Mudstone-hosted microfossils are a major component of the Proterozoic fossil record, particularly dominating the record of early eukaryotic life. Early organisms possessed no biomineralized parts to resist decay and controls on their fossilization in mudstones are poorly understood. Consequently, the Proterozoic fossil record is compromised—we do not know whether changing temporal/spatial patterns of microfossil occurrences reflect evolution or the distribution of favourable fossilization conditions. We investigated fossilization within the approximately 1000 Ma Lakhanda Group (Russia) and the approximately 800 Ma Svanbergfjellet and Wynniatt formations (Svalbard and Arctic Canada). Vertical sections of microfossils and surrounding matrices were extracted from thin sections by focused ion beam milling. Elemental mapping and synchrotron-based infrared microspectroscopy revealed that microfossils are surrounded by haloes rich in aluminium, probably hosted in kaolinite. Kaolinite has been implicated in Cambrian Burgess Shale-type (BST) fossilization and is known to slow the growth of degraders. The Neoproterozoic mudstone microfossil record may be biased to tropical settings conducive to kaolinite formation. These deposits lack metazoan fossils even though they share fossilization conditions with younger BST deposits that are capable of preserving non-mineralizing metazoans. Thus metazoans, at least those typically preserved in BST deposits, were probably absent from sedimentary environments before approximately 800 Ma.

泥岩中的微化石是元古代化石记录的主要组成部分，特别是在早期真核生命的记录中占主导地位。早期生物体不具有生物矿化部分来抵抗腐烂，并且对其在泥岩中化石化的控制知之甚少。因此，元古代化石记录受到损害——我们不知道微化石出现的时间/空间模式的变化是否反映了进化或有利的化石条件的分布。我们调查了大约 1000 Ma Lakhanda 群（俄罗斯）和大约 800 Ma Svanbergfjellet 和 Wynniatt 地层（斯瓦尔巴群岛和加拿大北极地区）内的化石形成。通过聚焦离子束铣削从薄片中提取微化石和周围基质的垂直部分。元素测绘和基于同步加速器的红外显微光谱显示，微化石被富含铝的晕包围，可能存在于高岭石中。高岭石与寒武纪伯吉斯页岩型 (BST) 化石有关，并且已知可以减缓降解剂的生长。新元古代泥岩微化石记录可能偏向于有利于高岭石形成的热带环境。这些矿床缺乏后生动物化石，尽管它们与能够保存非矿化后生动物的较年轻的 BST 矿床共享化石条件。因此，后生动物，至少是那些通常保存在 BST 沉积物中的后生动物，在大约 800 Ma 之前可能不存在于沉积环境中。