河北秦皇岛地区奥陶纪奇壁角石科分子的发现及意义

奇壁角石科（Ａｌｌｏｔｒｉｏｃｅｒａｔｉｄａｅ）分子在我华北东部属首次发现。它产于马家沟组下部。界于Ｄｅｆｏｒｍｏｎ－ｃｅｒａｓ－Ｐｅｒｉｐａｔｏｃｅｒａｓ与Ｐｏｌｙｄｅｓｍｉａ－Ｗｕｔｉｎｏｃｅｒａｓ－Ｅｏｓｉｏｔｅｌｕｓ组合带之间偏下。奇壁角石科是Ｆｌｏｗｅｒ，Ｒ．Ｈ．１９９５年创立的，至今科内已建立６属１３种和３个相似种。但以往所描述标本绝大多数仅保存有部分内体管，对其它特征所知甚少。文中描

DISCOVERY AND SIGNIFICANCE OF THE MEMBERS OF THE ALLOTRIOCERATIDAE FROM ORDOVICAIN QINHUANGDAO REGION, HEBEI, NORTH CHINA

Three new genera, including three new species and one sp. indet. of Allotrioceratidae from the lower part of Majiagou Formation between the Deformoceras Peripatoceras Zone and the Polydesmia Wutinoceras Eoistelus Zone were first discovered in North China. The Allotrioceratidae was established by Flower, R. H. in 1955. At present, it only contains 13 species and 3 conformis species assigned to 6 genera, and among the specimens described, mostly only a part of endosiphuncles were preserved. Our specimens described here, in addition to the preservation of endosiphuncles, contain ectosiphuncles and rare phragmocones with camera deposits and annulosiphonate deposits. From here we can see that these specimens are probably a kind of nautiloids between enodocerids and actinocerids. This discovery provides some new data for studying the evolution between endocerids and actinocerids. Genus Thecoceras gen. nov. Type species: Thecoceras typicum gen. et. sp. nov. Diagnosis:Shell small to moderately sized. Orthocone, expanded slowly in width, compressed in cross section. Its phragmocone, siphuncle and central pillar are compressed and oval in cross section. Siphuncle larger in near ventral position, 2/5 to 1/2 times as wide as its shell diameter. Central pillar very large, 3/5 to 4/5 times as wide as its siphuncle diameter and anterior part of the central pillar projects over endosiphocone. Septal neck hologhoaitic, connecting ring thicker, camera deposits absent or undeveloped. Remarks:The new genus differs from other genera of Allotrioceratidae in having larger siphuncle, larger and longer central pillar as well as phragmocone. Siphuncle and central pillar compressed as oval in cross section. Age and occurrence:late Early Ordovician at Qinhuangdao, Hebei. Genus Chazhuangoceras gen. nov. Type species: Chazhuangoceras densum gen. et sp. nov. Diagnosis:Shell smaller, orthocone, expanded slowly in width and with the rate of enlargment 1:10 to 1:6, oval in cross section. Siphuncle larger, 1/4 to 1/3 times as wide as the shell diameter, central to near ventral in position, plain ventral and convex dorsal in cross section, central pillar larger, nearly central in position, the rate of the central pillar diameter with the siphuncle diameter over 3:5.1/3 to 2/3 as the anterior part of the central pillar as the length of the endosiphocone. Camerae shorter, camera depsits developed, septal neck holochoanitic, connecting ring thin. Besides endocone deposits, annulosiphonate deposits also developed in siphuncle. Remarks:This genus is similar to Thecoceras, but it can easily be distinguished from Thecoceras by the shapes in cross section of their phragmocone, siphuncle and central pillar. Age and occurrence:Late Early Ordovician at Qinhuangdao, Hebei. Genus Qinhuangdaoceras gen. nov. Type species: Qinhuangdaoceras mecocameratum gen. et sp. nov. Diagnosis:Shell small to moderate, compressed, orthoceroid. Siphuncle very large, central or near ventral in position, 3/4 as width of dorsal ventral as the diameter of its shell. Central pillar depressed and plano vertral in cross section, central or near dorsal in position. Endosiphocone longer, anterior part of the central pillar project over the endosiphocone, but less than half length of the endosiphocone. Septal neck holochoanitic, connecting ring thin, camera deposits absent. Remarks:The new genus differs from other genera of Allotrioceratidae in having shorter phragmocone, large siphuncle and rounded triangular outline of plano convex ventral dorsal in cross section. Age and occurrence:Late Early Ordovician at Qinhuangdao, Hebei.