early Oligocene A. tjutkovae appear to be closely related elements of one lineage (Emry et al. 1995). A. zaissanense was once allocated to the Peradectinae because its preparacrista connects with stylar cusp B rather than A (Gabunia et al. 1990). Emry et al. (1995), however, pointed out that the buccal connection of the preparacrista is not a consistent feature at the species level, much less at the subfamily level. The upper molars of Asiadidelphis are significantly dilambdodont and have a dominant central stylar cusp, suggesting herpetotheriine rather than peradectine affinities (Emry et al. 1995).

The geographic occurrences of Junggaroperadectes and Asiadidelphis are very close. Faunal correlations show Junggaroperadectes to be slightly younger than A. zaissanense and about the same age as A. tjutkovae. However, significant morphological differences prevent us from considering a close affinity between Junggaroperadectes and Asiadidelphis.

Junggaroperadectes shares several similarities with two Asian Miocene peradectines, Siamoperadectes (Ducrocq et al. 1992) and Sinoperadectes (Storch and Qiu 2002). These three taxa all have strong buccally inclined paracones and metacones, significantly smaller and lower paracones relative to the metacones, and greatly reduced paraconules, metaconules, and stylar cusps. These characters distinguish the Asian peradectines from European and North American peradectines and are probably synapomorphic for Junggaroperadectes, Siamoperadectes, and Sinoperadectes. Considering its older geologic age and less reduced paraconule and stylar cusps, relative to Siamoperadectes and Sinoperadectes, Junggaroperadectes may represent an ancestral form of the Asian peradectines, a possible “intermediate form” between the Euro-American Paleogene peradectines and the Asian Early Miocene peradectines.

Siamoperadectes and Sinoperadectes both show a significant distal cingulum (Ducrocq et al. 1992; Storch and Qiu 2002), which is lacking in Junggaroperadectes. The presence of this distal cingulum is unusual among Tertiary marsupials and is totally lacking in the other peradectines although typically present in Late Cretaceous alphadontines (Storch and Qiu 2002). It has been suggested that the occurrence of this character in Siamoperadectes and Sinoperadectes may be primitive and retained from an alphadontine stock (Storch and Qiu 2002). If our suggestion that Junggaroperadectes represents an ancestral form of the Asian peradectine marsupials is correct, it suggests that the distal cingulum is virtually absent in primitive Asian peradectines. Thus, the presence of a distal cingulum in Siamoperadectes and Sinoperadectes is probably a convergence obtained through parallel or reversal evolution.

Although Asian Tertiary marsupials are extremely scarce, their teeth show a certain level of diversity by the earliest Oligocene. Asian Tertiary marsupials are unlikely the direct descendants of Mesozoic marsupials from Asia. It is more probable that they immigrated from either Europe or North America, or both, via multiple migration routes and events. (Ducrocq et al. 1992; Gabunia et al. 1990; Storch and Qiu 2002).

Acknowledgment

We thank Dr. Bi Shundong, Messrs. Su Jianfeng and Wei Yongpeng for field assistance; Profs. Daniel L. Gebo, Li Chuankui, and Qiu Zhuding for instructive and helpful comments on the manuscript. The work is supported by the National Natural Science Foundation of China (NSFC 40202003, 40472022) and the Chinese Academy of Sciences.

Copyright information

© Springer-Verlag 2006