陕北延长群一新古鳕类及其生物地层意义

本文记述了在陕西北部耀县铜川组发现的古鳕科一新属、种——延长三叠鳕(Triassodusyanchangensis,gen.et sp.nov.)。在对其形态特征作较详细描述的基础上,认为它既与美国晚三叠世的吐鲁瑟欧鳕(Turseodus)很接近,又与我国四川须家河组(晚三叠世)的蜀鳕(Sh-uniscus)相似。基于上述的理由,认为铜川组的时代是晚三叠世。根据迄今在我国晓三叠世发现的鱼类,探讨了我国晚三叠世的鱼群与北美的关系。

A NEW PALAEONISCOID FISH FROM THE YANCHANG GROUP OF NORTH SHAANXI AND ITS BIOSTRATIGRAPHIC SIGNIFICANCE

A new palaeoniscid genus and species,Triassodus yanchangensis,from the Ton-chuan Formation(Upper Triassic)of Yao Xian,North Shaanxi,is described.It at-tains a total length of 14—23cm,having elegantly fusiform body and large orbit an-teriorly placed.The greatest depth of the body,which is about 1/4.4 in the total length,is larger than the length of the head including opercular apparatus.The measurementsof the holotype are given in the text.The characters of the skull-bones and all the fins resemble those of Turseolus andShuniscus.Suspensorium oblique and mouth gape deep.Snout obtuse and single largepostrostral separating nasals.Circumorbital series consisting of supraorbital,three or fourinfraorbitals,anterorbital,and nasal.About two suborbitals present in front of thepreopercular.Single dermohyal present.Maxillary of Palaeoniscoid shape.Mandiblerobust,with broad symphysis,its outside ornamented with fine diagonal ridges.Maxil-lary and dentary with small pointed teeth.Operculum considerably larger than suboper-culum,much deeper than wide.Cleithrum and supracleithrum well developed,the latterwith diagonal ridges.Pectoral fin large,horizontal,consisting of about 20 lepidotrichiawhich not segmented proximally and bifurcated distally.Pelvic fin nearer pectoral thananal fin,cnsisting of about 2 unbranched and 20 branched lepidotrichia which segmentedcompletely.Dorsal fin remote,its origin slightly before that of anal fin,triangular,including about 2 unbranched and branched 20 lepidotrichia,wrich are segmentedthroughout.Anal fin long-based,with concave ventral margin,consisting of about 2unbranched and 53 branched lepidotrichia,which are segmented completely.Caudalfin heterocercal,deep cleft,nearly equilobate,cnsisting of about 60 lepidotrichia.Fulcraof moderate size and present on all the fins.Seales small,rhombic,and ornamentedwith diagonal ridges. The general characters of Triassodus resemble Turseodus Leidy and Shuniscus Su,but Triassodus differs from Turseodus in the following combination of characters:pos-trostral comparatively large,saddle-shaped.Cheek region(the distance between theorbit and the opercular)rather broad.Preopercular-maxillary angle much larger(ca-45°).The postorbital part of the maxillary relatively long and anterior part of thebone meeting the fellow below the rostrum.Branchiostegal rays few in number.Dermalbones ornamented with prominent tubercles or ridges of enamel.Anal fin long-based,butless than skull length.Scales more or less pectinated at the hind border,appear to bedestitute of dorsal articular process.Shuniscus differs from Triassodus in the followingcharacters:Body small.Preopercular-maxillary angle much smaller(ca.35°).Maxillaryexcluded from snout,not meeting the fellow below the rostrum.Ornamention on der-mal bones very weak.Supracleithrum short,stout and fusiform.Cleithrum with at-tenuated dorsal extension.Pelvic,dorsal and anal fins with comparatively long bases.The rays of all the fins sparsely segmented.Fin-fulcra very small,only present on un-paired fins.Scales without diagonal ridges,and with smooth posterior border.Triassodus retains the basic Palaeoniscoid condition.It is a typically fusiform fishwith strongly heterocercal tail,rhomboidal,enamel-covered scales and an oblique su-spensorium.Triassodus also exhibits some advanced characters,for instance,the lepido-trichia of tre pectoral fins unjointed poximally;Pelvic fins anteriorly placed;Caudalfin nearly equilobate cte.All these show the same characters as Turseodus and Shunis-cus.This new discovery is of great interest in the studying of the stratigraphic andgeographic distribution of fishes during the Late Triassic age.An interesting form ofPalaeoniscoids,Fukangichthys longidorsalis which was discovered(Su,1978)from theKaramy Formation(Upper Triassic)in Xinjiang(Sinkiang),bears characters closelyresembling Tanaocrossus kalliokoskii from the Chinle Formation of the United states.Another gracile Palaeoniscoid fish,Shuniscus longianalis which was discovered(Su,1983)from the Xujiahe Formation(Upper Triassic)in Sichuan,exhibits characters resem-bling Turseodus from the Chinle Formation and Newark Group in United States.Thediscovery of Triassodus and specimen of Coelacanths from the Tonchuan Formation ofNorth Shaanxi has further indicated the resemblance between the Upper Triassie fishfauna in Eastern Asia and North America.