Late Ordovician trilobites from the Taimyr Peninsula, Arctic Russia

Abstract

Ordovician trilobites from the important sequences of the Taimyr Peninsula, Arctic Russia have been poorly studied since the work by Balashova (1959, 1960). Newly collected and well-preserved specimens from Late Ordovician sections, along with the original collections of Balashova, form the basis of a reappraisal of 56 Upper Ordovician (Sandbian-Katian) trilobites. New species include Bronteopsis tenuirhachis, Dionide trigintasegmentata, Failleana superba, Pararemopleurides ornatissimus, Raymondella plastron, Robergia subtilis and R. nikolaiseni. Probable new species include: Stenopareia sp. aff. S. glaber, and Stygina sp. aff. S. latifrons. A new subgenus Bilobaspis of the monorakine genus Evenkaspis, and a new species, Evenkaspis (Bilobaspis) mirabilis, are proposed. Thoracic segments and pygidium are correctly associated for the first time with cephala of the hitherto poorly understood Taimyraspis. The genera Effnaspis and Yumenaspis are likely junior synonyms of Taimyraspis. A placement within the Ityophoridae is suggested for Taimyraspis, together with the closely related genera Ityophorus and Frognaspis. Comparison of Goldillaenoides taimyricus with Failleana suggests that these are also closely related. A provisional placement of the former in the Styginidae is adopted. Bronteopsis nannus Balashova could be a juvenile B. tenuirhachis, and is regarded as a nomen dubium. The genus Ceratevenkaspis dominates among the monorakines on Taimyr, while Monorakos itself has not been collected. Robergia nikolaiseni has a narrow cranidial border, which is also present in the type species of Robergia. Two biofacies are identified in the current collections. The first, the raphiophorid association, is widespread around low latitude Ordovician palaeocontinents, identifying marginal shelf sites; the association is not critical in defining palaeocontinents themselves. Inner shelf faunas of the monorakine-cheirurid-illaenid association have taxa that comprise a distinctive group with a strong link between Taimyr and the Ordovician Siberian craton. The trilobites described herein support reconstructions showing Taimyr peripheral to the Siberian craton during the Ordovician.

Trilobites of trinucleid, raphiophorid and cyclopygid associations from the Ordovician (Darriwilian – early Katian) of the west Balkhash region and Betpak‐Dala, Central Kazakhstan

“…The cranidial morphology of the described specimens strongly recalls that of Robergia microphthalma (Linnarsson, 1875) from the Andersö Shale Formation (Late Ordovician, Sandbian) of Sweden, as illustrated and discussed by Ebbestad & Fortey (2020), in general shape and proportions of the cranidium, including expression of the glabellar furrows and a narrow anterior border of uniform width. The only difference is in the transverse abaxial portion of S1 not bending backwards and the strongly posteriorly inclined S3.…”

“…The Kazakh specimens also recall cranidia described by Ebbestad & Fortey (2020) as Toernquistia sp. from the Povorotnaya Formation (early Katian) of the Fisherman's Bend section, in the Taimyr Peninsula, Arctic Russia, but differ in having a slightly wider cranidium (tr.…”

“…Throughout the Ordovician, raphiophorid biofacies are common in the offshore settings of continental margins. They are best known from peri‐Iapetus settings, in particular from Laurentia, and were recently documented from Siberia (Ebbestad & Fortey 2019, 2020). As noted by these authors, there is a distinct continuity in the distribution of the Late Ordovician taxa (e.g.…”