Fig. 8

Schematic drawing of the histomorphogenic model of opercular growth. a Opercular growth as was found in the outgroup species Polypterus bichir, Acipenser baerii, and Lepisosteus cf. L. oculatus. This model is in accordance with previously published results of opercular growth in other fish species (e.g., Perry and Casselman 2012). Numbers indicate hypothetical successive growth stages countable in the cross sections; b derived growth model of Saurichthys opercula. Note that the counting of successive growth stages is hampered by preferential growth so that growth marks cannot be followed throughout the sections. In the opercular bones, ornamentation and remodeling processes further decrease identification of growth marks in the bone tissue