双柱鼠科一新属在新疆的发现

PRODISTYLOMYS GEN.NOV.(DISTYLOMYIDAE,?CTENODACTYLOIDEA,RODENTIA) FROM XINJIANG,CHINA

本文记述了产自新疆准噶尔盆地上渐新统的双柱鼠科一新属——原双柱鼠属(Prodistylo-mys)。它具有一些较原始的特点,但代表不同于Distylomys的另一分支。它的颊齿是由下后方斜向前上方生长的。

A lot of fossil mammals were collected from Suosuoquan Formation at the northern edge of Junggar Basin,Xinjiang Uygur Autonomous Region,China,by the staffs of a field team of IVPP during the summer field season of 1982.Among them a left lower jaw representing a new genus,Prodistylomys,is recognized and described in this paper,The new one is quite different from the genus Distylomys,in the Family Distylomyidae.The discovery of it not only extends the distribution of Distylomyidae,but also reveals some primitive features.Meanwhile,it throws some light on the tooth grow way and the relationship between Distylomyidae and other cte- nodactyloids. SYSTEMATICS ?Ctenodactyloidea Simpson,1945 Distylomyidae Wang,1988 Prodistylomys xinjiangensis gen.et.sp.nov. Holotype A fragmental lower jaw with I_1 and DP_4—M_3(IVPP:V 7962). Locality and horizon IVPP:Loc.82503,Chibaerwoyi,the north bank of the Ulun- gur River,Junggar Basin,Xinjiang,China;Upper Oligocene,Suosuoquan Formation. Diagnosis A more primitive distylomyid close to Distylomys tedfordi in size.Differs from Distylomys in following features:cheek teeth rooted;DP_4 with metaflexid and metafos- setid,and shorter trigonid and talonid;molars trigonid short and wide rhombus in form and talonid lozenge formed,longer and narrower than trigonid;M_(1-2) with developed metaflexid and thin cement in ectoflexid and mesoflexid. Etymology Pro+di+stylo+mys,Greek,meaning a primitive distylomyid.Xinjiang, the name of the region where the fossil was collected. Description and comparison:V 7962 is similar to Distylomys in some main fe- atures of the lower jaw and the cheek teeth.It is close to D.tedfordi in size.DP_4 is similar to P_4 of Distylomys in shape and structure,but with shorter trigonid and talonid,and the metafle- xid and the circular restage of the metafossetid.The well developed ectoflexid and mesoflexid shallow towards the root.The crown surface is higher than those of the molars. The molars have great curvature of the vertical axes,both posteriorly and labially.The- refore,the root of M_1 extends below M_2,and that of M_2 below M_3. M_1 and M_2 are about the same in size and shape.The trigonid,being wide and short rhom- bus-shaped,is wider and shorter than the talonid which is lozenge-shaped.Well developed me- soflexid and ectoflexid are opposite to each other,and covered by thin cement at the bottom.The metaflexid is also very developed and extends to the middle of the talonid in the recent wear stage.It is more shallow on M_1 than on M_2.It seems that it gets shallow gradually and,at last,disappears with wear.Therefore,M_1 and M_2 slightly shorten in lenth towards the root. M_3 has just erupted.The crown surface without any cusp and crest structure has just be- gun to be worn.The trigonid is of vault compressed antero-posteriorly and with slightly nar- row and low top.The talonid is triangular in shape with lunar wear facet on it.The top is lower and narrower than that of the trigonid and does not join the latter until they are worn out sometime.In both lenth and width M_3 increases from the top to the root. In short,In V 7962 the hypsodont cheek teeth have roots.The trigonid is wider and shor- ter than the talonid.The metaflexid is well developed on M_(1-2) but disappears gradually with wear.The cement in the mesoflexid and metaflexid is thin etc.All these features are quite dif- ferent from those of Distylomys.It seems that it represents a distinct genus.We call it prodistylomys. DISCUSSION 1.The grow way of the cheek teeth The cheek teeth of Prodistylomys reveals some special grow way. First is about DP_4.The first one of the cheek teeth is heavily worn and much shorter than the molars in crown height.The metaflexid only remains vestage.It appears to be DP_4 rather than P_4.As mentioned above,the crown surface is slightly higher than those of the mo- lars.We do not know whether it is being replaced by P_4.There seems a hole under DP_4 in the mandible.We wonder if there exists a germ of P_4 in it during the lifetime,because the lower part of the mandible is broken.Anyway,it suggests,at least,that DP_4 of Prodistylomys is not replaced until M_3 erupts and begins to be worn,even if it does.Otherwise,ifit is true that the first tooth of the lower cheek teeth of Prodistylomys,molariformed just like P_4 of Diu- stylomys,is DP_4,it may prove that DP_4 of Prodistylomys is also molariformed as the other cte- nodactyloids. Second,it is interesting to point out that all lower molars of Prodistylomys have great cur- vature of the vertical axes,both posteriorly and labially.The roots extend downward and back- ward.Furthermore,the crown surface of the trigonid of M_3 is higher than that of the talonid. It seems that the lower molars grow forward and upward rather than upward only.Therefore, the lower jaw of Prodistylomys can hold much high crown cheek teeth without deeping the height of the mandible not only by the curvature of the teeth vertical axes,but also by the special grow way. 2.The relationships hetween Prodistylomys and Distylomys As pointed out above,Prodistylomys is more primitive than Distylomys in some teeth fea- tures.However,it does not mean that Prodistylomys is the ancestor type of Distylomys.First, Tong and all(in press)have pointed out that the Suosuoquan Formation yielding Prodistylo- mys is of Late Oligocene because the fauna including Sinolagomys sp.nov.(Tong,in press)and Tachyoryctoides sp.is similar to the Late Oligocene Taben-buluk fauna in Gansu Province.The Suosuoquan Formation is overlied by the Halamagai Formatoion which is considered to be of Middle Miocene Tunggurian stage.Although no distylomyid has been found in Halamagai For- mation so far,Distylomys tedfordi has been collected from Tunggur Formation.Therefore,it seems reasonable to consider Prodistylomys the ancestor of D.tedfordi based on the age.How- ever,the other species,Distylomys qianlishanensis,was collected from Upper Oligocene Yi- kebulag Formation.It means that both Prodistylomys and Distylomys qianlishanensis are of the same age——Late Oligocene.According to the age,Prodistylomys is too late to be ancestor of Distylomys.Second,the most important,is that Distylomys tedfordi and D.qianlishanensis share some derived characters(synapomorphy),for example,complete hypsodont rootless cheek teeth,triangular trigonid and talonid etc.Meanwhile,basides some primitive characters, Prodistylonmys also has some derived characters(autapomorphy),such as short and wide rhom- boid trigonid etc.As far as the characteristics are concerned,it appears that Prodistylomys re- presents a distinct lineage fron Distylomys rather than the ancestor of the latter.The time they split from each other might be,at least,in the Middle Oligocene or even earlier. 3.The relationships between Distylomyidae and other ctenodactyloids Discussing the phylogenetic status of Distylomys,Wang(1988)has pointed out that Dis- tylomys resembles some ctenodactyloids,but differs from the chapattimyids and yuomyids in the mandible and tooth characters,while in the molariform P_4 it is similar to the latter not to the former.Based on the molariform P_4,Distylomyidae has been thought belonging to the sis- ter group of Chapattimyidae-Yuomyidae rather than the Ctenodactylidae with non-molariform P_4.The similarities between the Distylomyidae and Ctenodactylidae were consisdered the result of parallel evolution.The characters of Prodistylomys give some new evidences for that,such as the talonid of M_3 is much small and has no any vestage of the metaflexid and M_2 is not lar- ger than M_1 in size.All these characters show that Prodistylomys has a distinct tendence from the Ctenodactylidae indeed.On the other hand,the only common character of the Distylomyi- dae and the Chapattimyidae-Yuomyidae is molariform P_4.The other characters,both in man- dible and cheek teeth,are quite differens between them.However,the molariform P_4 is a pri- mitive character for the rodents.It means that they have symplesiomorphy and compose a pa- raphyletic group.Therefore,Distylomyidae represents a distinct lineage from other ctenoda- ctyloids,even if it belongs to this superfamily.