记一新的无颌类化石兼论多鳃鱼类的分类地位

本文记述了云南宜良早泥盆世一新的无颌类化石,高棘四营鱼,新属、新种 (Siyingiu altuspinosa gen.et sp.nov)。在讨论中主要涉及鼻垂体凹的构造和多鳃鱼类的分类地位。笔者认为,多鳃鱼类和盔甲鱼类的鼻垂体孔位于鼻垂体凹的软骨成分中,它没有吸水的功能,只具感觉的功能。多鳃鱼目不应置于鳍甲鱼纲,而应放在头甲鱼纲的盔甲鱼亚纲中。在盔甲鱼亚纲中,南盘鱼不是接近于盔甲鱼目而是接近于多鳃鱼目。

ON THE POLYBRANCHIASPID AGNATHA AND THE PHYLOGENETICAL POSITION OF POLYBRANCHIASPIFORMES

The materials dealt with in this paper, named as a new form of polybranchiaspids, Siyingin altuspinosa gen. et sp. nov. were collected from the lower part of Cuifengshan Formation (Lower Devonian), Yilang district, Yunnan in 1979. The diagnosis of the new genus and type species is as follows: Head and anterior part of body covered by a cephalic shield, poster-dorsal margin of which a higher and laterally-compressed spine; Naso-hypophysial depression ellipselike in form, situated on the anterior-dorsal side of the shield; Orbital openings mediumsized, near rostro-lateral margin of carapace, behind the nasohypophysial depression; Oral opening inverted V-shaped located on the rostro-ventral margin of the head; Sensory system in dorsal plate coastituted by supraorpital system, infraorbital system and one median-dorsal commussure; Their distribution different from that of Order Heterostraci. Supraorbital system including V-shaped pineal canals that being in contact with supraorbital canals on each side by theirs ultimate canaticulns; Each infraor bital system including longitudinal lateral canal, transversal lateral canals with dichotomousends and infaoraorbital canal. Ornamentaion of carapace consising of minute polygonal tubercles. Remarks The new polybrachiaspid differ greatly from Polybranchiaspis in the form and size of the dorsal spine on the shield, the dichotomous ends of canals and the way of contact between a pineal canal and supraorbital canal. Discussion The "oral opening" explained by Liu in Polybranchiaspids was very much like naso-hypophysial opening in Eugaleaspids. from several polishs through the "oral opening" made recently by the authors, we found a developing thickened rim around the opening, on which there never was dermal cover closed. If Halstead has found occasionlly the dermal cover on the naso-hypophysial opening, then we have observed it on the orbital opening too. Therefore we think the exoskeletal fragments have been displaced during fossilization. Besids we have obsered a changing order in form from the transverse-elliptical "oral opening" in Polybranchiaspids to the Iong naso-hypophysial opening in Eugaleaspids, such as Polybranchiaspis—Huananaspis—Nanpanaspis—Yunnanaogaleapis—Sinogaleaspis—Eugaleaspis. According to the aforesaid fact, the "oral opening" of Polybranchiaspids must be still a naso-hyophysial opening. The naso-hypophysial opening in Polybranchiaspids and Eugaleaspids were much bigger than the other known forms of Agnatha. We don't think the nasal opening and the hypophysial opening could be such large when the animal lived. It is possible that in the naso-hypophysial opening of the live animal there was some cartilage that penetrated a very small nasal and hypophysial opening, the structure called a nasohypophysial opening must get a name of naso-hypophycial depresion, but the cartilage couldn't be conserved during fossilization. Then, it is impossible for the naso-hypophysial opening to lead drectly into the buccal cavity and to possess a inhalante function. We give several definite charactertics of Polybranchiaspids: the dorsal plate of head consisted of a single shield, the absence of electric fields, the way of distribution of the sensory system of the shield, the far separation of orbital openings from each other, the large naso-hypophysial depression, the situation of the pineal opening far behind orbits, the oral opening in ventral side, the numerous external branchial openings, the disposition of the interbranchial crest, the structure of the capsula auditus and the central nervous system, etc. From the abovementioned characteristics we can come to the conclution that polybranchipiformes was very much like Cephalaspides, but not Heterostaci; Among Cephalaspides, there are a lot of comon points between Polybranchiaspiformes and Eugaleaspiformes, which must be placed in a common subclass, Eugaleaspida; In comparison with Eugaleapiforms, Nanpanaspis is closer Polybranchiaspiformes.