Figure 13. Distribution of double cones among vertebrates. Given their widespread abundance among vertebrates, it appears double cones initially arose around the same time as calcified skeletons. Oil droplets probably arose shortly thereafter. Although double cones have been reduced in number and complexity in other lineages, eutherian mammals appear to be unique in having completely discarded the structures once they were acquired. The dashed line indicates the uncertainty in the phylogenetic positioning of  "Chelonia". This diagram is intended to hint at a broad pattern of photoreceptor evolution; beware that it masks much diversity within the lineages depicted.