Artem Kouchinsky

Chronology of early Cambrian biomineralization

Data on the first appearances of major animal groups with mineralized skeletons on the Siberian Platform and worldwide are revised and summarized herein with references to an improved carbon isotope stratigraphy and radiometric dating in order to reconstruct the Cambrian radiation (popularly known as the ‘Cambrian explosion’) with a higher precision and provide a basis for the definition of Cambrian Stages 2 to 4. The Lophotrochozoa and, probably, Chaetognatha were first among protostomians to achieve biomineralization during the Terreneuvian Epoch, mainly the Fortunian Age. Fast evolutionary radiation within the Lophotrochozoa was followed by radiation of the sclerotized and biomineralized Ecdysozoa during Stage 3. The first mineralized skeletons of the Deuterostomia, represented by echinoderms, appeared in the middle of Cambrian Stage 3. The fossil record of sponges and cnidarians suggests that they acquired biomineralized skeletons in the late Neoproterozoic, but diversification of both definite sponges and cnidarians was in parallel to that of bilaterians. The distribution of calcium carbonate skeletal mineralogies from the upper Ediacaran to lower Cambrian reflects fluctuations in the global ocean chemistry and shows that the Cambrian radiation occurred mainly during a time of aragonite and high-magnesium calcite seas.

Fossilized embryos are widespread but the record is temporally and taxonomically biased

SUMMARY We report new discoveries of embryos and egg capsules from the Lower Cambrian of Siberia, Middle Cambrian of Australia and Lower Ordovician of North America. Together with existing records, embryos have now been recorded from four of the seven continents. However, the new discoveries highlight secular and systematic biases in the fossil record of embryonic stages. The temporal window within which the embryos and egg capsules are found is of relatively short duration; it ends in the Early Ordovician and is roughly coincident with that of typical “Orsten”‐type faunas. The reduced occurrence of such fossils has been attributed to reducing levels of phosphate in marine waters during the early Paleozoic, but may also be owing to the increasing depth of sediment mixing by infaunal metazoans. Furthermore, most records younger than the earliest Cambrian are of a single kind—large eggs and embryos of the priapulid‐like scalidophoran Markuelia. We explore alternative explanations for the low taxonomic diversity of embryos recovered thus far, including sampling, size, anatomy, ecology, and environment, concluding that the preponderance of Markuelia embryos is due to its precocious development of cuticle at an embryonic stage, predisposing it to preservation through action as a substrate on which microbially mediated precipitation of authigenic calcium phosphate may occur. The fossil record of embryos may be limited to a late Neoproterozoic to early Ordovician snapshot that is subject to dramatic systematic bias. Together, these biases must be considered seriously in attempts to use the fossil record to arbitrate between hypotheses of developmental and life history evolution implicated in the origin of metazoan clades.

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.

Carbon isotope stratigraphy of the Precambrian-Cambrian Sukharikha River section, northwestern Siberian platform

A high-resolution carbon isotope profile through the uppermost Neoproterozoic-Lower Cambrian part of the Sukharikha section at the northwestern margin of the Siberian platform shows prominent secul ...

Cnidarian-like embryos associated with the first shelly fossils in Siberia

Phosphatized spheroids, similar to 0.5 mm in diameter, in the Lower Cambrian Manykay Formation at the Bolshaya Kuonamka River in northern Sakha (Yakutia) are interpreted as cnidarian embryos of late developmental stages, One of the poles has a double cro

An early Cambrian fauna of skeletal fossils from the Emyaksin Formation, northern Siberia.

An assemblage of mineralised skeletal fossils containing molluscs, hyoliths, halkieriids, chancelloriids, tommotiids, lobopodians, paleoscolecids, bradoriids, echinoderms, anabaritids, hyolithelminths, hexactinnelid, and heteractinid sponges is described from the early Cambrian Emyaksin Formation exposed along the Malaya Kuonamka and Bolshaya Kuonamka rivers, eastern flanks of the Anabar Uplift, northern Siberian Platform. The sampled succession is attributed to the Tommotian—Botoman Stages of Siberia and correlated with Stage 2 of Series 1—Stage 4 of Series 2 of the IUGS chronostratigraphical scheme for the Cambrian. Carbon isotope chemostratigraphy is applied herein for regional correlation. The fauna contains the earliest Siberian and probably global first appearances of lobopodians, paleoscolecids, and echinoderms, and includes elements in common with coeval faunas from Gondwana, Laurentia, and Baltica. For the first time from Siberia, the latest occurrence of anabaritids is documented herein from the Atdabanian Stage. Problematic calcium phosphatic sclerites of Fengzuella zhejiangensis have not been previously known from outside China. The sellate sclerites, Camenella garbowskae and mitral sclerites, C. kozlowskii are unified within one species, C. garbowskae. In addition to more common slender sclerites, Rhombocorniculum insolutum include broad calcium phosphatic sclerites. A number of fossils described herein demonstrate excellent preservation of fine details of skeletal microstructures. Based on new microstructural data, sclerites of Rhombocorniculum are interpreted as chaetae of the type occurring in annelids. A new mollusc Enigmaconus? pyramidalis Kouchinsky and Vendrasco sp. nov. and a hyolith Triplicatella papilio Kouchinsky sp. nov. are described.

Pre-Tommotian age of the lower Pestrotsvet Formation in the Selinde section on the Siberian platform: carbon isotopic evidence

Carbon isotopic data from the Selinde section in the southeastern part of the Siberian platform area are correlated with the reference isotopic profile from the Lower Cambrian stratotype sections of the Lena-Aldan region, but also show additional δ 13 C excursions unrecognized there. The chemostratigraphic correlation suggests that the geological and fossil record of the lower Pestrotsvet Formation in the Selinde section has a deeper history than the stratotype region. This conclusion is important for both constraining the age of the earliest Cambrian marine transgression on the Siberian platform and providing a clearer understanding of the pace and order of early Cambrian geochemical and biological events.

A middle Cambrian fauna of skeletal fossils from the Kuonamka Formation, northern Siberia

An assemblage of mineralized skeletal fossils containing molluscs, hyoliths, chancelloriids, protoconodonts, lobopods, paleoscolecids, bradoriids, echinoderms and hexactinellid sponges is described from the middle Cambrian part of the Kuonamka Formation, exposed along the Malaya Kuonamka and Bolshaya Kuonamka rivers, northern Siberian Platform. The sampled succession is attributed to the Kuonamkites and lower Tomagnostus fissus–Paradoxides sacheri biozones of the Amgan Stage of Siberia, correlated with Series 3, Stage 5—lower Drumian Stage of the IUGS chronostratigraphical scheme for the Cambrian. This work complements descriptions of molluscs from the same samples published by Gubanov et al. (2004) with additional material. It contains forms in common with coeval faunas from Australia, China, Western Gondwana, Avalonia, Laurentia and Baltica, increasing potential for global biostratigraphic correlation and understanding of palaeogeographic connections.

THE LOWER CAMBRIAN FOSSIL ANABARITIDS: AFFINITIES, OCCURRENCES AND SYSTEMATICS

Synopsis Anabaritids, or angustiochreids, are extinct organisms with mineralised tubular and mostly triradially symmetrical exoskeletons known from Lower Cambrian beds worldwide. They are particularly abundant and diverse on the Siberian Platform, from where their first representatives have been formally described. About 70% of the published species names and all valid genera are known from Siberia, which makes this region particularly important for revision of the group. A few species of anabaritids are also reported from such crustal units as Western Mongolia, Kazakhstan, South and North China, Eastern and Western Gondwana, Avalonia, Laurentia and Baltica. This study is mainly based on the extensive collections available from the Siberian Platform, including material illustrated in publications, and it provides a systematic review of the diversity of the group. The evaluation of taxonomically important features by different authors is assessed in order to find a balance between taxonomic oversplitting of anabaritids, which has resulted in 72 species being named to date, and unsubstantiated lumping. Of the 19 published genera we place 14 in synonymy, on the basis of our analysis of the type material. The genera Anbarites, Cambrotubulus,Selindeochrea,Aculeochrea and Mariochrea may be retained. We are inclined to regard anabaritids as diploblastic‐grade metazoans similar to, or located within, the Cnidaria. There is, however, no firm evidence for that and they are, therefore, collectively referred to herein as a group incertae sedis.

Chapter 13 Cambrian echinoderm diversity and palaeobiogeography

Abstract The distribution of all known Cambrian echinoderm taxa, encompassing both articulated specimens and taxonomically diagnostic isolated ossicles, is documented for the first time. The database described by 2011 comprises 188 species recorded from 65 formations from around the world. Formations that have yielded articulated echinoderms are unequally distributed in space and time. Only Laurentia and West Gondwana provide reasonably complete records at the resolution of Stage. The review of the biogeographical distributions of the eight major echinoderm clades shows that faunas from Laurentia and Northeast Gondwana (China and Korea) are distinct from those of West Gondwana and Southeast Gondwana (Australia); other regions are too poorly sampled to make firm palaeobiogeographical statements. Analysis of alpha diversity (species per formation) shows that diversity rose initially to Cambrian Stage 5, declined into Guzhangian and Paibian before returning to Stage 5 levels by the end of the Cambrian. This pattern is replicated in Laurentia and West Gondwana. We show that taxonomically diagnostic ossicles found in isolation typically occur significantly earlier than the first articulated specimens of the same taxa and provide important information on the first occurrence and palaeobiogeographical distribution of key taxa, and of the phylum as a whole. Supplementary material: Articulated Cambrian echinoderms and Isolated plates of Cambrian echinoderms are provided at: http://www.geolsoc.org.uk/SUP18668