Figure 12. The same mouth-opening characterizes all vertebrates except for adult lampreys. A, a in REVIEW Vertebrate origins are informed by larval lampreys (ammocoetes): a response to Miyashita et al., 2021

Figure 12. The same mouth-opening characterizes all vertebrates except for adult lampreys. A, a tunicate, is included as representing the closest relative of vertebrates, with the same mouth structures. Adult lampreys (C) do not have the typical, primary opening, but a secondary one in the oral funnel formed by their protrusive upper lips. The ammocoete lamprey (B) does not have a secondary mouth-opening, so the dashed blue line in (B) just shows the boundary corresponding to the adult lamprey's. Anaspids (E) are reconstructed with a primary mouth, despite the superficial similarity of their snout to that of adult lamprey; the anaspid snout does not project far enough forward (only three eye-diameters forward as opposed to five for the lamprey in Fig. 11J). Hagfish (D) also differ from adult lampreys in having the primary mouth-opening. In osteostracans (G) and galeaspids (H), the lips have not grown forward to form a secondary mouth, but have simply lain on the ground. I call their mouth openings 'pseudo-secondary.' Most of the pictures are retooled from Figures 10 and 11, but three are new: (A) is redrawn from Mallatt (2009), (E1) from Janvier (1996) and (E2) from Sansom et al. (2010).