重庆云阳中侏罗统三列齿兽类化石新发现及其古地理意义^*

New discovery of tritylodontids from the Milldle Jurassic in Yunyang area,Chongqing and their palaeogeographic significance

摘要

三列齿兽类(tritylodontids)是一类特化的、植食性的进步下孔类,在晚三叠世—早侏罗世呈全球性分布,之后趋于衰退,在亚洲东北部地区的分布可以延续至早白垩世。三列齿兽类被认为是与哺乳动物亲缘关系较近的下孔类,可为认识哺乳动物起源提供重要证据。借助CT扫描和三维重建技术,文中报道了2件发现于重庆云阳下沙溪庙组和新田沟组的三列齿兽类头骨化石,其中一件标本保存较完整的头骨,具有颧弓宽大且腹向扩展、上颊齿短宽、上颊齿的齿尖型式2-3-3等特征,而另一件标本仅保存下颌骨。经过形态特征对比和研究,认为这2件新标本应归入似卞氏兽属,但由于标本数量有限且保存不太完整,缺乏其他特征佐证,并且三列齿兽种内差异很大,所以笔者暂将2件新标本定为似卞氏兽属的2个未定种Bienotheroides sp.1和Bienotheroides sp.2。结合已有研究成果,讨论了三列齿兽类时空分布的演化,认为在早侏罗世三列齿兽类的明显灭绝可能与图阿尔期全球变暖相关,中—晚侏罗世三列齿兽类有明显的向北迁移的趋势。上述化石的发现不仅丰富了该地区的三列齿兽类化石材料,而且为探讨该类群的分类多样性、演化分异等提供了重要支撑。

Abstract

The Tritylodontidae represent a highly specialized group of herbivorous,derived synapsids that exhibited a global distribution during the Late Triassic and Early Jurassic. Following this era,their presence declined,with the last known occurrences recorded in northeastern Asia during the Early Cretaceous. Tritylodontids are considered one of the closest clades to mammals in terms of phylogeny,making them essential for understanding mammalian origins. This paper utilizes CT scanning and 3D reconstruction techniques to describe two new tritylodontid specimens from the Middle Jurassic Xintiangou Formation and Lower Shaximiao Formation in Yunyang,Chongqing. One specimen preserves a more complete skull,characterized by broad,ventrally extended zygomatic arches,short and broad upper cheek teeth,and an upper cheek tooth cusp formula of 2-3-3. The other specimen retains only the lower jaw. Comparative analysis of morphological features indicates that both new specimens should be classified as Bienotheroides. The number of specimens is limited,and the preservation is incomplete,with no additional characteristics available as evidence. The two new specimens are tentatively designated as Bienotheroides sp.1 and Bienotheroides sp.2. Furthermore,this paper discusses the spatio-temporal evolution of tritylodontids,suggesting that the apparent extinction of Early Jurassic tritylodontids may be linked to the Toarcian global warming event. Additionally,a clear northward migration trend is observed during the Middle to Late Jurassic. The discovery of these fossils not only enriches the tristichid fossil assemblage in this region but also provides significant support for discussions regarding the taxonomic diversity and evolutionary differentiation of this group.

关键词

重庆 / 中侏罗统 / 下沙溪庙组 / 新田沟组 / 三列齿兽类

Key words

Chongqing / Middle Jurassic / Lower Shaximiao Formation / Xintiangou Formation / tritylodontids

基金资助

*重庆市自然科学基金项目(编号: CSTB2022NSCQ-MSX1537)、重庆市规划和自然资源局《重庆市重要古生物化石资源保护研究》项目、重庆市市级人才计划“包干制”项目(编号: cstc2022ycjh-bgzxm0170)、国家自然科学基金(编号: 42122010,42072002)、中国科学院青年促进会项目(编号: 2019076)、北科萌芽计划(编号: BGS202201)共同资助