Fig. 10

Development of the caudal fin of A. burtoni. a, b Osteology of the adult caudal fin of A. burtoni as detected by Alizarin red staining (fluorescence image in panel a. A. burtoni has a typical acanthomorph caudal fin consisting of five hypurals (I-V), a parahypural, two epurals (I, II), a urostyle, and two haemal and neural spines, contributing to the support of the dermal fin rays. Note the highly modified neural spine on the second pre-ural centrum (PU-2) that forms a complex articulation with the first epural. c Alcian blue staining shows that the first elements to appear are the hypurals and parahypural aroud 120 hpf. This is followed by the appearance of the haemal and neural arches on pre-ural centra 2 and 3 around 132 hpf. The epurals appear later, between 156 hpf and 180 hpf. By 260 hpf (Alcian blue/Alizarin red double stain), the uroneural has developed as a directly ossifying membrane bone. The developing notochord is indicated using a grey asterisk in the 72 hpf specimen. Abbreviations (except those  explained at the bottom of the figure): hpf: hours post fertilization