Three groups of arthropods (Chasmataspidida, Offacolidae (?), and Euthycarcinoidea) cohabited a tidal zone in the late Cambrian paleobasins (495–488 Ma) of Eastern Siberia

Abstract

We examined arthropods from a newly discovered location, Kimiltei, in the Irkutsk region of Eastern Siberia. These arthropods belong to three distinct groups: basal euchelicerate (putative Offacolidae), Chasmataspidida, and Euthycarcinoidea. They were found together in a single bed. Chemical, mineralogical, and facial analyses revealed that this bed was formed within a very shallow lagoonal/tidal environment with nearby paleosols. Using the combined data, we present a taphonomic model explaining the uncompressed, three-dimensional preservation of the arthropods. Based on U-Pb isotopic dating of detrital zircon grains, we determined the age of this bed as 491.3 ± 4.2 Ma, placing it in the Furongian Series (Jiangshanian and Stage 10). Given this age, the arthropods from Kimiltei appear to be among the earliest known body fossils of Euthycarcinoidea and euchelicerates. An updated analysis of the geographical distribution of these groups indicates that by the late Cambrian, Euthycarcinoidea already had a wide distribution in tropical and possibly boreal regions of the Iapetus Ocean, while euchelicerates inhabited a tropical Siberian paleobasin. The evolution of these groups proceeded at the beginning in close ecological niches in a tidal zone.