内蒙古大青山地区二齿兽类化石的发现

THE DISCOVERY OF DICYNODONTS IN DAQINGSHAN MOUNTAIN,NEI MONGOL(INNER MONGOLIA)

本文记述内蒙古大青山地区脑包沟组中首次发现的脊椎动物化石——边缘大青山兽(Da-qingshanodon limbus)。它具有翼骨腭上枝、基蝶骨小侧架、卷曲的鳞骨等独特构造。依其颅基中轴长、翼间窝长、翼间孔窄,眶前部分短,泪骨与隔颌骨分离等特征,以及在较高层位中发现的大鼻龙类、兽头类等化石,确认大青山兽属典型的晚二叠世二齿兽类型。文中还讨论了二齿兽类鼻区、蝶筛区、翼骨区等构造的演变,“萨拉齐系”与“石千峰群”的岩性和生物的可比性。

A fossiliferous strata,Naobaogou Formation,outcrops in Daqingshan Mountain,Nei Mon- gol(Inner Mongolia).It stretches north-east-east for nearly 100 km by 10 km,between Baotou city and Huhehaote city.Naobaogou Formation have had no recognizable fossil until the first vertebrate fossil was found in 1982.This formation,beneath Laowopu Formation(Lowermost Triassic(?))and overlying Shiyewan Formation,consists of three continuously deposited red-brown sediments(deposit cycles)that are rich in fossil reptiles——dicynodonts,captorhinomorphs, theriocephalians,pareiasaurs,perhaps some labyrinthodonts.The total sediment reaches 1020 m thick. The first mammal-like reptile,Daqingshanodon limbus,is described in this paper.It is considered as a typical Late Permian dicynodont,distinguished from others by a upper-palatal ramus of the pterygoid,two small lateral ledges of the basisphenoid,and that the squamosals curved anteriorly.The structures of the nostril,sphenoid,and pterygoid regions are discussed. This study shows both similarities and differences of strata and animals between the so-called “Salaqi System”and“Shiqianfeng Group”. Description Family Dicynodontidae Owen,1859 Daqingshanodon limbus gen.et sp.nov. Holotype a skull with lower jaws in situ.IVPP number V7940. Locality Muhao Gully,Naobaogou Village,Shiguai District,Baotou City,Nei Mon- gol. Age Late Permian. Horizon Lower cycle of Naobaogou Formation. Diagnosis Body small size.Nasal boss longitudinally developed.Nasal-frontal boss small.Septomaxilla completely enclosed in nostril.Openings for the lachrymal duct,Jacob- son's organ,and lateral nasal gland lie on the suture line between the septomaxilla and facial bones.Canine process short,with a lip-like lateral extension.The palatine participates widely in the secondary palate,having a long suture line with the premaxilla.The ectopterygoid,pa- latine,palpal ramus of the pterygoid is well developed and extends anteriorly over the anterior end of the palatine.Labial fossae present.A lateral deltaic ledge is present on each side of basisphenoid.The squamosal curves anteriorly beside the temporal fossa.Preparietals pike- shaped,protruding above the surrounding bones.Parietals rise at the brim of the parietal fora- men.The dentary sulcus deep and narrow.Dentary table narrow. Discussion Major differences of Daqingshan specimens from the most similar genus Dicynodon are parasagittally developed nasal boss,well formed nasal-frontal boss,pike-shaped ridged prepa- rietal,larger exposed area of parietal on interpterygoid bar,rip-to-tip contact of pterygoid with maxilla,single vomer,no intertuberal ridge of basioccipital,anteriorly curved squamosal.Unique characters are small deltaic ledges on the sides of basisphenoid,upper-palatal ramus of pterygoid.Accordingly a new genus and species is named Daqingshanodon limbus. In extant reptiles,the duct of Jacobson's organ(accompanied by that of the lachrymal) usually opens into the choana,but when the palate extends by the fusion between the vomerine cushion and choanal fold of each side(as in Squamata),the duct opens into anterior oral ca- vity(Parsons,1970).This suggests that it be probable,considering the bony furrow of the la- chrymal duct leading to the nostril,for the duct to open into the nostril in dicynodonts,which had a well developed secondary palate.So among the three openings in the nostril of Daqing- shanodon,the connected upper two are openings of the lachrymal(the uppermost)and Jacob- son's organ(the middle);the lower solitary on,with ridges and furrows radiating from the vent anteriorly,is that of the lateral nasal gland(Fig.6).In Squamata,of these three organs, only the ducts of Jacobson's organ and lachrymal may be connected via a duct.The position of opening of lateral nasal gland was variable relative to the septomaxilla,and could be ranged as following(from back position forwards):Daqingshanodon——Dicynodon trigonocephalus —Kingoria(—Lystrosaurus).Probably the opening moved off the anterior edge of the septo- maxilla in Lystrosaurus.This might represent a trend of evolution(not necessary an ancester —descendant relationship). In the sphenoid region,Daqingshanodon had three isolatedly ossified pieces of bone.The upper two pieces have identical intrinsic lines.The lower one does not show visible intrinsic lines. These three pieces of bone seem to correspond to Olson's orbitosphenoid and mesethmoid (1944)respectively,therefrom,counting the basisphenoid and the presphenoid,Daqingshanodon had four bones on the basicranial axis.The phylogenetic and embryological significance of the bones in this region have been argued for years,but most researchers recognize more than four bones(Seeley,1898;Olson,1944;Cox,1959).If this stage recaptulated any evolutionary stage from reptile to mammal,it could be agreed that no less than four bones are presented on the basicranial axis of the ancesters of mammal. The space of the posterior part of interpterygoid fossa which is walled by pterygoids and named post-palatine vacuity here varies greatly among dicynodonts.In most Permian dicyno- donts,the vomerine is small,and extends backwards not as far as palatine does,and the spatial front of the postpalatine vacuity is V-shaped in lateral view.In some later genera(including Dinodontosaurus,Lystrosaurus,Kennemeyeria),the vomerine reached posteriorly beyond the pa- latine,the spatial front became nearly a slanting plane,the vacuity became smaller and shallo- wer,until it vanished.These changes could have evolutionary bearing. The upper-palatal ramus of the pterygoid which,such as appeared in Daqingshanodon,is rarely seen in dicynodonts corresponds to the anterior part of the pterygoid of pelycosaurs,whe- reas the palatal ramus of the pterygoid corresponds to the transverse flange.The upper-palatal ramus is well preserved;there is no fracture or matrix covering it.This ramus may be a vesti- gial character. The tusk has an abraded surface on the median side only.The abraded area shows conti- nuous parallel striae with a steep posterior surface and a flat anterior one.On the upper part of the abraded surface some striae swerve backwards.The striae couldn't be made by rough food as Hotton(1986)suggested,because there couldn't be any shearing and cutting between the distal half of the tusk and the lower jaw.When the jaws closed,the abraded surface was re- ached by the ventral edge of the lower jaw;hence the striae could only be made by the lower jaw smeared with residue of food and sands.If the striae were made by rough food,they should cover the whole length of the tusk.This suggests an intake model that tusked dicynodonts fed at or just below the surface of the substrate.The continuous striae with swerving upper ends in- dicate not only that the lower jaw of Daqingshanodon might have been pulled transversely to shear food,but also that it could cut food when the lower jaw was retracting. The percentage of interpterygoid space of Daqingshanodon is 85% in relation to the inter- nal nares;the spatial front of the post-palatine vacuity is V-shaped,without the characters of Tri- assic dicynodonts listed by Keyser(1974).Hence Daqingshanodon may be considered a typical dicynodont.Because it has some advanced features,such as a large part of the palatine con- tributing to a secondary palate,small ectopterygoid,invisible postfrontals,small ledge on the lateral side of the basisphenoid;and some primitive features,such as the long and narrow inter- pterygoid fossa,large post-palatine vacuity,and well developed upper-palatal ramus of ptery- goid;the age of Daqingshanodon should be older than the Daptocephalus zone,not younger than the Cistecephalus zone.This is in accordance with the data from upper beds which have captor- hinomorphs whose age is considered not later than“Daptocephalus”zone(Riqules and Taquet, 1982;Чудинов,1983). As a result of this study,two features seem important.First,it could be clear that the fau- nas of Xingjiang and North China are similar to some extent.Second,the animals from the Naobaogou Formation may represent more than a single fauna.No captorhinomorphs are known from the South African fauna.In the U.S.S.R.the fauna is characterized by dinocepha- lians.In India there are only dicynodonts and captorhinomorphs.In other places,such as the U.S.and Niger,there are captorhinomorphs,but no dicynodonts,therocephalians,or pare- isaurs.The fore-runners and relicts of different groups of reptiles both thrived in Naobaogou Formation,which is of great significance in investigating the evolutionary interactions between different groups of animals and their paleogeography.