New Palaeoscolecid plates from the Cambrian Stage 3 of northern Mongolia

New material of disarticulated paleoscolecid remains have been found in “Small Shelly/Skeletal Fossils” assemblages from Cambrian Stage 3 extracted from a section in the Khubsugul Lake region of northern Mongolia. The current material is composed of isolated phosphatic plates, rendering the whole-body reconstruction and comparisons difficult. However, the morphology of the plates is unique enough to warrant description of a new genus and species Floraconformis egiinensis. The new taxon is characterised by a stellate depression network spreading from the middle that separates numerous elevations. Floraconformis egiinensis gen. et sp. nov. represents one of the oldest records of isolated palaeoscolecid plates

Locating the BACE of the Cambrian: Bayan Gol in southwestern Mongolia and global correlation of the Ediacaran–Cambrian boundary

The diversification of animals during the Cambrian Period is one of the most significant evolutionary events inEarth’s history. However, the sequence of events leading to the origin of ‘modern’ ecosystems and the exacttemporal relationship between Ediacaran and Cambrian faunas are uncertain, as identification of the Ediacaran–Cambrian boundary and global correlation through this interval remains problematic. Here we review thecontroversies surrounding global correlation of the base of the Cambrian and present new high-resolutionbiostratigraphic, lithostratigraphic and δ13C chemostratigraphic data for terminal Ediacaran to basal Cambrianstrata in the Zavkhan Basin of Mongolia. This predominantly carbonate sequence, through the Zuun-Arts andBayangol formations in southwestern Mongolia, captures a distinct, negative δ13C excursion close to the top ofthe Zuun-Arts Formation recognized as the BAsal Cambrian carbon isotope Excursion (BACE). In this location,the nadir of the BACE closely coincides with first occurrence of the characteristic early Cambrian protoconodontProtohertzina anabarica. Despite recent suggestions that there is an evolutionary continuum of biomineralizinganimals across the Ediacaran–Cambrian transition, we suggest that this continuum is restricted to tubular forms,and that skeletal taxa such as Protohertzina depict ‘true’ Cambrian representatives that are restricted entirely tothe Cambrian. Employing the first appearance of the trace fossil Treptichnus pedum to define the base of theCambrian suffers significant drawbacks, particularly in carbonate settings where it is not commonly preserved.As T. pedum is the only proxy available to correlate the Cambrian Global boundary Stratotype Section and Point(GSSP) defined at Fortune Head, Newfoundland, we suggest that the GSSP be redefined elsewhere, in a newstratigraphic section that contains secondary markers that permit global correlation. We propose the nadir of theBACE as the favored candidate to define the base of the Cambrian. However, it is essential that the BACE becomplemented with secondary markers. In many global sections the nadir of the BACE and the first occurrence ofthe genus Protohertzina are closely juxtaposed, as are the BACE and T. pedum. Hence these taxa provide essentialbiostratigraphic control on the BACE and increase potential for effective global correlation. We also recommendthat an Auxiliary boundary Stratotype Section and Point (ASSP) be simultaneously established in order toincorporate additional markers that will aid global correlation of the Ediacaran–Cambrian boundary. The BAY4/5 section through the upper Zuun-Arts and Bayangol formations yields key shelly fossils and δ13C values and istherefore an ideal candidate for consideration as the GSSP for the Ediacaran–Cambrian boundary

Calcitic shells in the aragonite sea of the earliest Cambrian

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 Postacanthellashells 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 dependen