FIGURE 1.

The evolutionary history of fin diversity. (a) The evolutionary trajectory of paired fins in six major fish groups. From top; Rhyncholepis (Anaspida), Norselaspis (Osteostraci), Bothriolepis (Placodermi), Squalus (Chondrichthyes), Acanthodes (Acanthodii), and Danio (Actinopterygii). Rhyncholepis possessed fin‐like flaps with spines (s). The length of flaps varies depending on species. Endoskeleton of Norselaspis fin is unknown. Bothriolepis evolved the pectoral fin with nerves, muscles, and blood vessels, which seem to be used for active fin movement. Later, a tribasal fin, often exhibiting fin spines, evolved in chondrichthyans and actinopterygians. (b) Diversity of paired fin skeletons. While the number of fin rays is susceptible to change during evolution, the number of proximal radials generally do not go over four in most species. The left is hillstream loach (Beaufortia kweichowensis) with four broad proximal radials and the right top is jelly nose fish (Ijimaia antillarum) with the fusion of proximal radials into one bone. The right bottom is eel cod (Muraenolepis Kuderskii) with 13 radials, which is not the typical number of radials in Actinopterygii. (c) Innervation and musculature of the dorsal and pectoral fin. Top; the developing dorsal fin of sharks (Scyllium canicula). Each muscle bud (pink) associated with a radial is innervated by a spinal nerve (blue). Bottom; the adult pectoral fin of zebrafish. Four spinal nerves innervate the fin musculature that moves fin rays. Abductor superficialis (a.s.) articulate the girdle and proximal fin rays and arrector ventralis (a.v.) connects to the first fin rays. The number of somites in paired fin development could be related to the number of proximal radials seen (see the text). a.s.; abductor superficialis, a.v.; arrector ventralis, dr; distal radial, dsa; distal segment of exoskeleton fin armor, dse; distal segment of endoskeleton, m; muscle, meso; mesopterygium, meta; metapterygium, pr; proximal radial, pro; propterygium, psa; proximal segment of exoskeleton fin armor, pse; proximal segment of endoskeleton, r; radial, and s.n.; spinal nerve. All illustrations are after: (Balushkin & Prirodina, 2007; Goodrich, 1906; Grandel & Schulte‐Merker, 1998; Hara et al., 2018; Janvier, 1996; Kardong, 2012; Matsubara, 1963; Ritchie, 1980; Schaeffer & Williams, 1977; Stensiö, 1959; Stensio, 1964; Yano et al., 2012)