A dramatic shift in microbial reefs occurred around the Ediacaran–Cambrian boundary. Here we describe changes in the composition, construction, and texture of microbial reefs in the Zavkhan Terrane of Gobi-Altai Province, western Mongolia, during the late Ediacaran and early Cambrian. Stromatolites consisting of peloids, micritic clots, and homogeneous lime mud without calcified microbes (calcimicrobes) are characteristic of the upper Ediacaran (units 9 and 16A of the Zuun-Arts Formation). In contrast, abundant thrombolites with stromatolites occur in the lowest Terreneuvian (units 17A and 17 of the Bayan Gol Formation). These Cambrian microbial reefs are made up of micritic clots and homogeneous lime mud in close association with calcimicrobes including Korilophyton, Renalcis, and Tarthinia. The thrombolites and calcimicrobial reefs studied herein occur directly stratigraphically above strata that record a strong negative shift in δ¹³C values and are dominated by small shelly fossils; these are the earliest known calcimicrobial reef representatives of the Phanerozoic. These microbial reefs changed almost simultaneously with drastic fluctuations in environmental conditions (e.g., seawater chemistry, Ca concentration, carbonate saturation, and oxygen level). These changes would have been influenced by the evolution of calcimicrobes and skeletal metazoans across the Ediacaran–Cambrian boundary. The present work provides crucial geobiological information on substantial shifts at the Ediacaran–Cambrian boundary and how calcimicrobes and related textures appeared in tandem with the innovation of biomineralisation.

埃迪卡拉纪-寒武纪边界附近的微生物礁发生了戏剧性的变化。在这里，我们描述了在埃迪卡拉纪晚期和寒武纪早期，蒙古西部戈壁阿尔泰省扎夫汗地层微生物礁组成、构造和质地的变化。由球体、泥晶凝块和不含钙化微生物（钙微生物）的均质石灰泥组成的叠层石是埃迪卡拉纪上层（Zuun-Arts 组的 9 和 16A 单元）的特征。相比之下，在最低的 Terreneuvian（Bayan Gol 组的 17A 和 17 单元）中出现了丰富的凝块石和叠层石。这些寒武纪微生物礁由泥晶凝块和均质石灰泥与钙质微生物（包括苔藓藻、肾藻和塔提尼亚。本文研究的凝块石和钙微生物礁直接发生在地层之上，记录了 δ ¹³的强烈负移C值，以小型贝壳化石为主；这些是已知最早的显生宙钙质微生物礁代表。这些微生物礁几乎与环境条件（例如，海水化学、Ca 浓度、碳酸盐饱和度和氧含量）的剧烈波动同时发生变化。这些变化可能受到跨越埃迪卡拉纪-寒武纪边界的钙微生物和骨骼后生动物进化的影响。目前的工作提供了关于埃迪卡拉纪-寒武纪边界发生重大变化的重要地球生物学信息，以及钙微生物和相关结构如何与生物矿化的创新同步出现。