记真盔甲鱼类两新属——兼论真盔甲鱼类系统发育关系

本文记述了早泥盆世真盔甲鱼类两新属:憨鱼属(Nochelaspis)和翼角鱼属(Pterogonaspis).真盔甲鱼类是盔甲鱼类的一个单系类群,现有七属十二种,文中运用分支系统学原理对其系统发育关系进行了初步探讨.

TWO NEW EUGALEASPIDS, WITH A DISCUSSION ON EUGALEASPID PHYLOGENY

Two new Early Devonian eugaleaspids from Qujing, Yunnan in South China are described here. One is Nochelaspis maeandrine gen. et sp. nov., collected from the lower part of Xishantun Formation (early Early Devonian), the other Pterogonaspis yuhaii gen. et sp. nov. from Xujiachong Formation (late Early Devonian). Nochelaspis, the largest eugaleaspid so far described, is an element of the Early Vertebrate fauna in Xishancun Formation (Zhu, 1991), which includes polybranchiaspids, paleoniscoids, crossopterygian, arthrodire, antiarchs, petalichthyids (Zhu, dissertation; in preparation). Pterogonaspis, the huananaspid-like eugaleaspid, occurred in the same bed with Eugaleaspis xujiachongensis, Sanchaspis magalarostra, yunnanolepids and plant remains. Both genera throw lights on the phylogeny of eugaleaspids and galeaspids. Eugaleaspids are assumed to be a monophyletic galeaspid group by (1) the eugaleaspid pattern lateral line system and (2) the longitudinal oval median dorsal opening (if the median dorsal opening of Nanpanaspis is due to the parallelism). By now, there have been 7 genera including 12 species of eugaleaspids described, ranging from Middle Silurian to Early Devonian (Fig. 3). The character analysis of eugaleaspids has been done in view of Cladistics. With the principle of parsimony, a new testable cladogram is established (Fig. 5). Systematies Subclass Galeaspida Tarlo, 1967 Order Eugaleaspiformes (Liu, 1965) Liu, 1980 Nochelaspis gen. nov. Etymology From nocheles=slow, sluggish, retarded (Gr.); lepis=scale (Gr.). Type species Nochelaspis maeandrine sp. nov. Diagnosis Large sized eugaleaspid, cephalic shield triangular in shape, broader than long; cornua developed, spine-like; inner cornua very developed, leaf-like in form, its end extending slightly behind the end of the cornua; the breadth between the inner cornua shorter than the length of the inner cornua; median dorsal opening longitudinal slit-like,but its posterior margin in front of the level of the anterior margin of the orbital openings; eugaleaspid pattern of sensory line system with three pairs of lateral transverse canals; tubercular dermal ornanqent. Remarks The new genus has the longitudinal slit-like median dorsal opening. With re- gards to this character state, the new genus is similar to Eugaleaspis and Tridensaspis, but differs distinctly from the latter in its very developed inner cornua. Moreover, in Nochelaspis the posterior end of the slit-like median dorsal opening lies in front of the level of the anterior margin of the orbital openings, while in Eugaleaspis and Tridensaspis, the posterior end of the median dorsal opening lies between or behind the orbital openings. On the whole, the new genus is the most suggestive of Yunnanogaleaspis among the eugaleaspids so far described. But at least five differences can, be seen between them: (1) the median dorsal opening of Y. is longitudinal oval-like, shorter than that of Nochelaspis, which is slit-like; (2) both have developed inner cornua, but the inner cornua of N. lear-like, is much larger than that of Y.; (3) in N., the posterior end of inner cornu extends behind the posterior end of the cornu,where in y.,the posterior end of inner cornu lies in front of the posterior end of the cornu; (4) the breadth between the inner cornua is smaller than the length of the inner cornua in N., in contrast, the breadth between the inner cornua is bigger than the length of the inner cornua in Y.; (5) the cephalic shield of N. is broader than long, while the caphalic shield of Y. is longer than broad. Nochelapis maeandrine sp. nov. (Fig. 1; Pl. I-II) Etymology From maiandros=mealder, wander, roam (Gr.). Holotype V10106, a complete cephalic shield. Locality and Horizon Liaojiaoshan hill, Qujing, Yunnan; Lower part of Xishancun Formation (Lochkovian), Cuifengshan Group. Diagnosis As for genus (the only speces). Pterogonaspis gen. nov. Etymology From pteron=wing (Gr.), gonus=corner(Gr.), aspis (Gr.)=shield. Type species Pterogonaspis yuhaii sp. nov. Diagnosis Eugaleaspid with long rostral process and lateral slender cornua; inner cornua developed; median dorsal opening longitudinal slit-like, extending posteriorly between the orbital openings; eugaleaspid pattern sensory line system with three lateral pairs of transverse canals; ornament with small tubercles. Remarks This new genus has longitudinal slit-like median dorsal opening and typical eugaleaspid pattern sensory line system and can be referred definitely to Eugaleaspiformes. Among the eugaleaspids, it is the most suggestive of Tridensaspis (Liu, 1986) with its rostral process and lateral projecting cornua. But, it is difficult to make the crucial contrasts between Pterogonaspis and Tridensaspis because the cepahlic shield of T. is poorly preserved, especially its posterior half. By far, three di,ffernces could be seen between them. First, the median dorsal openig of T. has a pointed anterior end and round posterior margin, and is more or less spindie-like in form; while the median dorsal opening of P. has two parallel lateral margins,more similar to that of Eugaleaspis. Second, the median dorsal canal of T. is longer than that of the new genus relatively. Third, the caphalic shield of T. with a relatively large orbital openigs is smaller than that of Pterogonaspis. Pterogonaspis yuhaii sp. nov. (Fig. 2; Pl. III) Etymology After Dr. Liu Yuhai who described the first three genera of the galeaspids (Eugaleaspis, Nanpanaspis and Polybranchiaspis). Holotype V10105, a relatively complete cephalic shield. Loeality and Horizon Xujiachong, Xishan, Quqing, Yunnan; Xujiachong Formation, Cuifengshan Group, Early Devonian. Diagnosis as for genus (the only species).