Figure 4.

Co-evolutionary history of FVs and their vertebrate hosts. FV phylogeny (coloured) is superimposed onto the host phylogeny (black). The colours indicate if FVs are aquatic (blue), amphibian (purple), or terrestrial (red) FVs. Dotted branches are FV lineages that either have become extinct or have not yet been discovered. Cross-class transmissions are depicted by thick transparent vertical bars and the arrows indicate the direction. The exact transmission route among reptiles, birds, and mammals is unclear (‘?’) due to the limited data availability. The number on nodes are host evolutionary timescales, estimated elsewhere (Kumar et al. 2017) in the units of millions of years. The scale bar is in the units of millions of years.