Abstract

The morphology of Tuzoia is reinterpreted in the light of abundant new specimens from the Middle Cambrian Burgess Shale (British Columbia, Canada) and Kaili (Guizhou, China) Lagerstätten. Tuzoia was a very large (up to 180 mm long) bivalved arthropod with a nonmineralized domelike carapace strengthened by prominent pointed features and often flanked by a lateral ridge bearing a spiny frill. The reticulate pattern of Tuzoia is comparable with that of present-day crustaceans (e.g., myodocope ostracods) and is interpreted as a structural compromise between exoskeletal lightness and high resistance to mechanical stress. Tuzoia had a pair of large, stalked, spherical, possibly compound eyes facing forward. Flagella-like antennae protruded through the anterior notch. No other appendages are known except possible filamentous setae underlying the carapace. Tuzoia typically occurs as laterally (lc) or dorsoventrally (dvc) compacted carapaces or single valves. Each type (lc or dvc) emphasizes particular aspects of the morphology (e.g., spiny lateral ridge, ventral margin) that were often interpreted as specific differences by previous authors. A revision of Tuzoia validates only 7 of the 23 named species. Tuzoia is placed tentatively within a group of large bivalved arthropods along with Isoxys and the possible ancestors of Thylacocephala (Lower Cambrian–Upper Cretaceous). In the Middle Cambrian, Tuzoia occurs across Laurentia, South and North China, and the Perigondwanan area (Bohemia) within a relatively narrow subtropical belt, indicating a high dispersal capability and possible latitudinal control on its distribution. Functional morphology, taphonomy, and the distributional pattern indicate that Tuzoia was a free-swimming arthropod.

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