有壳变形虫(testate amoebae)的演化历史最早可追溯至新元古代早期, 以该时期北美、华北、挪威以及澳洲等多个地区浅海相碳酸盐岩、页岩中发现的瓶状微体化石(vase-shaped microfossils)为标志。此前认为, 显生宙的有壳变形虫最早出现在早泥盆世。长期以来, 早古生代的地层中未发现这类原生生物的明确化石证据。本研究通过对岩石样品进行常规孢粉酸泡分析处理和切磨岩石薄片, 获取原位保存的化石标本的技术方法, 从贵州东部剑河县交榜剖面出露的寒武系杷榔组(第2统第4阶)中获得数枚有壳变形虫(testate amoebae)化石标本。基于标本的显微形态特征, 并结合激光拉曼光谱等研究, 对原先记述为疑源类的Plagasphaera balangensis和P. sp. A两形态种进行重新认识和描述。由于它们在结构和形态上与一些现生的鳞壳虫目(Euglyphida)有壳变形虫极为相似, 因此将先前定为疑源类的Plagasphaera balangensis和P. sp. A两形态属、种名, 分别修订为Palaeoassulina balangensis gen. et sp. nov.和?Palaeoassulina sp. A。该发现不仅将显生宙有壳变形虫的原有化石记录从晚古生代泥盆纪向前延伸至寒武纪早期, 还为调查研究有壳变形虫的系统演化提供关键的生物化石证据。

The evolutionary history of testate amoebae can be traced back to the early Neoproterozoic. The earliest testate amoebae are represented by vase-shaped microfossils found in shallow marine carbonate rocks and shales in North America, North China, Norway, Australia, and other areas. In the Phanerozoic, the earliest fossil record of testate amoebae occurs in the Lower Devonian, whereas there has been no unambiguous fossil evidence of such protists known in the early Paleozoic rocks for a long time. By means of standard palynological acid maceration techniques, we obtained fossil specimens preserved in situ by grinding thin slices of rocks. We found several organic microfossils bearing strange ornamentations from the Balang Formation (Stage 4, Series 2, Cambrian) at the Jiaobang section in Jianhe County, eastern Guizhou. Based on their micromorphological characteristics and the results of laser Raman spectroscopy, these organic-walled microfossils are proposed as remains of the early Cambrian testate amoebae, and they are morphologically very similar to those of Euglyphida. In particular, they can be morphologically compared with extant testate amoebae Assulina Greef, 1888. Thus, our specimens, previously described as acritrach Plagasphaera balangensis and Plagasphaera sp. A, are now re-classified here as Palaeoassulina balangensis gen. et sp. nov., and ?Palaeoassulina sp. A, respectively, based on their morphological similarity to extant testate amoeba Assulina Greef, 1888. This discovery not only extends the fossil record of testate amoebae from the previously known late Paleozoic to the early Cambrian, but also provides key fossil evidence for understanding the evolution of testate amoebae.

国家自然科学基金(E250020027)、中国科学技术部项目(2015FY310100-5)和贵州省科技计划项目(黔科合支撑[2020]4Y241号)联合资助