Abstract:

Evidences for tooth replacement of known euharamiyidans are reported based on eight specimens of four species from the Jurassic Yanliao Biota, Liaoning Province, China. Tooth morphologies, eruptional and wear condition, and tooth germs are directly observed and/or revealed by Micro CT or slab CL scan. The euharamiyidan dentition has definite number of cheek teeth and monophyodont molars that are related to precise occlusion. Incisor germs are found in three specimens of Arboroharamiya but not in Shenshou lui and Xianshou linglong. The incisor germs in the upper jaw, presumably I2, have a large crown with two or three cusps; those in the lower jaw, interpreted as the permanent i2, are positioned dorsal to the root of the erupted incisor, interpreted as di2. Comparing dental development within various ontogenetic stages, the incisor tooth germs in Arboroharamiya and Vilevolodon would replace the deciduous incisors in a much later time than when ultimate molars became fully erupted and functional, if it did happen. The available evidence indicates presence of diphyodonty in the loci of the ultimate lower premolar and incisor, which are common mammalian features potentially related to lactation and parental care. The prolonged or delayed eruptions of incisors and ultimate molars in Arboroharamiya and Vilevolodon are probably associated with the specialization of dentition, with emphasis on the P4/p4 chewing function, which should be an autapomorphy and represent a heterochronic shift of tooth replacement in terms of ontogenetic timing comparing to other “haramiyidians”. The heterochronic incisor replacement is probably owing to developmental suppression related to expansion of the premolars, and may have evolved independently multiple times within mammaliaforms.

Key words: Jurassic, Yanliao Biota, euharamiyidans, tooth replacement, diphyodonty

摘要：

贼兽是一已绝灭的哺乳动物型类,它们与多瘤齿兽和相关类群的系统关系尚存争议,很大的原因在于这类动物的牙齿和头骨形态的特征呈哺乳动物冠群和似哺乳动物之间的镶嵌状态。通过显微CT和平板CL技术,对辽宁省侏罗纪燕辽生物群4种真贼兽的8件标本的牙齿形态、磨损状况和牙齿替换信息进行观察和描述,并与贼兽已发表属种牙齿替换进行对比,对二出齿这一被认为可能与哺乳和亲代养育行为演化有关的重要特征在贼兽中的存在与否进行了讨论。结果显示,所有贼兽的颊齿数恒定,在任何萌出的臼齿下均无齿胚保存,且无替换证据,可以确定为单出齿。前臼齿具一次替换,为典型的哺乳动物二出齿。所有真贼兽只有一对下门齿,一对上门齿,仅玲珑仙兽例外,其在增大的I2内侧,有一残留的I1。神兽和仙兽上下颌骨中,都不具有门齿牙胚,而树贼兽和翔齿兽的上下颌骨中,都具有替换牙胚。其中上门齿齿胚具2-3个齿尖,位于萌出的乳门齿背后方;下门齿齿胚根部始于m2下方,齿尖向前延伸至乳门齿的后背方;牙胚随着不同个体发育阶段向前推进的位置不同,较成熟的个体,齿根有逐渐愈合的现象。门齿的替换应晚于最后臼齿的完全萌出,显示了明显的滞后性,相比其他贼兽的门齿替换应为异时发育。牙胚的形态、相对颊齿大小和后端位置与神兽和仙兽已萌出的门齿更为相近,而较树贼兽和翔齿兽已萌出的门齿显得更大且更复杂,因此神兽和仙兽的门齿更可能为恒齿,贼兽类的门齿则更可能为二出齿。树贼兽和翔齿兽极度膨大的前臼齿很可能加长和推迟门齿的替换过程,甚至影响了替换的发生,同时也在一定程度上抑制了最后臼齿的萌出和功能作用。树贼兽和翔齿兽的这些特征显示它们可能具有某种特殊的食性,更倾向于强调前臼齿对食物的摄取和咀嚼处理。而贼兽具有的典型的哺乳动物二出齿特征、稳定的齿式和高度分化的牙齿等,增加了其为哺乳动物的可能性。

关键词: 侏罗纪, 燕辽生物群, 真贼兽类, 牙齿替换, 二出齿