Fig. 9

Development of the pelvic fins and girdle of A. burtoni. a Alizarin red/Alcian blue stain of the adult paired pelvic girdle (ventral view, both left and right side elements are present). The pelvic girdle consists of a single element (the basipterygium) that directly articulates with the fin rays, without presence of proximal or distal radials (see zoom in on the dissected left fin). The most anterior fin ray is unbranched and unsegmented, and has the identity of a spine (indicated with “S”), while the other rays have soft-ray morphology. b, c Alcian blue (198 hpf-240 hpf) and Alcian blue/Alizarin red stained (12 dpf − 16 dpf) pelvic girdle/fin complexes. The first elements of the pelvic fin skeleton appear around 198 hpf as a split chondrogenic condensation, which at 240 hpf has elongated and developed a distinct “forked” appearance. Between 240 hpf (10 dpf) and 12 dpf, the forked ends become connected through intermediate occurring cartilaginous growth, possibly homologous to the endoskeletal elements of the pectoral fins (blue arrowhead, indicated “endoskeletal disk”). Fin rays develop from 240 hpf onwards (indicated black arrowhead). Abbreviations: dpf: days post fertilization; hpf: hours post fertilization; S: spine