Three-dimensionally preserved appendages of the early Cambrian trilobite Yunnanocephalus yunnanensis

Advances in micro-computed tomography have brought new insights into the non-biomineralized three-dimensional morphologies of various euarthropods from the Chengjiang biota. The application of this technique to Yunnanocephalus yunnanensis, as described herein, also helps reveal the well-preserved 3D morphologies of this early Cambrian trilobite for the first time. The two study specimens bear a pair of uniramous antennae, four post-antennal cephalic appendages, and up to sixteen thoracic appendages. The first post-antennal appendage has a well-developed endopodite and exopodite, both diverging from a protopodite—an organization that could aid in respiration, feeding, and locomotion, but has not been known in any other Cambrian redlichiids. All the following biramous appendages consist of a protopodite transversely elongated into a robust gnathobase, a bipartite exopodite with sagittally broad podomeres bearing dumbbell-shaped marginal lamellae, and an endopodite composed of seven endite-bearing podomeres that become more elongated distally. The present study not only adds to the diversity of appendage morphologies in redlichiids but also helps understand how trilobites may have survived under different selective pressures by “upgrading” their feeding, sensory, and locomotory apparatus.