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In this paper, we demonstrate the Mongolian Precambrian and Cambrian stratigraphic classification of sedimentary, sedimentary-volcanogenic, volcanogenic, and metamorphic rock sediments spread over the territory of Mongolia according to the new stratigraphic classification approved by the International Stratigraphic Commission. The Mongolian republic is situated in the central part of the Asiatic continent. Geological studies in different parts of Mongolia began in the middle of the twentieth century, but regular and extensive examination started in the 1970s and 1980s along with stratigraphic analyses of the oldest deposits within the sedimentary basins. The Joint Russian-Mongolian Scientific Research Geological and Palaeontological Expeditions have played a leading role in the study of the region. These studies commenced more than 40 years ago and are still in progress to some extent. Precambrian rock units, exposed mainly in northern Mongolian, have been divided into three regional metacomplexes (Baidrag, Buteel, and Bumbuger) and four sedimentary formations (Muren, Khug, Darkhad or Zavkhan, and Khubsugul or Tsagaanolom). Isotopic age determinations on rocks and minerals from Archean granulitegneiss blocks demonstrate an evolution from *3.1 to 2.6 Ga. The Palaeoproterozoic sedimentary complex is well exposed in northern and central Mongolia and is distinguished by marbles, various calciphyres, amphibolites, and iron quartzites. The age of the complex is 1600-2050 Ma. Detailed studies of the middle Neoproterozoic and early Cambrian faunal (comeosiliceous polyactinal sponges, ichnofossils, softbodied fauna, medusa, chiolites, anabarites, archaeocyathids, trilobites) and floral (stromatolites, microphytolite assemblages, cyanobacterial mats, microfossils, calcareous algae) fossils provide the opportunity to construct the first regional stratigraphic subdivision of different facies sediments. Glaciogenic deposits are widely distributed in the Neoproterozoic successions around the world, but only a few of them contain unequivocal macroscopic fossils. Precambrian glacial marine deposits (Ediacaran Maikhanuul Formation) were discovered in the Zavkhan region of

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The initial acquisition of calcium carbonate polymorphs (aragonite and calcite) at the onset of skeletal biomineralization by disparate metazoans across the Ediacaran-Cambrian transition is thought to be directly influenced by Earth’s seawater chemistry. It has been presumed that animal clades that first acquired mineralized skeletons during the so-called “aragonite sea” of the latest Ediacaran and earliest Cambrian (Terreneuvian) possessed aragonite or high-Mg calcite skeletons, while clades that arose in the subsequent “calcite sea” of Cambrian Series 2 acquired low-Mg calcite skeletons. Here, contrary to previous expectations, we document shells of one of the earliest helcionelloid molluscs from the basal Cambrian of southwestern Mongolia that are composed entirely of low-Mg calcite and formed during the Terreneuvian aragonite sea. The extraordinarily well-preserved Postacanthella shells have a simple prismatic microstructure identical to that of their modern low-Mg calcite molluscan relatives. High-resolution scanning electron microscope observations show that calcitic crystallites were originally encased within an intra- and interprismatic organic matrix scaffold preserved by aggregates of apatite during early diagenesis. This indicates that not all molluscan taxa during the early Cambrian produced aragonitic shells, weakening the direct link between carbonate skeletal mineralogy and ambient seawater chemistry during the early evolution of the phylum. Rather, our study suggests that skeletal mineralogy in Postacanthella was biologically controlled, possibly exerted by the associated prismatic organic matrix. The presence of calcite or aragonite mineralogy in different early Cambrian molluscan taxa indicates that the construction of calcium carbonate polymorphs at the time when skeletons first emerged may have been species dependent.