Liu Yao, Zeng Han, Zhao Fangchen, Zhu Yuyan, Li Yimeng, Yin Zongjun, Zhu Maoyan
State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, 210008, China.
University of Chinese Academy of Sciences, Beijing, 100049, China.
Sci Rep. 2025 May 31;15(1):19115. doi: 10.1038/s41598-025-03544-0.
The mandibulate euarthropods are the most speciose animal group, but the evolutionary gaps in origin of mandibulate body plan remain unresolved. Marrellomorphs, a common Paleozoic euarthropod group, had a long evolutionary history from Cambrian to Devonian. With computed microtomography, here we report the fine-scale soft-bodied morphoanatomy of the oldest marrellomorph Primicaris larvaformis, a millimeters-sized euarthropod from the ~ 518-million-year-old Chengjiang biota, China. Primicaris possesses a body plan featuring morphologically similar post-antennular biramous appendages, but also mandibulate diagnostic features including multi-segmented exopodites, a well-developed and differentiated hypostome-labrum complex, and a pancrustacean-like topological configuration of frontalmost three pairs of appendages. Phylogenetic analysis resolves Acercostraca and Marrellida as stem-Mandibulata. The undifferentiated post-antennular appendages in Primicaris suggest a possibility that the head appendages acquired a crown-mandibulate configuration before their morphological specialization in mandibulate origin. The emergence of novel appendage morphotypes in Acercostraca and Marrellida reveals that the complexity of limb tagmatization evolved independently in different Euarthropoda clades.
有颚节肢动物是物种最为丰富的动物类群，但有颚身体结构起源方面的进化空白仍未得到解决。奇虾类是常见的古生代节肢动物类群，从寒武纪到泥盆纪有着漫长的进化历史。借助计算机断层扫描技术，我们在此报告了最古老的奇虾类——幼虫形始虾的精细软组织形态解剖结构，这是一种来自中国约5.18亿年前澄江生物群的毫米级节肢动物。始虾具有一种身体结构，其触角后双分支附肢在形态上相似，但也具有有颚类的诊断特征，包括多节外肢、发育良好且分化的下咽 - 唇复合体，以及最前端三对附肢类似泛甲壳动物的拓扑结构。系统发育分析将桨足纲和奇虾目解析为有颚类干群。始虾中未分化的触角后附肢表明，在有颚类起源过程中，头部附肢在形态特化之前就有可能获得了冠有颚类的结构。桨足纲和奇虾目中新附肢形态类型的出现表明，附肢分节的复杂性在不同的节肢动物类群中是独立进化的。
