Fish Evolution is a slow process, which began about 530 million years ago in the early Cambrian. Fish were the first creatures that were jaw. Pisces presumably are the ancestors of all tetrapods. Gnathostomatous first appeared in the Silurian period. They were placoderms, large predatory fish that are protected by a bony shell. It is assumed that the jaws evolved from jawless vertebrate gill arches.
Since these fossil fishes have jaw but do not have bones it can be assumed that they appeared about 395 million years ago in the middle of the Devonian. Cartilaginous fish include sharks, rays and chimaeras. In cartilaginous fish, unlike bone, no swim bladder. Therefore, in order not to sink to the bottom, cartilaginous fish should be in motion. And cartilage, unlike bony fish, gills open outwards gill slits; no gill covers.
See attached image;
A fossilized fish is found that has jaws but no true bones. This fossil belongs to D on the cladogram in the image attached.
Answer; D.
Explanation;
A Cladogram is a branching treelike diagram used to illustrate evolutionary (phylogenetic) relationships among organisms. A point of divergence, has two branching lines of descendance, indicating evolutionary divergence from a common ancestor.
