记江西一新的中华弓鳍鱼化石

NOTE ON NEW SINAMIA FROM JIANGXI, SOUTHEAST CHINA

本文记述了在江西弋阳石溪组发现的一中华弓鳍鱼。其形态特征虽与华北的师氏中华弓鳍鱼很相似,但又有很明显的差别,因此建立一新种——Sinamia poyangica, sp. nov.。在此基础上,对中华弓鳍鱼的形态变异和石溪组的时代作了讨论,并对师氏中华弓鳍鱼的复原图提出了修正的建议。

The specimens of the sinamiid fishes described here were collected by Chang Limin and Li Haochang of East China College of Geology from Yiyang Xian, N. E. Jiangxi. All specimens, about 21 individuals, were sent to the IVPP for determination, and are reffered to a new species, S. poyangica, of Sinamia. It attains a length of about 120 to 270 mm, having elongate fusiform body. The maximum depth of the body, which about 1/4 in length of the body, is almost equal to the length of the head including opercular apparatus. The length of the head, which is longer than its depth, is about 1/3.9 in the length of the body. The measurements of the holotype are given in the text. The dermal bones of the skull roof and cheek area exhibit the characters of Sinamia. The rostral is small and V-shaped. The frontals are very large. The parietal is a large unpaired plate. The supratemporals are numerous. The orbit is of moderate size. The supraorbitals are numerous (5—7). The two infraorbitals behind the orbit are small, of which the upper one appears to be much smaller. There is also a wide gap among them and the preopercular, resembling that in Sinamia. The mouth is large and the jaws long. The premaxillary and maxillar are very strong, the former has developed ascending process, the posterior part of the latter deepening gradually. A small low supramaxillary is present on the porterior part of the maxillary. The mandibular consists of dental-splenial, angular and supraangular. The oral borders of both upper and lower jaws bear large and styliform teeth. The opercular apparatus resembles that in Sinamia, the operculum is a large four-sided plate, which is higher than broad. The subopercular is much smaller than the opercular. The preopercular is high and narrow, and almost crescent-shaped. The interopercular is very small and trianglar in shape. The branchiostegal rays are imperfectly preseved, which are observed 10 in number in specimen P012, and are rather robust. The median gular plate is large and almost isoceles triangular. All dermal bones of the head covered with thick enamel, and with developed ornamentation. The prominent ornamentation of the skull-bones make the interpretation of the sensory canal system of the head difficult, but judging from its impression, it essentially resemble that in Sinamia zdanskyi. The structure of the hyomandibular and parasphenoid resemble those in Sinamia zdanskyi. The present specimens show that parasphenoid is destitute of teeth, its posterior end is deeply notched in its median part for aorta, which is just like the hypothesis of Stensio (1935). The centra of the trunk, which can be seen in specimen P003, are higher than length, with lateral ridges and pits. The dermal shoulder girdle, which covered with thick enamel and ornamented with enamel striae, is very developed. The post-temporals are very large and almost triangular in shape. These two bones have wavy borders. The supracleithrum is comparatively low and very robust. The cleithrum is rather narrow, with a dorsal and a broad ventral limb. There are two postcleithrum plates, of which the upper one is large and deep, the lower one is much smaller and almost triangular in shape. The characters of the paired and unpaired fins resemble those in Sinamia. The pectoral fin rather long, with 9 lepidotrichia. The pelvic fin is small, with 7 lepidotrichia. A basipterygium can be seen in the holotype, which is dumbbell-shaped in outline, resembling that in lkechaoamia. The dorsal fin is very long, which occupies more than one half length of the back, consisting of 28 lepidotrichia, each of them situated from one another fairly far apart. The anal fin is short based, with 9 lepidotrichia. The caudal fin is abbreviate hemiheterocercal, with a convex hinder border, and consists of Ⅱ+11+Ⅱ lepidotrichia. The bases of the epaxial and hypaxial lobes bear basal fulcra. The scales are rhombic and covered with thick enamel on their exposed area. All scales are lower than long, destitute of peg and socked articutioh, having internal medial ridges generaly. The exposed area of the scales is almost smooth. The flank-scales have serrated hind borders. The total number of transverse scales is about 48, with,about 25 rows in maxmum depth of the body. As described above, Sinamia poyangica (sp. nov.) is very similar to Sinamia zdanskyi and S. huananensis, but it differs from the latter two species in following characters: Dermopterotics relatively broad and short, its anterolateral extension comparatively short and obtuse; Dermal bones of skull roof, opercular apparatus and dermal shoulder girdle ornamented with prominent and radiant ridges of enamel and tubercles of enamel; Opercular and subopercular with radiating enamel ridges running to posterior borders, which obtusely pectinated; Premaxillary, maxillary, dental-splenial and dermopalatine with large styliform teeth, closely set; Hind borders of cleithrum generally with serrations. In addition, this new species also differs from Sinamia zdanskyi in its shorter and higher head, pronounced snout and longer pectoral fins. It also differs from S. huananensis in its more lepidotrichia of dorsal fin (only 22—23 in S. huananensis) and its flank-scales with serrated hind borders. Here it should be pointed out that some of the dermal bones in Sinamia poyangica as well as in S. zdanskyi show much variation in the various individuals. For instance, the supratemporal and supraorbital series of bones show variation both in number and shape; the lateral marginal process on each side of the parietal exhibits variation in the size; the anterior process of the parietal displays variation in the length. All these indicate a rather great range of the variation in the various individuals and species of Sinamia. Therefore, we consider that those variation should not be considered as specific diagnosis of Sinamia. Sinamia poyangica occurs in Shixi Formation of Xinjiang Basin, N. E. Jiangxi, the age of the fisk-bearing beds is considered as Early Cretaceous.