Figure 5:

Jaw patterning in cartilaginous fish classes Placodermii, Chondrichthyes, and Acanthodii. The fossil placoderm Entelognathus has a bony dermal head skeleton with evidence of an opercle and dentary. Other elements are also present such as maxillary bones, but not shown. Modern Squalus (dogfish shark) features cartilaginous elements only. Acanthodes has a cartilaginous endoskeleton with overlying dermal bone elements, including a hyoid gill covered in bony branchiostegal rays (grey outlines). The hyomandibula is beneath the palatoquadrate and the ceratohyal is beneath the Meckel’s cartilage. Note the presence of an interhyal and symmetry between epibranchial and ceratobranchial cartilages. Adapted with copyright permission (Janvier, 1996; Kardong, 2012; Zhu et al., 2013).