The SPICE carbon isotope excursion in Siberia: a combined study of the upper Middle Cambrian-lowermost Ordovician Kulyumbe River section, northwestern Siberian Platform

An integrated, high-resolution chemostratigraphic (C, O and Sr isotopes) and magnetostratigraphic study through the upper Middle Cambrian–lowermost Ordovician shallowmarine carbonates of the northwestern margin of the Siberian Platform is reported. The interval was analysed at the Kulyumbe section, which is exposed along the Kulyumbe River, an eastern tributary of the Enisej River. It comprises the upper Ust’-Brus, Labaz, Orakta, Kulyumbe, Ujgur and lower Iltyk formations and includes the Steptoean positive carbon isotopic excursion (SPICE) studied here in detail from upper Cambrian carbonates of the Siberian Platform for the first time. The peak of the excursion, showing δ13C positive values as high as+4.6‰and least-altered 87Sr/86Sr ratios of 0.70909, is reported herein from the Yurakhian Horizon of the Kulyumbe Formation. The stratigraphic position of the SPICE excursion does not support traditional correlation of the boundary between theOrakta and Labaz formations at the Kulyumbe River with its supposedly equivalent level in Australia, Laurentia, South China and Kazakhstan, where the Glyptagnostus stolidotus and G. reticulatus biozones are known to immediately precede the SPICE excursion and span the Middle–Upper Cambrian boundary. The Cambrian–Ordovician boundary is probably situated in the middle Nyajan Horizon of the Iltyk Formation, in which carbon isotope values show a local maximum below a decrease in the upper part of the Nyajan Horizon, attributed herein to the Tremadocian Stage. A refined magnetic polarity sequence confirms that the geomagnetic reversal frequency was very high during Middle Cambrian times at 7–10 reversals per Ma, assuming a total duration of about 10 Ma and up to 100 magnetic intervals in the Middle Cambrian. By contrast, the sequence attributed herein to the Upper Cambrian on chemostratigraphic grounds contains only 10–11 magnetic intervals

The two phases of the Cambrian Explosion

Abstract The dynamics of how metazoan phyla appeared and evolved – known as the Cambrian Explosion – remains elusive. We present a quantitative analysis of the temporal distribution (based on occurrence data of fossil species sampled in each time interval) of lophotrochozoan skeletal species (n = 430) from the terminal Ediacaran to Cambrian Stage 5 (~545 – ~505 Million years ago (Ma)) of the Siberian Platform, Russia. We use morphological traits to distinguish between stem and crown groups. Possible skeletal stem group lophophorates, brachiopods, and molluscs (n = 354) appear in the terminal Ediacaran (~542 Ma) and diversify during the early Cambrian Terreneuvian and again in Stage 2, but were devastated during the early Cambrian Stage 4 Sinsk extinction event (~513 Ma) never to recover previous diversity. Inferred crown group brachiopod and mollusc species (n = 76) do not appear until the Fortunian, ~537 Ma, radiate in the early Cambrian Stage 3 (~522 Ma), and with minimal loss of diversity at the Sinsk Event, continued to diversify into the Ordovician. The Sinsk Event also removed other probable stem groups, such as archaeocyath sponges. Notably, this diversification starts before, and extends across the Ediacaran/Cambrian boundary and the Basal Cambrian Carbon Isotope Excursion (BACE) interval (~541 to ~540 Ma), ascribed to a possible global perturbation of the carbon cycle. We therefore propose two phases of the Cambrian Explosion separated by the Sinsk extinction event, the first dominated by stem groups of phyla from the late Ediacaran, ~542 Ma, to early Cambrian stage 4, ~513 Ma, and the second marked by radiating bilaterian crown group species of phyla from ~513 Ma and extending to the Ordovician Radiation

Small shelly fossils and carbon isotopes from the early Cambrian (Stage 3-4) Mural Formation of western Laurentia

The extraordinary window of phosphatised and phosphatic Small Shelly Fossils (SSFs) during the early and middle Cambrian is an important testament to the radiation of biomineralising metazoans. While SSF are well known from most Cambrian palaeocontinents during this time interval, western Laurentia has relatively few SSF faunas. Here we describe a diverse SSF fauna from the early Cambrian (Stage 3-4) Mural Formation at three localities in Alberta and British Columbia, Canada, complemented by carbon isotope measurements to aid in a potential future bio-chemostratigraphic framework. The fauna expands the recorded SSF assemblage diversity in western Laurentia and includes several brachiopods, four bradoriids, three chancelloriids, two hyoliths, a tommotiid and a helcionellid mollusc as well as echinoderm ossicles and specimens of Microdictyon, Volborthella and Hyolithellus. New taxa include the tommotiid genus Canadiella gen. nov., the new bradoriid species Hipponicharion perforata sp. nov. and Pseudobeyrichona taurata sp. nov. Compared to contemporaneous faunas from western Laurentia, the fauna is relatively diverse, particularly in taxa with originally phosphatic shells, which appear to be associated with archaeocyathid buildups. This suggests that the generally low faunal diversity in western Laurentia may be at least partly a consequence of poor sampling of suitable archaeocyathan reef environments. In addition, the tommotiid Canadiella filigrana appears to be of biostratigraphic significance in Cambrian Stage 3 strata of western Laurentia and the unexpected high diversity of bradoriid arthropods in the fauna also suggests that this group may prove useful for biostratigraphic resolution in the region

Shell microstructures in Early Cambrian molluscs

The affinities of a considerable part of the earliest skeletal fossils are problematical, but investigation of their microstructures may be useful for understanding biomineralization mechanisms in early metazoans and helpful for their taxonomy. The skeletons of Early Cambrian mollusc-like organisms increased by marginal secretion of new growth lamellae or sclerites, the recognized basal elements of which were fibers of apparently aragonite. The juvenile part of some composite shells consisted of needle-like sclerites; the adult part was built of hollow leaf-like sclerites. A layer of mineralized prism-like units (low aragonitic prisms or flattened spherulites) surrounded by an organic matrix possibly existed in most of the shells with continuous walls. The distribution of initial points of the prism-like units on a periostracurn-like sheet and their growth rate were mostly regular. The units may be replicated on the surface of internal molds as shallow concave polygons, which may contain a more or less well-expressed tubercle in their center. Tubercles are often not enclosed in concave polygons and may co-occur with other types of textures. Convex polygons seem to have resulted from decalcification of prism-like units. They do not co-occur with tubercles. The latter are interpreted as casts of pore channels in the wall possibly playing a role in biomineralization or pits serving as attachment sites of groups of mantle cells. Casts of fibers and/or lamellar units may overlap a polygonal texture or occur without it. They may reflect an inner layer consisting of aragonitic fibers fused into more or less well-developed lamellar units. It seems that nacreous and crossed-lamellar aragonitic microstructures evolved in the Cambrian from such lamellar aragonitic microstructures independently in different groups of molluscs

The tube wall of Cambrian anabaritids

Celestite/barite−replaced and phosphate−replicated tubes of Early Cambrian anabaritids from the northern part of the Siberian Platform (Anabar Shield) give new evidence on the wall−structure of these enigmatic fossils. The walls consist of fibres, interpreted as reflecting an original aragonitic fabric. Bundles of fibres are arranged in growth lamellae, and the latter form an angle of at least 45° with the inner tube wall. Where the outer tube surface projects into annular flanges, the lamellae have a chevron−like section due to the backwards deflection of the outer parts. Anabaritids are usually referred to the Cnidaria or left without systematic assignment, but earlier suggestions included affinity to the serpulid polychaetes. The chevron structure resembles that previously exclusively known from serpulids, but the presence of internal tooth−like structures in anabaritid tubes, perhaps compromising up−and−down movement through the tubes, continue to make a direct assignment to the Serpulida questionable

A new tannuolinid problematic from the lower Cambrian of the Sukharikha River in northern Siberia

A new species of tannuolinid, Tannuolina pavlovi, is reported from the basal Krasnoporog Formation cropping out along the Sukharikha River at the northwestern margin of the Siberian Platform. The new material expands the geographic range of tannuolinids onto the Siberian Platform and extends their stratigraphic range into the basal Tommotian Stage of the traditional Lower Cambrian. The oldest appearance of tannuolinids in the fossil record is shown herein to have occurred as early as the oldest appearance of the linguliformean brachiopods, of which tannuolinids have been suggested to be a stem group. The sellate sclerites of T. pavlovi sp. nov. are different from those of the other known species of Tannuolina in the consistent absence of a pronounced sella and duplicature on the respective sides of sclerites, as well as in having regularly distributed large setal pores on the lateral edges. The mitral sclerites of T. pavlovi sp. nov. do not clearly show asymmetry of left and right forms, manifested in other species by the presence of the carina on the interior surface. Large pores aligned along the lateral margins of sellate sclerites regularly coalesce below the wall surface from an apically and an aperturally directed basal branch, forming a nearly right angle

A new tannuolinid problematic from the lower Cambrian of the Sukharikha River in northern Siberia

A new species of tannuolinid, Tannuolina pavlovi, is reported from the basal Krasnoporog Formation cropping out along the Sukharikha River at the northwestern margin of the Siberian Platform. The new material expands the geographic range of tannuolinids onto the Siberian Platform and extends their stratigraphic range into the basal Tommotian Stage of the traditional Lower Cambrian. The oldest appearance of tannuolinids in the fossil record is shown herein to have occurred as early as the oldest appearance of the linguliformean brachiopods, of which tannuolinids have been suggested to be a stem group. The sellate sclerites of T. pavlovi sp. nov. are different from those of the other known species of Tannuolina in the consistent absence of a pronounced sella and duplicature on the respective sides of sclerites, as well as in having regularly distributed large setal pores on the lateral edges. The mitral sclerites of T. pavlovi sp. nov. do not clearly show asymmetry of left and right forms, manifested in other species by the presence of the carina on the interior surface. Large pores aligned along the lateral margins of sellate sclerites regularly coalesce below the wall surface from an apically and an aperturally directed basal branch, forming a nearly right angle

The problematic early Cambrian fossil Tumulduria incomperta represents the detached ventral interarea of a paterinid brachiopod

The organophosphatic early Cambrian (Terreneuvian, Cambrian Stage 2) fossil Tumulduria incomperta has been problematic ever since its original description in 1969. Comparison of abundant specimens from the Lower Cambrian of Siberia with co-occurring brachiopod valves show that T. incomperta represents the central portion of the ventral interarea of a paterinid brachiopod similar to Cryptotreta neguertchenensis, and that the domed central portion of typical Tumulduria specimens represents the ridge-like pseudodeltidium of the interarea