Earliest trace fossil evidence of wood-eating termites (Isoptera) and mites (Oribatida) in circumpolar environments of Australia

Abstract

Termites (Suborder Isoptera) first appear in the fossil record in the Upper Jurassic and quickly gained global distribution afterwards. Here we describe and diagnose a coalified conifer log filled with a complex suite of xylophagy-related trace fossils. Sourced from the upper Strzelecki Group (Hauterivian-Albian) of southeastern Australia, the log comes from a unit that records the sedimentary history of a massive braided river system, featuring volcaniclastic sandstones interbedded with conglomerates, mudstones, and prolific thin coal sheets. We employed multiple imaging methods to analyse the specimen, such as thin sectioning, CT scanning, X-ray fluorescence microscopy, and Raman spectroscopy. Trace fossils consist of two distinct sizes of tunnel filled with abundant coprolites and post-burial clastic sediment, with one type an order of magnitude smaller than the other. The larger coprolites within tunnels are carbonaceous with hexagonal cross-sections, and are identifiable as Microcarpolithes hexagonalis, an ichnotaxon commonly related to modern drywood termites (Family Kalotermitidae). Oribatid mites (Acari) are identified as the trace makers of the smaller coprolites and tunnels. The log itself served as a vessel for transporting these trace fossils, having likely originated from an upland forest before its deposition on a rift-valley fluvial floodplain. The paleobiogeographic and behavioural implications of these trace fossils are significant, as this specimen constitutes the earliest record of Isoptera in Australia, the southernmost record of Isoptera in the fossil record and the only record of Isoptera within a circumpolar environment. The presence of termites in the upper Strzelecki Group (within the Antarctic Circle) suggests they had a global distribution by the Early Cretaceous. These termite trace fossils also support times when permafrost was likely absent in the Early Cretaceous if modern termites' vulnerability to prolonged freezing conditions is ancestral.