Behavioural imprint of the Ordovician Radiation: Evidence from Middle–Upper Ordovician deep-sea trace fossils in western Inner Mongolia, North China

Trace fossils are unique evidence of the body plan and behavioural complexity of animals. Major radiation periods in the Phanerozoic were associated with not only changes in biodiversity, biomass, and ecosystems but also transformations in the relatively implicit, behavioural world. Here we report on a diverse ichnoassemblage from the Middle–Upper Ordovician deep-marine deposits of Inner Mongolia, North China, totalling 24 ichnogenera (including two undetermined forms). They include Alcyonidiopsis, Bifasciculus, Bifungites, Chondrites, Circulichnis, ?Cochlichnus, Dendrorhaphe, Diplocraterion, Halopoa, Gyrophyllites, Monofungites, Multina, Nereites, Paleodictyon, Phymatoderma, Ptychoplasma, Rhizocorallium, Sinusichnus, Taenidium, Thalassinoides, Trichichnus, Zoophycos, bag-like trace, and closed ring trace. Highly regular and complex burrow systems have been discovered, i.e., the branched circular pattern of Circulichnis sinensis, elaborate radial graphoglyptid tunnel system Dendrorhaphe racemosa ichnosp. nov., sinusoid mazework Sinusichnus sinuosus, and the boxwork bag-like trace consisting of highly symmetric horizontal galleries and numerous semi-equidistant vertical shafts. The occurrence of these complex trace fossils suggests distinct behavioural complication and novelty in deep-sea communities during the Middle–Late Ordovician. We speculate that along with the onshore–offshore displacement of ichnofauna and significant expansion of deposit-feeding structures possibly driven by accelerated competition in the shallow-marine niches and aided by ameliorated deep-sea benthic food level, there was underlying behavioural evolution towards more delicate, geometric, and efficient feeding patterns. The Late Ordovician may represent the early peak not only in ichnodiversity but also in ichnodisparity and behavioural complexity of deep-sea ichnofauna, which probably reflect the complex interplay of biodiversity, biomass, and ecological changes during the Ordovician Radiation.