Yunnan Key Laboratory for Palaeobiology, Yunnan University, South Waihuan Road, Chenggong District, Kunming 650500, China; MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Yunnan University, Kunming 650500, China.
Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA.
Curr Biol. 2020 Aug 3;30(15):3057-3061.e2. doi: 10.1016/j.cub.2020.05.085. Epub 2020 Jun 25.
The euarthropod head is a highly versatile and functionally specialized body region composed of multiple appendage-bearing segments and whose complex evolution has been scrutinized through anatomical, developmental, and paleontological approaches [1-4]. Exceptionally preserved Cambrian fossils have allowed for the reconstruction of critical stages of the evolutionary history of the head, such as the origin of the labrum-an anteromedian flap-like structure that overlies the mouth opening in almost all extant representatives-from an ancestral pair of pre-ocular (protocerebral) appendages [3-5]. The highly conserved position of the labrum makes it a valuable anatomical landmark for understanding the anterior segmental organization among extant and extinct euarthropods [2]. However, the labrum is seemingly absent in the megacheirans, a major extinct group characterized by enlarged raptorial "great appendages" with a central role in competing hypotheses on the early evolution of the head [1-3, 6-8]. Here, we used micro-computed tomography to demonstrate the presence of a three-dimensionally preserved labrum associated with the mouth opening in juvenile specimens of the megacheiran Leanchoilia illecebrosa from the early Cambrian Chengjiang biota, Southwest China. The position of the labrum relative to the pre-oral great appendages of L. illecebrosa indicates that these limbs correspond to the deutocerebral segment and are therefore serially homologous with the first appendage pair of extant euarthropods [1, 2, 4, 6, 8]. The reduced labrum and deutocerebral great appendages of L. illecebrosa also strengthen the affinities of megacheirans as stem-group chelicerates, in line with recent paleoneurological fossil data from the early to mid-Cambrian of China and North America [6, 9].
真节肢动物的头部是一个高度通用和功能专业化的身体区域，由多个附肢承载的节段组成，其复杂的进化通过解剖学、发育和古生物学方法进行了仔细研究[1-4]。保存异常完好的寒武纪化石使人们能够重建头部进化历史的关键阶段，例如，唇瓣的起源——一种覆盖在几乎所有现存代表的口部开口前缘的瓣状结构——是由一对前眼（原脑）附肢的祖先演变而来[3-5]。唇瓣的高度保守位置使其成为理解现存和已灭绝真节肢动物头部前节段组织的有价值的解剖学标志[2]。然而，巨型栉蚕类中似乎没有唇瓣，巨型栉蚕类是一个主要的灭绝类群，其特征是具有中央作用的特化捕食“大附肢”，在头部早期进化的假说中占据核心地位[1-3,6-8]。在这里，我们使用微计算机断层扫描技术，证明在中国西南早寒武世澄江生物群的幼年 Leanchoilia illecebrosa 标本中，与口部开口相关的三维保存的唇瓣的存在。唇瓣相对于 L.illecebrosa 的前口大附肢的位置表明，这些附肢对应于后脑节，因此与现存真节肢动物的第一对附肢具有同源关系[1,2,4,6,8]。L.illecebrosa 的缩小的唇瓣和后脑大附肢也加强了巨型栉蚕类作为原始螯肢动物的亲缘关系，与来自中国和北美的早至中寒武纪的最新古神经化石数据一致[6,9]。
