Fig. 2

Diversity of the opercular/branchiostegal series (in red outlines; branchiostegal rays in light red fill) in osteichthyans (skulls in left lateral view); a Dialipina^† (Devonian; [60]). The region of the cheek and the gill cover is studded with multiple bony plates that makes it impossible to delineate an opercular/branchiostegal series [56]. b Guiyu^† (Silurian; [32]) showing the “standard pattern” of the opercular/branchiostegal series, including (from dorsal to ventral) operculum, suboperculum, a number of branchiostegal rays, and gular. c The recent paddlefish Polydon ([5]) without operculum, the larger bone being the suboperculum and the smaller one a single branchiostegal ray. d Saurichthys^† (Triassic; [11]) with a single element, the suboperculum. e The gar Lepisosteus ([5]) with operculum and suboperculum and three branchiostegals. f The zebrafish Danio rerio ([61]); as in all cypriniforms its opercular series consists of three elements. g The salmon Salmo ([6]), with variable number of branchiostegal rays (9–13), even within the same species. h The Australian lungfish Neoceratodus ([5]), with a small suboperculum and no branchiostegal rays. Elements from the opercular series may be missing (e.g. the operculum and the gulars in paddlefish, the branchiostegals in lungfish, all elements in saccopharyngiforms ([62] not shown in Fig. 2)