记湖南衡东盆地的平顶鳄一新种

本文记述了产自湖南衡东的一鳄类头骨化石——衡东平顶鳄Planocrania hengdongensissp.nov.并对这一属的分类位置进行了讨论,依据次生腭的性质将它归入真鳄亚目鳄科。

A NEW SPECIES OF PLANOCRANIA FROM HENGDONG,HUNAN

In 1970's two form of crocodiles were found in Nonshan Formation(LatePaleocene)of Nanxiong basin,Guangdong and red beds((?)Paleocene)of Hengdongbasin,Hunan respectively.The former,associated with some mammals,such as arc-tostylopidid,Ernanodon,Altilambda etc.was studied and identified as a new genus andspecies Planocrania datangensis by the present author.In this paper the latter form,discovered together with arctostylopidid,is described as a new species and assigned tothe same genus.Crocodylidae Cuvier 1807Planocrania Li 1976Emended Diagnosis Skull deep and laterally compressed;nasal placed horizon-tally while the sides of snout almost vertically.A notch for reception of the 4th dent-ary tooth present.Postorbital bar subdermal.Secondary palate well developed,palatal fenestra large,palatine tube narrow and long,internal nares posteriorly placedand surrounded by pterygoid.Teeth laterally compressed and serrated on anterior andposterior margings,and displying a high degree of differentiation in size and shape;5 premaxillary.11—16 maxillary and 13—15 dentary teeth on each side.Mandibularsymphysis short with a splenial inclusion.Type Species Planocrania datangensis Li 1976Planocrania hengdongensis(sp.nov.)Type An incomplete skull and lower jaw(V 6074).Horizon and Locality Red beds((?)Paleocene)of Hengdong basin,HunanProvince.Diagnosis 5 premaxillary,11—12 maxillary and 12—13 (?) dentary teeth,thecrown narrowly and shallowly fluted from tip to base on both lingual and labial sides.Frontal excluded from the margin of supratemporal fenestra.Quadratojugal-surangu-lar articulation present.Remarks The new form is close to Planocrania datangensis in shape of skull,structure of palate and position of interal nares.but it differes from the latter by theless number of teeth(16 maxillary teeth in P.datangensis,11—12 in P.hendongensis).the presence of radiated shallow flute on the surfaces of the teeth,instead of beingsmooth or rugose as in the P.datangensis,and by its frontal excluding from themargin of supratemporal fenestra. Besides the Planocrania,only three dinosaur-toothed crocodiles,a medial sizedskull with lower jaw and a series of caudal scutes called Hsisosuchus chunkingensis,aleft ramus of lower jaw named Wanosuchus atresus and several isolated teeth recogni-zed as Pristichampsus aff.rollinati have been reported in China prior to this paper.The Upper Jurassie form Hsisosuchus chungkingensis seems,by its preservasion of an-torbital opening,and incipient secondary palate to be more primitive than the others.Planocrania is similar to Wanosuchus in size and in the connection of the quadrate-jugal and surangular,although the shapes of the surfaces of their articulations arequite different.In Wanosuchus it is a longitudinal groove on the widened uppersurface of the surangular,while a shallow depression on the upper-inner margin of thebone of Planocrania.But the external mandibular foramen appears to be more im-portent for distinguishing one from another(present in Planocrania and absent inWanosuchus).Because of the limitation of the material for comparing Wanosuchuswith Planocrania,it would be better at the moment to keep them in the differentgenera,as suggested by Zhang(1981).Pristichampsus aff rollinati was original dis-cribed by Chow et all(1973)based only on four isolated teeth,without anyillustration.Comparing Plamocrania with the North American Pristichampsus vorax,however it is to be noted that there exist similarities between them:a flat roofmaking up of the skull table and almost horizontal nasals,and well developted second-ary palate;and differences:diversity size,different development of the serrations onthe teeth,and fused in Planocrania or unfused(in Pristichampsus)parietal bones.Worthy of mantion here,the serrations in Planocrania are neither complete nor asclear as that in the other,lacking on the top of crown and less protrudent on the an-terior and posterior margins of the teeth.The close relationship between the two generawould imply their inclusions in the same family of Eusuchia rather than of Sebeco-suchia.It seems to me that the developement of secontary palate is more significantin the systematic classification of crocodiles,whereas the ziphodont teeth present inSebecosuchia and Eusuchia could be parallel or convergent development in the twobranches of crocodiles.