辽宁大连晚更新世马类牙齿釉质结构的研究

ENAMEL STRUCTURE OF THE EQUID CHEEK TEETH FROM THE LATE PLEISTOCENE OF DALIAN, LIAONING

大连晚更新世的马类共包括三个种类:野驴(Equus hemionus)、小型的普氏野马中国马亚种(E. przewalskyi sinensis)、大型的大连马(E. dalianensis)。从它们颊齿的釉质层微观结构、釉柱大小及釉质层的厚度分析,晚更新世的野驴和现生的野驴是十分接近的,可以归于同一个种。大连马与现在的普氏野马也比较接近,可归于同一个大的类别。然而,普氏野马中国马亚种却与现生的普氏野马有着比较显著的区别:中国马颊齿釉质层的釉柱平均直径比现生的普氏野马大得多,而釉质层的厚度又比现生的普氏野马更薄,所以,原来的分类未必合适,中国马似不应归于普氏野马,它可能代表一个比较特殊的种类。

Fossils of three species of Equus are known in the Gulongshan Cave Site at Dalian, i. e. E. hemionus (the wild ass), E. przewalskyi sinensis (the small wild horse) and E. dalianensis (the large wild horse) Though the small wild horse is close to the living species E. przewalskyi in size of the body and metapodials, their upper and lower cheek teeth are smaller than those in the living species E. przewalskyi. These characters can distinguish the small wild horse from the living species E. przewalskyi. Therefore the small wild horse was tentatively named E. przewalskyi sinensis in 1990. We have recently observed the enamel structure of the cheek teeth of E. przewalskyi sinensis, E. dalianensis. E. hemionus, and the living species (E. przewalskyi caballus and E. hemionus) with Scanning Electron Microscope (SEM). This paper reports the result of a SEM study of the enamel structure of the equid forms mentioned above. 1. Method For comparision, we have selected M_3 or M_1 accordingly, from the crown, and used the Protoconid and Hypoconid positions. Three sections were examined, as shown in Fig. 1. 2. Observation and Result The study of the Longitudinal Facet and the Transverse Facet enabled us to gain the pictures that reflect the pattern of the arrangement of enamel rods. Lets see Plate Ⅰ, Ⅱ and Ⅲ. (1) Observation of the Longitudinal Facet Pl. Ⅰ, 1, 2 show the structure of buccal lateral and lingual lateral enamel of Hypoconid. The structure of the Longitudinal Facet of the Protoconid enamel has been depicted before (Ouyang Lian and Zhang Wending, 1988). From Pl. Ⅰ, 1, 2 we can discover that both the buccal lateral and lingual lateral enamel have two layers: the inner layer (near detain), indicated by arrow A, and the outer layer (near surface), indicated by arrow B. We magnify the graphs of position A and B. The enlargement of A is like P1. Ⅰ, 3, 4, a sawtooth and crisscross pa'ern, telling the cross growing process of enamel rods in the inner enamel. From the enlargement of B we can see that the rods in the buccal lateral outer layer show a parallel pattern but the lingual lateral outer layer has the Hunter-Schreger bands, as shown in P1. Ⅰ, 5 and Pl. Ⅱ, 1 respectively. (2) Observation of the Transverse Facet The enamel structures are of two layers in both buccal and lingual lateral. The inner part has a pattern unlike the uniserial enamelpattern of rodents. The rods cross each other approximately at right angle, forming the approximate Hunter-Schreger bands with only a rod's width, as shown in P1. Ⅱ, 2, local enlargement of P1 Ⅲ, 2. The outer part has radial pattern which can be seen in P1. Ⅲ, 1. The enamel structure described above is common in horses which indicates their relation in evolution. (3) Observation of the Tangential Facet We studied the Tangential Facets of different depth from the enamel surface at the radiated part of buccal lateral and got the pictures in different depths: on the surface the rods present a mainly random pattern, sometimes show a conic pattern; on the section 60 μm from the surface the enamel rods show mainly hexagon pattern (The former two kinds are called Pattern Ⅰ, see P1. Ⅳ 1—5); on the sections 100—200 μm from the surface the rods have the pattern shown in P1. Ⅴ which are called Pattern Ⅲ. (4) The average hexagon diameters of the enamel rods of different horse tooth are given. Table Ⅰ shows the differences of values in each species. The differences can also be seen in P1. Ⅴ in which the figures are all 2100 times enlarged. (5) Enamel Thickness Measurement Besides distributed in buceal and lingual lateral, a rather larger part of the enamel is rolled into the dentine and the thicknesses of the enamel can be very different in the same tooth. For comparison, the thicknesses were all measured at the buccal lateral position as shown in Fig. 1. The results are listed in Table 2. The values are also different in each species. 3. Conclusion (1) The average diameter of enamel rods and the width of buccal lateral enamel of E. hemionus in both the Late Pleistocene and the living species is quite similar (Table 1 and 2). So both fossil and living forms belong to the same species, i. e. E, hemionus. (2) The average diameter of enamel rods and the width of buccal lateral enamel of E. dalianensis is close to the data of the living species E. przewalskyi (Table 1 and 2). Perhaps the relationship between the two species may be ancestor-descendant one. (3) The thickness of buccal lateral enamel of the small wild horse (E. przewalskyi sinensis) is much less than that in the other species (Table 2). And the average diameter of enamel rods of the small wild horse is much longer than that of the other species (Table 1). It seems that the small wild horse probably do not belong to E. przewalskyi and should be considered as a new species.