Ancestry and Relationships of Major Groups of Fishes

Ancestry and Relationships of Major Groups of Fishes
The fishes are of ancient ancestry, having descended from an unknown free-swimming protochordate ancestor (hypotheses of chordate and vertebrate origins are discussed in Chordates ). The earliest fishlike vertebrates were a paraphyletic assemblage of jawless agnathan fishes, the ostracoderms (Figure 25-14). One group of the ostracoderms gave rise to the jawed gnathostomes (Figure 26-1).

The jawless agnathans, the least derived of the two groups, include along with the extinct ostracoderms the living hagfishes and lampreys, fishes adapted as scavengers or parasites. Although hagfishes have no vertebrae and lampreys have only rudimentary vertebrae, they nevertheless are included with the subphylum Vertebrata because they have a cranium and many other vertebrate homologies. The ancestry of hagfishes and lampreys is uncertain; they bear little resemblance to the extinct ostracoderms. Although hagfishes and the more derived lampreys superficially look much alike, they are in fact so different from each other that they have been assigned to separate classes by ichthyologists.

All remaining fishes have paired appendages and jaws and are included, along with the tetrapods (land vertebrates) in the monophyletic lineage of gnathostomes. They appear in the fossil record in the late Silurian period with fully formed jaws, and no forms intermediate between agnathans and gnathostomes are known. By the Devonian period, the Age of Fishes, several distinct groups of jawed fishes were well represented. One of these, the placoderms, became extinct in the following Carboniferous period, leaving no direct descendants. A second group, the cartilaginous fishes of the class Chondrichthyes (sharks, rays, and chimaeras), lost the heavy dermal armor of early jawed fishes and adopted cartilage rather than bone for the skeleton. Most are active predators with sharklike or raylike body forms that have undergone only minor changes over the ages. As a group, sharks and their kin flourished during the Devonian and Carboniferous periods of the Paleozoic era but declined dangerously close to extinction at the end of the Paleozoic. They staged a recovery in the early Mesozoic and radiated to form the modest but thoroughly successful assemblage of modern sharks and rays (Figure 26-1).

The other two groups of gnathostome fishes, the acanthodians and the bony fishes, were well represented in the Devonian period. Acanthodians somewhat resembled bony fishes but were distinguished by having heavy spines on all fins except the caudal fin. They became extinct in the lower Permian period. Although the affinities of the acanthodians are much debated, many authors believe that they are the sister group of the bony fishes. The bony fishes (Osteichthyes, Figure 26-2) are the domi-nant fishes today. We can recognize two distinct lineages of bony fishes. Of these two, by far the most diverse are the ray-finned fishes (class Actinopterygii), which radiated to form the modern bony fishes. The other lineage, the lobe-finned fishes (class Sarcopterygii), although a relic group today, carry the distinction of being the sister group of the tetrapods. The lobefinned fishes are represented today by the lungfishes and the coelacanth— meager remnants of important stocks that flourished in the Devonian period (Figure 26-1).