Abstract:We redescribe the morphology of Yohoia tenuis (Chelicerata sensu lato) from the Cambrian Burgess Shale Lagerstätte. The morphology of the most anterior, prominent, so-called great appendage changes throughout ontogeny. While its principal morphology remains unaltered, the length ratios of certain parts of the great appendage change significantly. Furthermore, it possesses a special jack-knifing mechanism, i.e. an elbow joint: the articulation between the distal one of the two peduncle elements and the most proximal of the four spine-bearing claw elements. This morphology might have enabled the animal to hunt like a modern spearer-type mantis shrimp, an analogy enhanced by the similarly large and protruding eyes. For comparison, details of specimens of selected other great-appendage arthropods from the Lower Cambrian Chengjiang Lagerstätte have been investigated using fluorescence microscopy. This revealed that the morphology of the great appendage of Y. tenuis is much like that of the Chengjiang species Fortiforceps foliosa and Jianfengia multisegmentalis. The morphology of the great appendage of the latter is even more similar to the morphology developed in early developmental stages of Y. tenuis, while the morphology of the great appendage of F. foliosa is more similar to that of later developmental stages of Y. tenuis. The arrangement of the elbow joint supports the view that the great appendage evolved into the chelicera of Chelicerata sensu stricto, as similar joints are found in various ingroup taxa such as Xiphosura, Opiliones or Palpigradi. With this, it also supports the interpretation of the great appendage to be homologous with the first appendage of other arthropods.

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