An early Cambrian bivalved euarthropod with specialised limbs and highly sclerotised abdomen

ABSTRACT With extraordinary preservation, the bivalved Cassicaris clarksoni gen. et sp. nov. from the early Cambrian (Stage 3) Xiaoshiba Lagerstätte in Kunming, China, is characterised by having an anterior cardinal spine and a heavily segmented body, including an abdomen with a heavily sclerotised shell. Anatomically, the antennulae are small and the antennae are robust with seta-bearing podomeres, possibly of predatory function, followed by the other five pairs of biramous cephalic limbs. There are about 11 pairs of thorax segments, each corresponding to a pair of biramous limbs, including a multi-segmented endopod with feather-like podomeres and terminal spines, and a small paddle-shaped exopod fringed with setae. The bulbous, stalked eyes, which exhibit fineness of vision, infer adaptation to a vagile epibenthic lifestyle. Functionally, such assorted appendages may indicate an efficient suspension-feeding strategy for capturing tiny zooplankton. The median eye is presumably not a typical ocellar system but another compound eye, which may offer further insights into the evolution of compound eyes. Cladistic analysis implies that Cassicaris is a sister taxon to Pectocaris and Jugatacaris ; these intriguing euarthropods are critical for discerning their body plan and living habits. Our findings offer fresh insights into the early evolution of Cambrian euarthropods, characterised by notable morphological disparity and ecological diversity. These fossils, including not only many intact individuals but also a few with well-preserved soft parts, form well-characterised groupings, making the broad pattern of Cambrian arthropod systematics increasingly consensual.