A Reduced Labrum in a Cambrian Great-Appendage Euarthropod.

The euarthropod head is a highly versatile and functionally specialized body region composed of multiple appendage-bearing segments and whose complex evolution has been scrutinized through anatomical, developmental, and paleontological approaches [1-4]. Exceptionally preserved Cambrian fossils have allowed for the reconstruction of critical stages of the evolutionary history of the head, such as the origin of the labrum-an anteromedian flap-like structure that overlies the mouth opening in almost all extant representatives-from an ancestral pair of pre-ocular (protocerebral) appendages [3-5]. The highly conserved position of the labrum makes it a valuable anatomical landmark for understanding the anterior segmental organization among extant and extinct euarthropods [2]. However, the labrum is seemingly absent in the megacheirans, a major extinct group characterized by enlarged raptorial "great appendages" with a central role in competing hypotheses on the early evolution of the head [1-3, 6-8]. Here, we used micro-computed tomography to demonstrate the presence of a three-dimensionally preserved labrum associated with the mouth opening in juvenile specimens of the megacheiran Leanchoilia illecebrosa from the early Cambrian Chengjiang biota, Southwest China. The position of the labrum relative to the pre-oral great appendages of L. illecebrosa indicates that these limbs correspond to the deutocerebral segment and are therefore serially homologous with the first appendage pair of extant euarthropods [1, 2, 4, 6, 8]. The reduced labrum and deutocerebral great appendages of L. illecebrosa also strengthen the affinities of megacheirans as stem-group chelicerates, in line with recent paleoneurological fossil data from the early to mid-Cambrian of China and North America [6, 9].