Rods and cones in an enantiornithine bird eye from the Early Cretaceous Jehol Biota

Extant birds have an extensive spectral range of colour vision among vertebrates, but evidence of colour vision among extinct birds has hitherto been lacking. An exceptionally well-preserved extinct enantiornithine fossil bird from the Early Cretaceous Jiufotang Formation (120 Ma) of Liaoning, China, provides the first report of mineralised soft tissue of a bird eye. Cone cells are identified, which have preserved oil droplets falling between wide ranges of size that can be compared with an extant house sparrow. The size distribution of oil droplets of extant birds demonstrates good correlation between size and the detectable wavelength range of the cone cells: UV-sensitive cones contain the smallest oil droplets, while red-sensitive cones possess the largest. The data suggests that this Early Cretaceous bird could have possessed colour vision.