The late Mesoproterozoic to Tonian (∼1100 Ma to ∼720 Ma) witnessed a critical evolutionary transition in Earth history. Several fossil taxa, including acritarchs and vase-shaped microfossils, have been suggested as potential diagnostic fossils for this time interval. The acanthomorphic acritarch Trachyhystrichosphaera aimika has become a focus of recent biostratigraphic investigations, showing great potential to assist the definition of the Tonian Global Boundary Stratotype Section and Point (GSSP). Although T. aimika has been extensively reported in the Proterozoic sequences, little is known about its phylogenetic interpretation. Its palaeogeographic and stratigraphic distributions need to be critically scrutinized with recently emended diagnosis of the taxon and new published age constraints. In this study, we report new palaeobiological data of T. aimika specimens from the Tonian Liulaobei Formation in the Huainan region, North China, using transmitted light microscopy, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Our analyses reveal a multilayered ultrastructure from the vesicle wall of T. aimika that has experienced advanced diagenesis to low-grade metamorphism with the peak palaeotemperature of ∼212 °C. In addition, a compilation of palaeogeographic occurrences of T. aimika shows the taxon is preserved in a wide range of palaeoenvironments and has a broad palaeogeographic distribution with a relatively limited stratigraphic range from ∼1150 Ma to ∼720 Ma, suggesting that T. aimika has great potential to become an index fossil for the late Mesoproterozoic to Tonian interval.

中元古代至通尼时期（约1100 Ma至约720 Ma）见证了地球历史上的关键性演化过渡。有人建议使用几种化石类群，包括顶角生物和花瓶状微化石，作为该时间间隔的潜在诊断化石。棘手的头针准直器（Trachyhystrichosphaera aimika）已成为最近生物地层学研究的重点，显示出巨大的潜力可协助定义Tonian Global Boundary Stratotype Section and Point（GSSP）。虽然T. aimika已在元古代序列中广泛报道，对其系统发育解释了解甚少。需要对它的古地理和地层分布进行严格审查，包括最近修订的分类单元诊断和新的年龄限制。在这项研究中，我们使用透射光显微镜，扫描电子显微镜，透射电子显微镜和拉曼光谱法报告了华北淮南地区Tonian Liulaobei组的T. aimika标本的新的古生物学数据。我们的分析揭示了T. aimika囊泡壁的多层超微结构经历了成岩作用向低等变质的高级成岩作用，古温度峰值约为212°C。此外，古地理出现的汇编T. aimika示出了分类单元在很宽的范围古环境的保存和具有相对有限的范围内的地层从~1150马到~720马宽的古地理分布，这表明T. aimika具有具有成为中元古代晚期至Tonian间隔的索引化石的巨大潜力。