Taphonomic bias in Cloudina distribution data from Siberia
Sesgo tafonómico en los datos de distribución de Cloudina en Siberia

G. Markov^1,2, V. Rogov¹, G. Karlova¹, D. Grazhdankin^1,2

¹Trofimuk Institute of Petroleum Geology and Geophysics, Novosibirsk, Russia. Email: markovge@ipgg.sbras.ru; ORCID ID: https://orcid.org/0000-0002-7391-5629, https://orcid.org/0000-0003-1880-5635, https://orcid.org/0000-0001-5914-6870, https://orcid.org/0000-0003-0797-1347

²Novosibirsk State University, Novosibirsk, Russia

IntroductionTOP

Representatives of the fossil genus Cloudina, a potential terminal Ediacaran index-taxon, are preserved in the form of conico-tubular calcareous structure built of numerous nested funnel-shaped elements, with concentrically placed circular to ellipsoid layers in cross section. It is one of the most geographically widespread and numerically abundant fossil in uppermost Ediacaran strata; however, the paucity of this taxon in Siberia has been puzzling (Zhuravlev et al., 2012; Grazhdankin et al., 2015). Fossiliferous strata in the north-western slope of the Olenek Uplift, arctic Siberia offer an exceptional view of terminal Ediacaran and Terreneuvian palaeobiology. In particular, the Ediacaran strata contain the youngest assemblage of Ediacaran Avalon-type macrofossils preserved as mouldic imprints in limestones; diverse carbonaceous compression macrofossils; probably the planet’s oldest known occurrence of diverse taxonomically identifiable skeletal fossils; taxonomically, numerically and ecologically abundant trace fossils; and key radiometric and chemostratigraphic records – all in a single 400-m-thick continuous succession (Fig. 1). Fragments of conico-tubular calcareous moulds found throughout the Turkut Formation and predating the lowest occurrence of fossil anabaritids in the section have been traditionally regarded as poorly preserved fossils of Cambrotubulus decurvatus (Nagovitsin et al., 2015).

Results and discussionTOP

The first occurrence of Cloudina-morph small skeletal fossils in the Olenek Uplift is found in the uppermost Syhargalakh Formation of the Kessyusa Group (Fig. 1). Here the Cloudina-morph fossils are all allochthonous (Fig. 2A-D) forming a laterally continuous coquina deposit that rests upon the stratiform breccia of the Tas-Yuryakh Volcanic Complex and appears to be coeval with the lowest stratigraphic occurrence in the section of the Cambrian index fossil Treptichnus pedum. Furthermore, the coquina deposit is immediately overlain by alternating sandstone and siltstone of the lowermost Mattaia Formation that yielded arthropod trace fossils Rusophycus avalonensis (Fig. 1). Cloudina-morphs in South China, Maly Karatau and Siberia have been known to occur together with small skeletal fossils of early Cambrian appearance (Zhuravlev et al., 2012; Yang et al., 2016). The section of the Olenek Uplift in Siberia provides the first record of the Cloudina-morph fossils postdating the Cambrian index-taxon Treptichnus pedum. Supposing our interpretation of the earliest ‘Cambrotubulus’ fossils from the Turkut Formation is correct, the stratigraphic range of Cloudina-morph fossils in Siberia commences with the strata predating the first occurrence of Anabarites trisulcatus and extends into the Purella antiqua Assemblage Zone. Some of the Cloudina-morph fossils occur in strata that are demonstrably part of the Cambrian Stage 2 (Fig. 2E-G).

Concluding remarksTOP

Cloudina-morph fossils in Siberia have always been interpreted as a taphonomic mode of Anabarites tests inserted one into another under specific hydrodynamic conditions. Clusters of telescoped conical tests are ubiquitous in the Kessyusa Group and coeval strata across Siberia but not all of them can be easily interpreted as a result of simple mechanical stacking (Fig. 3). We suggest that these fossils represent internal moulds of disarticulated Cloudina specimens. Disarticulation, internal mould formation, diagenetic dissolution and recrystallisation of tubes represent an under-explored taphonomic pathway in the preservation of Cloudina. If borne out by future studies, the entire early Nemakit-Daldynian fossil record of Siberia must be reevaluated to remove the bias in the Cloudina distribution data.

ACKNOWLEDGEMENTSTOP

This study was supported by the Russian Foundation for Basic Research Grant 18-05-70110. The findings and conclusions have immediate implications for understanding early evolution of Metazoa (Russian Science Foundation Grant 17-17-01241). Fieldwork was conducted with the financial support of the National Geographic Society’s Committee for Research and Exploration (grants 8227-07, 8637-09, 9031-11, NGS-372R-18).

ReferencesTOP