Fig. 4. Evolutionary reconstruction of the Root effect in jawed vertebrates. The underlying phylogenetic tree is based on the species and branching pattern shown in Fig. 3. The Root effect has been colour coded and its magnitude in ancestral species has been reconstructed on the z-plane of the structure by linear parsimony from values measured in living species as shown in Fig. 1. (A) The three-dimensional structure has been rotated to visualise the gradual increase of the Root effect in early ray-finned fishes (nodes c–f) after their ancestors diverged from the lineages of sharks (a) and lobe-finned fishes (including tetrapods, b). The red bar indicates the origin of the choroid rete mirabile in the branch leading to the bowfin and teleosts only after the Root effect had increased. (B) Enlarged part of the structure in A after rotation, showing two examples of secondary reductions of the Root effect in Ostariophysi. The Root effect is only ever reduced when the choroid rete mirabile has been lost. The latter is indicated by red bars. The oriental weather loach still has a swimbladder rete mirabile, whereas the two catfishes lack both types of rete (Fig. 3), consistent with a complete loss of the Root effect in the latter group. Ma, million years. Modified from Berenbrink et al. (Berenbrink et al., 2005).