First occurrence of caryocaridids (Crustacea, Phyllocarida) in the Ordovician of North China

Although caryocaridids experienced a significant biodiversity boost during the Ordovician and spread over broad trans-equatorial areas, they have been rarely reported from Siberia, Tarim and North China. Herein, we describe a new species of caryocaridids, Soomicaris ordosensis sp. nov., from the Upper Ordovician of western Inner Mongolia, China, which represents the first report of caryocaridids from the North China craton and extends the palaeogeographical distribution of caryocaridid phyllocarids to the northwestern margin of the Palaeo-Tethys. Detailed morphological and morphometric studies show that these specimens can be ascribed to different ontogenetic stages. After close inspection, we infer that Soomicaris ordosensis sp. nov. should have experienced a long pre-adult time, with several moulting stages, during which time the posterior margin of carapace gradually extends backward to form the posterodorsal spine, and the posterior margin of carapace gradually transitions from convex, linear, and finally to a rightward-sloping sigmoid curve. The taphonomic features of caryocaridids from the Upper Ordovician in the western Inner Mongolia suggest in-situ and parautochthonous preservation. The carapaces have experienced weak phosphatic mineralization during diagenesis. The Ordovician caryocaridids were most likely stenothermic which preferred to inhabit the cool- to cold-water environments from the upper mesopelagic to the lower epipelagic zones, and showed the polar emergence phenomenon.