间型三棱齿象(Trilophodon connexus Hopwood,1935)属于豕棱齿象类而非嵌齿象

Reassessment of Trilophodon connexus Hopwood,1935 and attributing it to the Choerolophodontidae

间型三棱齿象(Trilophodon connexus Hopwood,1935)长期以来被认为是中国嵌齿象属(Gomphotherium)的一个代表种。然而,由于其下颌联合部与下门齿的形态未知,这一归入存疑。重新研究了来自新疆准噶尔盆地北缘乌伦古河地区哈拉玛盖组的一件此前归为陕西嵌齿象相似种(Gomphotherium cf.G.shensiensis)的下颌。该下颌联合部伸长,呈深槽状,下门齿缺失,因此确定可归入豕棱齿象科(Choerolophodontidae)。进一步将间型三棱齿象的正型标本与其相比较,两者颊齿的关键特征完全一致,包括:高度丘型化,m3伸长,具有四脊,上下颊齿第二脊“人字型”(chevron)很弱,第二脊中附锥与前中心小尖不愈合,釉质褶皱、齿谷中小锥及白垩质发育弱或缺失。因此,间型三棱齿象事实上是一种豕棱齿象类而非嵌齿象。综上所述,暂将其改定为间型“豕棱齿象”(“Choerolophodon”connexus(Hopwood,1935))。同时,以上特征与北美的索普颌门齿象(Gnathabelodon thorpei)比较接近。此外,在颌门齿象属和间型“豕棱齿象”中,颊齿第二脊呈“人字型”,釉质褶皱、齿谷中小锥及白垩质发育强这些典型的豕棱齿象属(Choerolophodon)的特征较弱甚至缺失,但m3齿脊数变多,这表明颌门齿象属可能起源于东亚的间型“豕棱齿象”。

Trilophodon connexus Hopwood,1935 has long been considered a typical species of Gomphotherium in China.However,due to the unknown state of the mandibular symphysis and tusks,there is no definite evidence to assign“T.connexus”to Gomphotherium.Here we describe and reevaluate a hemimandible from the Halamagai Formation,Ulungur region,northern Junggar Basin,which was previously identified as Gomphotherium cf.G.shensiensis.The mandibular symphysis is deeply troughed and lacks mandibular tusks;therefore,it undoubtedly belongs to the Choerolophodontidae.Further comparison revealed that the cheek tooth morphology is identical to that of the type specimen of Trilophodon connexus.The characteristic features include high bunodonty,elongation of the m3 with four lophids,an only weakly chevroned lophid 2,enlargement of the posterior pretrite central conule 2,unfused state of the pretrite mesoconelet 2(if present)and anterior pretrite central conule 2,as well as the absence of ptychodonty,choerodonty,and cementodonty.Therefore,T.connexus Hopwood,1935 is a choerolophodontid rather than a species of Gomphotherium.Based on the above features,we provisionally refer to it as“Choerolophodon”connexus.“Choerolophodon”connexus is characterized by the following features:weak or absent ptychodonty,choerodonty,and loph chevron(which were all strong in the typical species of Choerolophodon),as well as multiplication of the lophids in the m3,which were similar to that of the North American Gnathabelodon.Therefore,Gnathabelodon might represent a distinct lineage within the Choerolophodontidae,and may be derived from the East Asian“Choerolophodon”connexus.