戈壁中新乳齿象(Miomastodon gobiensis)头骨的发现及生物地层学分布

The first cranium of Miomastodon gobiensis and its biostratigraphic distribution

玛姆象科是象型类两个基本分支之一.然而由于发现材料的限制,学术界对早期欧亚大陆玛姆象类的解剖学特征及系统演化所知甚少.来自甘肃临夏盆地老沟地点的一件中新乳齿象头骨,是目前发现的第一件中新乳齿象头骨,也是欧亚大陆发现的第二件早-中中新世玛姆象科头骨,另外一件是发现于法国的苏黎世轭齿象(Zygolophodon turicensis),但保存不佳,因此,该标本对研究玛姆象类在早-中中新世的演化具有重要意义.新发现的标本颊齿轭型化程度比经典的苏黎世轭齿象(Zygolophodon turicensis)稍低,例如,齿脊比较粗壮,压缩齿谷,主齿柱新月嵴粗,侧视高度达齿谷的一半,牙齿釉质层厚.因此可归入之前在内蒙古通古尔中中新世报道过下颌的戈壁中新乳齿象(Miomastodon gobiensis).该头骨具有玛姆象类中的独特的衍征,例如,眼眶位置比苏黎世轭齿象靠后,且略偏向头顶.后一特征与豕脊齿象类相近,它的外鼻孔位置非常靠后,开口偏向上方,这些特征说明中新乳齿象在玛姆象科中具有独特的演化地位.中新乳齿象具有较显著的时代意义,早期较原始的同心中新乳齿象的时代为早中新世晚期到中中新世早期,约17~14 Ma;后期较进步的戈壁中新乳齿象的时代为中中新世晚期,在13.7~11.8 Ma左右.

The Family Mammutidae represents one of the two fundamental phylogenetic bifurcations of the elephantiforms.Yet due to the scarcity of well-preserved fossil materials,including that of largely complete crania,we know little concerning the anatomy,evolution and systematics of early Eurasian mammutids.In the present article,we report the first Miomastodon gobiensis cranium from the Laogou locality in the Linxia Basin,Gansu Province.It is only the second known cranium of Eurasian mammutids which pre-dates the Late Miocene,and preserved in almost pristine conditions.As the previously known pre-Late Miocene mammutid skull from Eurasia is in a poor condition of preservation,this specimen reported herein is of crucial importance for understanding the early evolution of mammutids in the Early-Middle Miocene.The molars of the Laogou skull are less accentuated in their zygodonty than those of Zygolophodon turicensis,the classic representative of Eurasian mammutids in the pre-Late Miocene age.Namely,the cheek teeth possess relatively more inflated lophs,compressing the interloph valleys to some extent,the thicker pretrite crescentoids which reach half the height of the interloph in side view.Furthermore,enamels are also thicker compared with Zygolophodon.These features warrant the skull’s attribution to Miomastodon gobiensis,the type specimen of which is a mandible from the Middle Miocene of Tunggur,Inner Mongolia.The new Miomastodon cranium displays several distinct apomorphs of Mammutidae,for example,the orbit is more posteriorly positioned than that of the pre-Late Miocene mammutids,i.e.,Eozygodon morentoensis and Zygolophodon turicensis.The orbit is at a high level on the cranium,recalling the condition in Choerolophodon,and the external nares are strongly posteriorly positioned,with an opening that is slightly dorsally oriented.These features indicate a special evolutionary position of Miomastodon in Mammutidae.However,it resembles Mammut(Pliomastodon)nevadanus of North America in several aspects,for example,the low cranium,and the posterior position of the external nares.At the present time,it is difficult to judge whether the terminal taxon,Mammut americanum,was derived from the endemic Mammut(Pliomastodon),or migrated from a certain Mammut clade from Eurasia.Miomastodon is a fossil genus of relatively strong biochoronologic significance in the Early-Middle Miocene of China.The ancestral species Miomastodon tongxinensis possibly spanned approximately 17–14 Ma of the late Early to early Middle Miocene(based on the unpublished palaeomagnetic data).However,the derived species,M.gobiensis,is between 13.7–11.8 Ma of the Late middle Miocene.The latter result is based on the correlation of the evolutionary levels of Platybelodon grangeri from both Laogou and Tunggur region—Platybelodon grangeri from the Laogou locality display similar evolutionary level with that from the Moergen Fauna of the middle Tungger strata.