记陕西蓝田晚中新世灞河组4种跳鼠(Dipodidae,Rodentia)化石

描述了陕西蓝田晚中新世早期灞河组的4属4种跳鼠:蓝田原跳鼠(新种)(Protalacta- ga lantianensis sp.nov.)、副跳鼠(未定种)(Paralactaga sp.)、原始三趾心颅跳鼠(新种)(Sal- pingotus primitivus sp.nov.)和小五趾心颅跳鼠(新种)(Cardiocranius pusillus sp.nov.)。蓝田原跳鼠兼有原跳鼠和副跳鼠的特征,可能是原跳鼠向副跳鼠进化过程中产生的一种过渡类型。副跳鼠和原始三趾心颅跳鼠及小五趾心颅跳鼠分别代表了副跳鼠属在中国的最早记录和心颅跳鼠亚科(Cardiocraniinae)在地史上的首次出现。4种跳鼠指示了蓝田地区当时的自然环境可能比现代更加干旱。

NOTE ON FOUR SPECIES OF DIPODIDS (DIPODIDAE, RODENTIA) FROM THE LATE MIOCENE BAHE FORMATION, LANTIAN, SHAANXI

Summary Although discussion on the phylogenetic classification of jumping mice, birch mice and jerboas is still ongoing, we follow the opinion that the two families Zapodidae and Dipodidae are included in the superfamily Dipodoidea and four subfamilies in the family Dipodidae ( Qiu, 1996; Daxner-Hock, 1999), i. e. Allactaginae Vinogradov, 1925, Dipodinae Fischer de Waldheim, 1817, Cardiocraniinae Vinogradov, 1925 and Euchoreutinae Lyon, 1901. The oldest record of Dipodidae is the genus Protalactaga from the middle Miocene of Quantougou, Gansu and Tunggur, Nei Mongol, which might be derived from a Plesiosminthus-like Oligocene Zapodidae ancestor (Young, 1927; Qiu, 1996, 2000). At the present, Dipodidae are restricted in the Palearctic Region. They are adapted to an arid environment and distributed in semi-desert or desert regions of northern Africa, southern Europe, central and northern Asia. In China, there are 7 genera and 13 species mainly living in the arid northwestern regions (Wang, 2003). The dipodids described below were collected by the scientists of the Sino-Finish cooperative project in vertebrate paleontology and stratigraphy from Lantian, Shaanxi, during the field seasons of 1997 -2000. Protalactaga Young, 1927 Protalactaga lantianensis sp. nov. (Fig.1) Etymology Named after the Lantian area, from where the new species was collected. Holotype A left Ml; IVPP V 14435. Type locality Loc. 12, Lantian County, Shaanxi Province (China). Stratum typicum Bahe Formation, early Baodean, early late Miocene. Paratypes Loc. 12: 44 isolated teeth (5P4, 6M1, 8M2, 4M3, 7ml, 9m2, 5m3) , IVPP V 14436. 1 -44; Loc. 19: 2 M2, V 14436.45 -46; Loc. 6: anterior part of a damaged m2, V 14436.47; Loc. Ms 14: 1m2, V 14436.48. Diagnosis Large-sized Protalactaga with bunodont-lophodont teeth. On Ml -2 protoloph posteriorly-directed and close to mesoloph; metaloph remarkably postero-situated; posteroloph short and posterolingually-directed. On ml - 2 hypolophid anteriorly-directed and close to mesolophid; M3 and m3 relatively more simplified and reduced. Remarks The specimens are referred to Protalactaga due to possession of the following characters: small size; the brachyodont and bunolophodont cheek teeth; the weak anteroloph and anterocone, and the separate protoloph from the mesoloph on Ml ~ 2; the separate mesolophid from the hypolophid on ml ~2; the strong ectomesolophid of ml. Three species of Protalactaga have so far been recognized. They are P. grabaui and P. major of middle Miocene from Gansu and Nei Mongol of China, and P. moghrebiensis (Jaeger, 1977) of middle Miocene from Jebilet of Morocco. Protalactaga lantianensis sp. nov. can be distinguished from the three known species in the protoloph and metaloph posteriorly directed on Ml ~2, the hypolophid anteriorly directed on ml ~2. In addition, the new species differs from P. grabaui in its distinctly larger size,the un-isolated main cusp on P4 and the stronger lophs, the reduction or absence of mesoloph ( id) on M3 and m3. Compared with P. major, it has more robust and rounder cusps on cheek teeth, weaker mesoloph(id) on M3 and m3 than P. major. Its cusps and lophs are stronger than those of P. moghrebiensis. In P. grabaui, the protoloph on Ml transversely connects to the endoloph, while on M2 transversely connects to the protocone; on Ml of P. major and P. moghrebiensis, it slightly posterolingually connects to the endoloph, while on M2 transversely connects to the endoloph; however, on M1 of P. lantianensis, it distinctly posterolingually connects to the endoloph or the base of mesoloph; while on M2 connects to the base or middle of the mesoloph. The metalophs on Ml ~2 of all known species transversely connect to the hypocones or slightly posterolingually connect to the hypoconules; while in P. lantianensis they distinctly posterolingually connect to the hypoconules or posterolophs. The hypolophids on ml ~2 of the three known species do not directly connect to the mesolophids, while in P. lantianensis they directly anterolabially connect to the base of the mesolophids. The directions of the protolophs and metalophs on Ml ~2 and the hypolophids on ml ~ 2 of P. lantianensis are similar to those of Paralactaga, but the protolophs and metalophs on Ml ~ 2 of Paralactaga posteriorly connect to the middle of mesolophs and posterolophs, respectively; the hypolophids on ml ~2 anteriorly connect to the middle of the mesolophids. In some teeth, the mesoloph on M3 and mesolophid on m3 are absent, consequently the number of the sinuses of these teeth decrease, which differ from those of P. grabaui and P. major, but are similar to P. moghrebiensis and Paralactaga. The features similar to Paralactaga, namely the distinctly posteriorly oriented protolophs and metalophs on Ml ~2, the anteriorly oriented hypolophids on ml ~2, the reduction of the third molars, are interpreted as derived characters in Protalactaga. It is likely that P. lantianensis represents a progressive species of Protalactaga at the intermediate stage between Protalactaga and Paralactaga. "Paralactaga minor" from Loc. 80007 of Tianzhu, Gansu (Zheng, 1982) is similar to P. lantianensis in size and morphology. A minor difference includes the direct connection of the hypolophid to the middle part of mesolophid in the m2 of "P. minor". The smaller size, lower crown and less robust cusps and lophs seem to group "P. minor" into the genus Protalactaga. Paralactaga Young, 1927 Paralactaga sp. (Fig. 3.1) Material A left M3, IVPP V 14437. Locality Loc. Ms 36, Lantian County, Shaanxi Province (China). Remarks The M3 is larger than that of all known species of Protalactaga in size (Fig.4) and lacks a mesoloph. Accordingly, we refer it to Paralactaga rather than Protalactaga. Differences of the M3 from that of other known species of Paralactaga include the narrower than length proportions, the distinctly developed anteroloph relative to the paracone, and the narrower sinus. The morphology shown in the M3 is considered primitive characters of the genus. Paralactaga. If our determination is correct, this single M3 represents the earliest appearance of Paralactaga. Salpingotus Vinogradov, 1922 Salpingotus primitivus sp. nov. (Fig.3.2,5.2) Etymology Primitivus (Latin) means original. Holotype A left ml; IVPP V 14438. Type locality Loc. 19, Lantian County, Shaanxi Province (China). Stratum typicum Bahe Formation, early Baodean, early late Miocene. Diagnosis Small-sized Salpingotus with weaker lophs related to cusps, and relatively short ectolophid on ml. Remarks This ml falls into that of Cardiocraniinae in size and structure: tiny size and Dipus-like pattern of molar. As a kind of dwarf jerboa, Cardiocraniinae includes so far only two extant genera Cardiocranius Satunin, 1903 and Salpingotus Vinogradov, 1922. Review of the extant specimens of Cardiocraniinae shows that ml of Salpingotus differs from that of Cardiocranius in its longer and narrower outline with less anteriorly situated metaconid relative to protoconid, and the remarkably elongated ectolophid ( see Fig. 5). The morphology of this ml is more similar to that of Salpingotus than Cardiocranius. Differences of the ml from that of the extant S. kozlovi are the weaker cusps and lophs and narrower posterosinusid. Fig. 6 shows that the fossil tooth is definitely smaller than that of S. kozlovi. In view of its extremely small size and highly resemblance to that of S. kozlovi, the tooth is treated as a new species of the genus Salpingotus. Cardiocranius Satunin, 1903 Cardiocranius pusillus sp. nov. (Fig.3.3,5.4) Etymology Pusillus (Latin) means tiny. Holotype A right ml; IVPP V 14439. Type locality Loc. 19, Lantian County, Shaanxi Province (China). Stratum typicum Bahe Formation, early Baodean, early late Miocene. Diagnosis Small-sized Cardiocranius with weaker lophs related to cusps, and remarkable narrower anterior portion on ml. Remarks Compared with the only two genera of Cardiocraniinae, this right ml differs from that of Salpingotus in having a distinctly anteriorly situated metaconid relative to the protoconid, and a more developed hypoconulid. In morphology, it is rather closely similar to the ml of the extant Cardiocranius paradoxus, but differs from the latter in its weaker cusps and lophs, and narrower anterior portion of the tooth. In view of its extremely small size ( Fig. 6) and resemblance to that of C. paradoxus ( Fig. 5) , it is considered to be a new species which is referred to the genus Cardiocranius. Conclusions The dipodids from the Lantian assemblage include four taxa Protalactaga lantianensis sp. nov. , Paralactaga sp. , Salpingotus primitivus sp. no. , and Cardiocranius pusillus sp. nov. The former two belong to the subfamily Allactaginae, while the latter two are attributed to the subfamily Cardiocraniinae. Salpingotus primitivus sp. nov. , and Cardiocranius pusillus sp. nov. are the first known fossil records of the subfamily. P. lantianensis is the most advanced species of the genus Protalactaga, representing an intermediate form between Protalactaga and Paralactaga. Paralactaga sp. might be a primitive species of this genus. It is difficult to assess a more precise age of Bahe Formation using the dipodids, because of the inadequate knowledge of biochronology for these animals. The presence of Protalactaga lantianensis, however, suggests that Bahe Formation is later than the later Middle Miocene, because the Lantian sample seems to be the youngest for its advanced dental characters, while the three known species of the genus Protalactaga are of Middle Miocene age. An earlier Late Miocene age is also suggested by the presence of Paralactaga sp. , which is close to P. suni from the later Late Miocene Ertemte, Nei Mongol. Allactagines and cardiocraniines are widely distributed in temperate arid area and adapted to the desert and semi-desert environments. Generally, they form a typical desert rodent fauna together with gerbils ( Ma et al. , 1987; Zho