Four camera-type eyes in the earliest vertebrates from the Cambrian Period

Vertebrate vision is mainly accommodated by a pair of lateral image-forming camera-type eyes and is supplemented in non-mammalian vertebrates by a dorsal pineal complex (pineal and parapineal organs) functioning as photoreceptive and/or endocrine organs1. The pineal complex shares a common genetic and embryological basis with the lateral eyes, both derived from evaginations during the development of diencephalon2. Despite being widely heralded as the ‘third eye’ in crown vertebrates3, the nature of the pineal complex and its presumed visual capability in early vertebrates2 remain unknown. Here we describe two pigmented features situated between the lateral eyes in two species of myllokunmingids, the earliest known fossil vertebrates (approximately 518 million years ago), and interpret these as pineal/parapineal organs. In both myllokunmingid species, the pineal complex contains abundant melanin-containing melanosomes identical to those in the retinal pigment epithelium in the lateral eyes, together with a distinctive, regularly ovoid structure interpreted as a lens. Our results indicate that the lateral eyes and pineal complex in myllokunmingids probably functioned as camera-type eyes capable of image formation. Thus, we propose that the four camera-type eyes represent an ancestral vertebrate character, corroborating hypotheses about the deep homology between the eyes and pineal complex.