盔甲鱼类Antiquisagittaspis cornuta(新属、新种)在广西六景下泥盆统的发现

本文记述了盔甲鱼类三岔鱼科 (sanchaspidae) —新属 Antiquisagittaspis 并对含鱼层时代作了对比;同时对中背孔和盔甲鱼类的生活环境作了评述.

A GALEASPID (AGNATHA), ANTIQUISAGITTASPIS CORNUTA GEN. SP. NOV., FROM LOWER DEVONIAN OF GUANGXI, CHINA

The here described fossil of huananaspid galeaspid, Antiquisagittaspis cornuta gen. et sp. nov., was collected from Lower Devonian Nagavlin Formation of Lujing, Hengxian, Guangxi. Occurred in the sanme bed are another huananaspid, Asiaspis expansa, and some brachiopod and mollusk shells. The new form, estimated beyond 300 mm in the length of the dorsal shield, is the biggest of known galeaspids. It is based on an imperfect dorsal shield, witheut right half, the rostral part and Posterior mediodorsal spine. Despite of this, the shield could be essentially restored in dorsal view (fig. 1). That the lateral margins of the shield in front of the orbits run forward convergcntly rather than archedly suggests that a rostral process Were present. The cornua are with long bases and might be quite strong . They are anteriorly placed in the Position, situated at about the midway from the Posterior margin of the mediodorsal opening to the end of the lateral posterior spine. The maximum breadth of the shield is across the cornea. From there both folrward and backward the width of the shield reduces. Behind the cornua the lateral margins of the shield convergently run backward and smoothly continue with the lateral margines of the lateral posterior spines. On each side of the shiel4 a lateral posterior spine is strongly develped. The Posterior mediodorsal spine is poorly preserved, but its upward arched base hints that it might be very strong. The mediodorsal opening, preserved in the left part, could be on the symmetrieal principle restored as a cirele in the shape. As notches in the edges of the shield, the orbits are laterally placed. The sensory canal system is ill preservred. Nevertheless, the supraorbital and some remains of other canals indicate that the system is of the polybranchiaspid pattern. The ornamentation of the shieid consists of fiat-topped tubercles, which are about 40-60 per cm~2 in the density. On the area Mong the left side of the shield the endoskeleton is exposed in dorsal surface. as result of the split of the exoskeleton. On the surface the subaponeurotic vascular plexus is clearly dismayed. In fact, it is quite common that the exoskeleton of galeaspids peels off from the endoskeleton. One of the best examples is a dorsal shield of Polybranchiaspis yulongssus (Liu 1975, pl. II, 1), in which the endoskeleton is almost entirely exposed in dorsal surface. While the exoskeleton is kept in the counterpart and exposed in hasal surface (V4416b). It seems that the well-developed plexus causes the looseness of the eonnection between the exo- and endo- skeleton. The section made by Wang and Wang (1982, fig. 4; refigured in fig. 2, in this paper), of the dorsal shield of Diandongaspis comfirms that, as shown in Polybranchiaspis liaojiaoshanensis (Liu1975, pl. III, 2; fig. 5C), the mediodorsal opening perforates the roof of the orobranchial chamber ventrally and is not covered by any plate dorsally. Thus, the hypoplaysial canal of Galeaspida seems to possesses sueh a fashion as that of myxinoids or that hypothesized by Stensi? in Hetcrostraci, as suggested by Janvier (1981). But about hypophysial region of galeaspids it is still obscure, for puzzlingly the brain in front of the diancephalon as well as the nasal opening have not yet been identified. Although in the roof of the orobranchial chamber in Polybranchiaspis liaojiaoshanensis a Pair of recesses was previously designed by the author (Liu, 1975) as nostrils, they ate best interpreted to aceommodate venous sinuses. As for to the habitat, at least six galeaspid genura have been found together with brachiopods or trilobites at various localities. They are Hanyangaspis in Guodingshan Formation of Wuhan, Latirostraspis in Fentou Formation of Chaoxian, Asiaspis and Antiquisagittaspis in Naguoling Formation of Hengxian, Duyunolepis in Danlin Formation of Duyun and Wuemngshanensis in Suotoflshan Formation of Daguan. Even in Cuifengshan Formation famous for the plenty of galeaspids some shark teeth and Lingula were known Therefore, it seems that at least a number of galeaspids dweIt in the seacoast, including delta and estuary.