Fig. Ext. 1

a,b, Van Gieson Verhoeff?s elastic staining on sections of adult zebrafish gill filaments (a) and jaw hyaline cartilage of the same animal (b). Blue-black fibers (white arrow) indicate positive staining. c, Phylogenetic tree depicting evolutionary relationships of fish species tested for elastin staining in their gill cartilages. d, Alcian blue counterstained with nuclear fast red on sections of gill-filament cartilage from adult zebrafish, gar, bowfin, bichir, and Atlantic salmon. e, Miller?s elastic staining on sections of jaw hyaline cartilage from adult gar, bowfin, and bichir. f, Whole-mount Alcian staining followed by gill dissection from 10 and 14 dpf zebrafish. Boxed regions are magnified to highlight late emerging Alcian blue-positive cartilage matrix in the gill filaments at 14 dpf when compared to the strongly staining ceratobranchial hyaline cartilage above. White dashed lines outline the primary gill-filament mesenchyme. Scale bars, 20?µm (a,b), 50 um (d-f).