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(a) Polypterus. (b) Gogonasus23. (c) Tiktaalik24. (d) Ventastega25. (e) Ligulalepis (modified from Basden et al.27). (f) Guiyu (reconstructed based on study material from Zhu et al.28). (g) Onychodus29. Polypterus is shown with both the spiracular valve closed (left) and open (right).