Abstract
True crabs (Brachyura) are one of the few groups of arthropods to evolve several types of compound eye, the origins and early evolution of which are obscure. Here, we describe details of the eyes of the Cretaceous brachyuran Callichimaera perplexa, which possessed remarkably large eyes and a highly disparate body form among brachyurans. The eyes of C. perplexa preserve internal optic neuropils and external corneal elements, and it is the first known post-Paleozoic arthropod to preserve both. Additionally, a series of specimens of C. perplexa preserve both the eyes and carapace, allowing for the calculation of the optical growth rate. C. perplexa shows the fastest optical growth rate compared with a sample of 14 species of extant brachyurans. The growth series of C. perplexa, in combination with the calculation of the interommatidial angle and eye parameter, demonstrates that it was a highly visual predator that inhabited well-lit environments.