中国尖齿兽牙齿替换方式及其对哺乳动物进化的意义

中国云南下禄丰组的早侏罗世中国尖齿兽的新材料显示出从前未曾认识到的早期哺乳动物牙齿特征．中国尖齿兽的门齿和犬齿，像非哺乳类犬齿兽一样，是多次替换的。新的中国尖齿兽幼体标本保存有前臼齿，前臼齿替换一次之后，在较大（更老）的标本上就永久消失。犬后齿分化为前臼齿与臼齿，以及前臼齿仅替换一次是现代哺乳动物的衍生特征。中国尖齿兽前边的臼齿可能无后继齿替换而消失；后边的臼齿替换一次．中国尖齿鲁当其成体的牙齿还在经受替换期间，头骨仍在继续生长。中国尖齿首是所有其他哺乳动物的姊妹类群。中国尖齿兽的牙齿替换方式可解释为从爬行类原始多出齿系向哺乳动物进步的二出齿系进化的中间过渡类型。由中国尖齿兽的多出齿系和其头骨无期限式生长推知，该类动物尚不具备现代哺乳动物所具有的有期限生长方式和哺乳特征。

PATTERN OF DENTAL REPLACEMENT OF SINOCONODON ANDITS IMPLICATION FOR EVOLUTION OF MAMMALS

This paper reports new features of the dentition of Sinoconodon, an EarlyJurassic mammal from the lower Lufeng Formation of Yunnan China Incisors andcanines were replaced multiple times in Sinoconodon as in many non-mammalian cynodonts.New juvenile specimens of Sinoconodon have premolars which were replaced once beforebeing permanently lost in the larger (thus older) specimens. Differentiation of premolarsfrom molars and a single replacement of premolare are the derived features of modemmammals. The anterior molare (M1-M2) were probaby lost without replacement. Theposterior molare (M3-M5) were replaced once. The skull experienced indeterminategrowth while the teeth were being replpaced in adult specimens. Sinoconodon is the sistertaxon to all othr mammals. The dental replecement of Sinoconodon could be interpretedas an intermediate stage in the chaseter evolution from the primitive pattem ofpolyphyodont replacement seen in most cynodnts to the derived diphyodont replacementof mammals. From the poyphyodont replacement and the substantial growth of the adultskulls of Anoconodon, we infer that this taxon lacked the lactation and determinategrowth typical of living mammals.