云南曲靖早渐新世真古仓鼠化石

本文记述了在云南曲靖蔡家冲地区下渐新统蔡家冲组中首次发现的真古仓鼠化石: Eucrtcetodon meridionalis sp. nov. 和 E. leptaleos sp. nov. 认为仓鼠可能在亚洲由类似 Cocomys 的祖先类型起源的; Eucricetodon 在渐新世时广布欧亚大陆;欧洲的 Eucricetodon 是在"大间断"之后由亚洲迁去的; Simimys 代表从仓鼠科基部分出的旁枝, 应归入 Muroidea.

EUCRICETODON (RODENTIA, MAMMALIA) FROM THE LOWER OLIGOCENE OF QUJING, YUNNAN, CHINA

In the winter of 1980, while visiting the classical Lower Oligocene Caijiachong locality, Yunnan, the senior author discovered some Eucricetodon teeth. They represent the first record of that genus in the Early Oligocene of Asia. In 1982 and 1983 more specimens were collected, so that the total number of isolated cheek teeth, which are dealt with in the present paper, amounts to 61. Two new species are described: Eucricetodon meridionalis and E. leptaleos. Eucricetodon meridionalis sp. nov. Holotype LM~1 (V 7952. 1). Diagnosis In size elose to Eucricetodon atavus; teeth brachyodont, with main cusps blunt and transverse lophs short; M~1 wide, its anterior lobe very small, anterocone single-cusped, anterolophule usually very developed; metaloph transverse, mesostyle distinct and mesolophid developed. M_1 about equal to M_2 in length, with narrow and small trigonid, with low and small anteroconid which stands closely to the protoconid and metaconid; anterior cingulum weak; on M_(2-3) posterior arm of protoconid longer than mesolophid; M_(3~3) less reduced. Comparison The simple structure of the cheek teeth (blunt main cusps, convex external border of M~1 and indistinct metacone of M~3) suggests that the specimens in question should be referred to Eucricetodon, rather than to Eumys or Pseudocricetodon. In Eucricetodon the new species is more similar to Eucricetodon atavus, However, it possesses some distinctive characters: M~1 is less enlarged relative to M~2 and possesses very small anterior lobe, single-cusped anterocone and well developed anterolophule; M_1 is about equal ta M_2 in length, with less developed anteriot cingulum and small anteroconid situated closely to the protoconid and metaconid. These characters render the new species distinet not only from E. atavus, but also from the other species from Europe. Furthermore, E. meridionalis differs from E. atavus in possessing more developed mesostyle and mesoloph on upper molars, but lacking metalophid I on M_1 and less degree of reductian of M~(3_3) etc. Among the four Asian Eucricetodon species E. schaubi is more similar to E. meridionalis. But the former is larger in size and possesses better developed lophs and larger entoconid on M_3. Eucricetodon leptaleos sp. nov. Holotype LM~1 (V 7953. 1). Diagnosis Size small, teeth with sharper ridges than in Eucricetodon meridionalis; M~1 with isolated anterocone; metaloph extends foreward to join anterior arm of hypocone; on M~1 sinus shallow and symmetric; mesostyle absent and mesoloph short on upper molars; on M_1 anteroconid very small, without anterolophulid; mesolophid very weak or even absent on lower molars; posterior parts of M~(3_3) reduced. Comparison Eucricetodon leptaleos differs from E. meridionalis in its smaller size, in having sharper ridged teeth, long transverse lophs, in lacking mesostyle, possessing short and weak mesoloph; M~(1_1) with isolated anterocone (id), lacking anterolophule (id); and in having low and weak mesolophid and ectolophid and more reduced M~(3_3) etc. E. moguntiacus differs from E. leptaleos in possessing larger anterecone on M~1 more developed mesoloph and some accessory structure, and less reduced M_(3~3). E. schaubi differs from our species in being of larger size and in having more developed lophs. Discussion Almost all the Asian Paleogene cricetids, except Cricetops and Selenomys, used to be referred either to Cricetodon of Europe or Eumys of North America. In 1966 Thaler first proposed to erect a new genus Eucricetodon for the European Oligocene and Early Miocene Cricetodon. This has raised the question of how to deal with the Asian Crieetodon. In recent years the general tendency has been not only to reassign to Eucricetodon the Asian Paleogene Cricetodon but also the Asian Eumys (Vianey-Liaud, 1972, p. 40; Lindsay, 1977 and 1978; Li et Ting, 1983, p. 47). The discovery of Eucricetodon in Qujing suggests that Eucricetodon spread to Eurasia during Oligocene and Early Miocene time, but no real remain of the North American Eumys has ever been found in Asia during that time. Eucricetodon from the Lower Oligecene of Qujing with its many primitive characters may represent a primitive member of the genus, Based on its characters some aspects of the evolutionary transformation of the early cricetids may be inferred. These may be listed as follows: 1. The size of M~(1_1) increased progressively; 2. Anterior lobe and anterocone changed from small to large, then the anterocone becane separated into double cusps; 3. Protoloph joins the anterior arm of the protocone in primitive form, while in progressive ones it joins the posterior arm of protocone; 4. Anteroconid of M_1 became increasingly larger. It seems that the primitive cricetids possess an M~1 lacking anterocone and anterior lobe and an M_1 lacking anteroconid. Since the earliest ericetids as well as dipodoids have been so far discovered from the Late Eocene of China, it appears that the two groups might have their origins in Asia and appeared probably not later than the Middle Eocene or even earlier. The ear- liest rodent in Asia is Cocomys (Cocomyidae, Ctenodactyloidea) from Hengdong, Hunnan, China. Cocomys is quite distinct in Showing a series of primitive characters, such as a protrogomorphous skull, still retaining p~3 and P_4, the progressive enlargement of the cheek teeth from P_4-M_3, the metalophid I forming an anterior wall, a prominent hypoconulid, etc. Neither Cricetidae nor Dipodoidea shared the above liSted characters of Cocomys. However, Cocomys and Dipodoidea share one common character: P~4 with single buccal cusp, which is quite different from that in Sciuravidae, Theridomyidae and Isehyromyidae. It is not improbably that cricetids and dipodoids derived fron a Cocomys-like ancestor, through losing P~3 and P_4, adding anterior cingulun and by changing the skull structure, etc. It may be that Cocomys represents their, common ancestor. Unfortunately no intermediate forms have ever been found. Eucricetodon might have immigrated from Asia into Europe after the "Grand Coupure". As to the taxonomic position of Simimys, which has been a long standing dispute, Wood (1980, p. 56) regarded it as a new family, the Simimyidae, which he included in the Dipodoidea. At about the same time Emry (1981) placed Simimys in the Muroidea incertae sedis. If our hypothesis as to the nature of the evolutionary transformations in the early cricetids, as cited above, is true, then the primitive cricetids may have possessed M~1 without an anterior lobe and anterocone. This is just the ease in Simimys which may represent the primitive stage of the cricetids. Furthermore, Simimys and cricetids sharc the following synapomorphies: The both lack P~4, i.e. their dental formula is 1003/1003, which is different from that of Dipodoidea. We agree with Emry (1981) concerning the allocation of Simimys to the Muroidea.