云南文山大窗鱼(Macrothyraspis)属一新种

记述了产于云南文山古木早泥盆世坡松冲组华南鱼类大窗鱼一新种——长矛大窗鱼(Macrothyraspis longilanceus sp.nov.)。新种头甲呈头盔形,吻突与角均很发育,中背孔呈后缘前凹的桃形,头甲背面具一对蚕豆形的背窗,纹饰为细小的粒状瘤点,这些特征与属型种——长角大窗鱼(Macrothyraspis longicornis)相似。但新种吻突特别长,约为头甲中长的2 倍,眶孔背位,两个背窗之间脑腔和迷走腔区域相对较宽,两个侧向延伸的角向后倾斜,末端不前翘等,与属型种区别明显,应为该属的一新种。

A NEW SPECIES OF MACROTHYRASPIS (GALEASPIDA, AGNATHA) FROM WENSHAN, YUNNAN, CHINA

Summary A new species of galeaspid agnathan, Macrothyraspis longilanceus, is described from the Early Devonian Posongchong Formation ( Upper Pragian) at Zhichang village, Gumu Town, Wenshan District, southeastern Yunnan, China. As a typical non-marine deposit, the Posongchong Formation in southeastern Yunnan yields abundant remains of early vascular plants, antiarchs, sarcopterygians and galeaspids, the latter including Kwangnanaspis subtriangularis (Cao, 1979) , Gantarostrataspis gengi, Gumuaspis rostrata (Wang and Wang, 1992), Macrothyraspis longicornis (Pan, 1992), Wenshanaspis zhichangensis and Sanqiaspis rostrata (Zhao et al. , 2002). Subclass Galeaspida Tarlo, 1967 Superorder Polybranchiaspidida Janvier, 1996 Order Huananaspidiformes Janvier, 1975 Family Macrothyraspidae Pan, 1992 Genus Macrothyraspis Pan, 1992 Macrothyraspis longilanceus sp. nov. (Figs. 1 -4) Etymology longus ( L. ) = long, lanceus ( L. ) = lance, in reference to the long rostral process of cephalic shield. Holotype External and internal moulds of a complete cephalic shield, IVPP V 13592. 1A, B. Referred specimens External and internal moulds of a nearly complete cephalic shield, IVPP V 13592.2A, B. Horizon and locality Posongchong Formation, Pragian, Early Devonian, Zhichang village, Gumu Town, Wenshan, southeastern Yunnan, China. Diagnosis A middle-sized galeaspid, cephalic shield armet-shaped with a small median spine at the posterior margin; rostral process lance-like, slender and about twice as long as the cephalic shield proper (i. e. the cephalic shield excluding rostral process and corners) ; corner projecting laterally and slanting backward; inner corner small but obvious; orbital opening dorsally placed and close to lateral margin of cephalic shield; median dorsal opening fairly large and peach-like; pineal opening situated slightly behind orbital openings, postpineal region of cephalic shield longer than prepineal region ( excluding rostral process) ; dorsal fenestra on the dorsal side of cephalic shield very large and broad bean-shaped; region between paired dorsal fenestrae for cranial cavity and labyrinth cavity relatively broad; sensory canal system only with anterior supraorbital canal, posterior supraorbital canal and dorsal commissure; six ~ sever pairs of branchial chambers; ornament with evenly-distributed, minute round tubercles. Remarks The type species of Macrothyraspis, M. longicornis was found from the Posongchong Formation of Guangnan, southeastern Yunnan (Pan, 1992). Additional material of M. longicornis from the Posongchong Formation of Gumu, Wenshan District, southeastern Yunnan, was studied to clarify its defining characters (Zhao et al. , 2002). The specimens described here were found from the same horizon and locality as those described by Zhao et al. (2002) , but represent a new form. The new form resembles M. longicornis in such characters as cephalic shield armet-shaped, rostral process and corners well developed, no spines along posterior margin of corner, median dorsal opening large and peach-like, paired dorsal fenestrae on the dorsal side of cephalic shield large and broad bean-shaped, and ornament with minute round tubercles. So it can be definitely referred to Macrothyraspis. However the new form is distinct from M. longicornis in characters as follows. 1 ) The rostral process of the new form is about twice as long as the cephalic shield proper, whereas the rostral process of M. longicornis is as long as the cephalic shield proper. 2) The orbital opening of the new form is situated at the dorsal portion of cephalic shield, and shows a round opening in dorsal view, whereas the orbital opening of the latter lies at the lateral margin of cephalic shield, and shows a semicircle in dorsal view. 3 ) The region between dorsal fenestrae for cranial cavity and labyrinth cavity in the new form is fairly broad, and about one-sixth of the cephalic shield in width. The region in the latter is narrow, and only one-tenth of the cephalic shield in width. 4) The corner of the new form projects laterally and slants backward. In M. longicornis, the corner projects laterally but turns forward at the end (Pan, 1992; Zhao et al. , 2002). Based on these differences, a new species of Macrothyraspis, M. longilanceus sp. nov. is erected.