Fig. 4

Osteology of the axial and median fin skeleton of A. burtoni. a, b Alizarin red/Alcian blue stained and cleared skeleton of an adult fish shows a typical teleost skeleton, consisting of a spinal column with associated elements, and pectoral, pelvic, dorsal, anal and caudal fins. The axial skeleton consists of pre-caudal, caudal and ural regions. Vertebral centra and associated arches and spines are drawn as fused units. Two anterior ribless vertebrae followed by around 10 rib-bearing vertebrae form the pre-caudal region. The caudal region consists of around 12 ribless vertebrae. The vertebral column terminates posteriorly in the ural region, which consists of highly modified vertebral elements and arches supporting the caudal fin rays. Other accessory elements present to the vertebrae are the epicentrals, also known as dorsal ribs (see main text), which are membrane bones without homologs in tetrapods. In panel a note the presence of the pelvic fins on the anterior abdomen at the same anterior-posterior level as he pectoral fins. Dorsal and anal fins consist of an anterior domain containing fin spines and a posterior domain with soft fin rays, each shown for the dorsal fin in a zoom-box in panel a