论Yangtzechiton Yu与Paracarinachites Qian et Jiang的区别

ON THE DIFFERENT AFFINITIES BETWEEN YANGTZECHITON YU AND PARACARINACHITES QIAN ET JIANG

从窄长的形状和背侧具棘刺来看,Yangtzechiton与Paracarinchites似乎有些类似,但不同点有:(1)Yangtzechiton的壳体是由头板、中间板和尾板组成,而Paracarinachites是一个弯曲的窄长骨片;(2)前者的壳板是由盖层和连接层组成,后者的整个骨片是为多层平行的薄片层组成;(3)Yangtzechiton的头板长宽近等,背侧后缘有宽深的缺四,中间板矩形,有一椭圆形孔,可能是空心棘断损后遗留下来的,后一亮板为前一壳板覆盖约1/2,尾板的后亮顶锥形;Paracarinachites前端弯曲部分有6—8个瘤粒突起,骨片后部具10—14根左右交错排列的、末端向后、向左(右)倾斜的棘刺;和(4)前者头板腹侧有一肾形凹沟和“W”形弯曲的条纹,中间板腹侧徽凹见有叠覆缝;后者腹侧分为前端弯曲部分和后部。前端弯曲部有一近圆形的盘状体,近中部有一个孔,后部空凹。因此,它们是不同门类的代表,Yangtzechiton属于软体动物门多板纲,Paracarinachites是分类位置未定的骨片。

Yu (1984) described Yangtzechiton elongatus Yu and Luyanhaochiton spinus Yu, two ofthe earliest Cambrian polyplacophorans in the world, which have aroused attention of col-leagues both at home and abroad and have been cited in their publications. For instance, inSmith and Hoare's monograph entitled 'Paleozoic Polyplacophora: A Checklist and bibliog-raphy' (1987), Yangtzechiton elongatus and Luyanhaochiton spinus have been listed as repre-sentatives of Early Cambrian polyplacophoran fossils. Subsequently, the writer further stud-ied and reported some other earliest Cambrian chitons from the Meishucun Stage, includingTchangsichiton notabilus Yu, Runnegarochiton modestus Yu, Meishucunchiton vulgarus Yu,Gotlandochiton? minimus Yu, Yunnanoplera biformis Yu, etc. The discovery of such abun-dant polyplacophorans from the earliest Cambrian of China fully demonstrates the occurrenceof polyplacophorans in the earliest Cambrian and their origin from the Proterozoic. TheYangtze Region of China is the old home and birth place of the Polyplacophora. Paracarinachites sinensis Qian et Jiang is also one of the important fossils in the upperZhongyicun Member of the Lower Cambrian Tongying Formation at Meishucun andShangssu of Jinning, Baizai and Xianfeng of Xundian, and Dahai of Huize, eastern Yunnan;among them,the specimen obtained from the Baizai section is the best one in this area. At a glance, both Yangtzechiton elongatus Yu and Paracarinachites sinensis Qian et Jiangpossess a narrow shell, with processes or spines in the middle of the dorsal side; this is whysome scholars are apt to regard Yangtzechiton elongatus as Paracarinachites sinensis Qina etJiang (Qina & Bengtson, 1989). Some scholars even lumped together the three different gen-era Yangtzechiton, Luyanhaochiton and Paracarinachites (Kerber, 1988); while some otherscholars considered Yangtzechiton as belonging to Class Polyplacophora of the Phylum Mol-lusca but Paracarinachites to some other group of problematic fossils (He & Xie, 1989). Inorder to further expound the essential distinction between these two genera, the writerwould like to make a discussion on the following points: 1)In Yangtzechiton, the shell iscomposed of head, intermediate and tail valves in complete coverage pattern, while in Paracarinachites, it is an elongated and narrow sclerite with its adapical part strongly curved(pl. I, figs. 10, 16, 18); 2)In the former, each valve is composed of tegmentum and articula-mentum, while in the latter, the whole sclerite is composed of many lamellae arranged paral-lel to each other (pl. I, figs. 10, 11; He & Xie, 1989,pl. I, figs. 11, 12; Qina & Bengtson,1989,fig. 28); 3)In Yangtzechiton, the head valve is nearly equal in length and width, theposterior margin has a broad, deep sinus, the intermdiate valve has a greater length thanwidth, with one half of the valve overlapped by the preceding one, the middle part of the an-terior end has an elliptical hole which may be the remains of empty spines after falling off(pl. I, figs. 1, 3); while in Paracarinachites, the outer surface of the sclerite bears 5--6nodular projections in the adapical curved part and 10--14 alternately arranged spines in theposterior part (pl. I, figs. 7, 15, 17); 4) In the former, a nephroid depression appears nearthe anterior margin on the ventral side of the head valve, with a row of W-shaped curvedstriae in the posterior part of the depression, the intermediate valves are slightly concave onthe ventral side, with contiguous fissures between individual valves clearly visible (pl. I,fig.2), while in the latter, the lower side of the sclerite is distinctly divided into two parts-the adapical curved part with a subcircular dish bearing an opening in its subcentre, and pos-terior part widely concave (pl. I,figs. 8, 9, 12-14). From the above comparison, Yangtzechiton and Paracarinachites ar representatives oftwo entirely different biotic groups, with the former belonging to Class Polyplacophora ofPhylum Mollusca and the latter to some other fossil group. Observations on the morphologi-cal characterist