贺风三趾马头骨的发现及其系统关系的讨论

贺风三趾马是根据下颊齿和零星肢骨建立的。本文记述的头骨和一贺风三趾马的下颌属于同一个体。新材料表明:贺风三趾马在头骨和上牙方面也与其它已知种有明显的区别。在系统关系上,它和 Hipparion hippidiodum 及 H.turkanense 最为接近。它和后一个种在进化水平上大体相当。它们是从 H.hippidiodum 发展起来的两个不同的支系。

NOTES ON THE FIRST DISCOVERY OF THE SKULL OF HIPPARION HOUFENENSE

Hipparion houfenense Teilhard de Chardin et Young,1931,the well-known speciescharacteristic of the post-pontian and pre-nihowanian Chinglo Formation of China,unfortunately,was originally based on isolated lower cheek teeth and limb bones only.No record of skull material of this species has been reported up to now.During theprocess of joint re-examination of the Equidae material collected by E.Licent fromYushe Basin,Shansi,the authors happened to find a skull associated with a lowerjaw belonging undoubtedly to Hipparion houfenense.Both the skull and the lowerjaw bear the same catalogued number:10508.In Licent's catalogue system specimensbearing the same number,as far as we know,always belong to one individual.Thesuggestion that both the discovered skull and the lower jaw belong to one individualof H.houfenense is further confirmed by their striking correspondence in size,in teethwear degree,and by their quite similar state of preservation.The specimens were collected at Loc.26:Matzekou,Baihai villager,and now signedas THP.10508.SKULL.It is compressed laterally,with the hinder part(posterior to postglenoidprocess)broken away.Size large(see Tab.1 in the Chinese text).The leadingcharacteristic of the skull is the absence of a true preorbital fossa,which is representedon the left side by a poorly-defined and shallow depression,but on the right side bya rather deep one,owing evidently to the downward sliding of the right frontal bone.Nasal bones extend foreward a little beyond the level of upper canine,while theposterior border of the nasal notch lies above the anterior 1/3 of P~2.Both supra-andinfraorbital foramina are large in proportion.The bone-bar separating the orbit fromthe temporal fossa,and the posterior part of the zygomatic arch are strongly built,their narrowest widths are 27 and 31 mm respectively.INCISORS.All the preserved incisors(right I~(2-3),I_(2-3),left I_3 and partly brokenI_2)show no sign of cup,an evidence of advanced stage of wear.All of thempossess marked longitudinal grooves on both lingual and labial sides,a characterseldom met with in the genus Hipparion.CANINES.All damaged,but the anterior and the posterior ridges are clearlyshown,especially on the lingual side.CHEEK TEETH.No P~1.Parastyles and mesostyles increasingly widened fromM~2 to P~3.Mesostyles,and in a lesser degree,the parastyles bifurcated,leaving longi-tudinal grooves on the labial side.Inner borders of the para-and metacones are finely wrinkled.Fossette plications are moderate in amplitude,pli caballine extremely weakand short,simple on P~4-M~2,doubled on P~3 and M~3.Protocones large and compressed,that of M~3 being the narrowest in proportion.The lower cheek teeth are nearly duplicates of the type specimens described byTeilhard de Chardin and Young in 1931,with the only exception of cement beingthinner.Hipparion houfenense was once supposed by V.Gromova as an intermediate formlinking Hipparion with Proboscidipparion.However,the present skull material failsto support such a point of view.Rather unexpectedly,many characters observable onthe skull enable us to link it with Chinese Pontian H.hippidiodum and the recentlydiscovered African H.turkanense.All the three forms are characterized by lacking atrue preorbital fossa,rather long muzzle,similar form of nasal notch on the skull,weak fossette plication and very simple pli caballine on the upper cheek teeth.Amongthem H.hippidiodum seems to be the most primitive species,both on account of itsearlier occurrence in geological age as well as its primitiveness in cheek teeth morphology(protocones smaller,transversely oriented enamel layer very thin,etc.)On the otherhand,H.houfenense and H.turkanense may stand approximately on the same stage ofspecialisation.Their upper cheek teeth are quite similar in pattern:protoconesenlarged and para-and mesostyles widened.However,the two forms can easily beseparated by the following features:1.Protocones in the African species are rounderin shape,often flattened internally,while in the Asian species they are longer and moreelliptical in shape.2.In the former species the para-and mesostyles are widened fromP~2 to M~1,while in the latter-from P~3 to M~2.3.incisors of H.houfenense are deeplygrooved,while for the African species such a character has never been mentioned.To sum up,it seems quite probable that both H.houfenense and H.turkanensewere evolved from H.hippidiodum,or from some closely related forms.The formerhad been confined to East Asia only and had become extinct before the beginning ofNihowan age,while the latter species invaded sometime in Pontian age,or a little later,into Africa,and then gave rise to the peculiar African Pleistocene Eurygnathohippus(Stylohipparion).