Pygmaclypeatus daziensis, a unique lower Cambrian arthropod with two different compound eye systems

More than half a billion years ago, a high diversity of organisms appeared in the fossil record. All major clades we know today already existed, and arthropods dominated the marine faunas. Many were already equipped with a pair of elaborated compound eyes on top of movable eye stalks. Some of them also possessed 3-4 small single-aperture eyes, so-called median eyes. Just trilobites possessed sessile dorsal eyes. One pair of compound eyes/lateral eyes is considered plesiomorphic and is a common trait for euarthropods. Here, we describe an arthropod that possessed two independent compound eye systems-a pair of stalked and a pair of tiny sessile dorsal trilobite-like compound eyes, unique in the arthropod kingdom so far. A competition between prey and predators for the capacity of vision triggered the evolution of visual systems, and we discuss this newly described system(s) in its evolutionary context and ecological significance. Regarding its eye system phylogenetically, P. daziensis reinforces the position of a now non-missing link between the non-trilobite artiopodans and trilobites.