PousazPower

In the lineage that gave rise to modern fish and fortopods, the ancestral featherlike fins became compact and less concentrated around a single axis, or flagellum.  Instead, they became fanlike, with a row of small ossicles at the base of the pelvic or pectoral girdle, another row of ossicles distal to that and a series of long, cartilaginous or bony rays coming off of that.  In the ancestors of the fortopods, the ossicles anchoring the fin rays became elongated and limb-like, allowing for greater mobility and muscular attachment for getting around underwater. In the fortopods, however, the cartilaginous fin rays became ossified and jointed,