Fishes

Agnathans or jawless fish (Class Agnatha) include about 60 living species assigned to two groups, lampreys and hagfish, all marine scavengers. Many Paleozoic species have been discovered, suggesting that agnathans once inhabited the oceans in large numbers. Many of these species (called ostracoderm fish) show characteristic armors covering the head that have been hypothesized to be antipredator structures or structures storing phosphates or calcium. The oldest fossil hagfish and lampreys are about 300 Ma.

Placoderms or armored fish (Class Placodermi) and acanthodians or spiny fish (Class Acanthodii) are both extinct. Placoderms evolved a variety of bony plates over their heads, and many species are considered to have been bottom feeders. Placoderms are probably related to sharks. Acanthodians, probably related to bony fish, were freshwater fish up to 20 cm in length and covered with small scales. Little is known about their skeleton because they tend to be preserved crushed against the rocks, but they had bony jaws and could be the oldest known group ofjawed vertebrates.

Cartilaginous fish (Class Chondrichthyes) appear as fossils in rocks dated to 360 Ma. There are about 625 species of chimeras, sharks, and rays. Chimerans are the most conservative

Figure 2.8 A tree ot the vertebrate classes. Fossil taxa have been excluded, except tor the two classes of extinct fish, placoderms and acanthodians. Key innovations included in this tree apply to all the lineages above their location. Notice the convergent evolution ot endothermy in mammals and birds.

Figure 2.9 The fossil record of the various classes of vertebrates. Pikaia and the conodonts, both suspected to be chordates, are included for comparison.

Origin and Evolution of Animals 51 living cartilaginous fish. Sharks possess a bony jaw, power musculature, and a streamline body shape, suggesting these animals evolved as predators. By contrast, rays are specialized as bottom feeders and have evolved flattened bodies.

Bony fish (Class Osteichthyes) are the most numerous today with some 20,000 catalogued species. The skull of bony fish includes many jointed bony plates permitting a substantial flexibility that probably supported the evolution of new predatory abilities. Two groups are distinguished by the structure of their fms: actinopterygians or ray-fm fish (Actinopterygii) and sarcopterygians or lobe-fm fish (Sarcopterygii). They were already differentiated about 400 Ma.

Ray-fm fish are characterized by fms supported by a series of rods radiating from the base. Three major groups are recognized, each with its own adaptive radiation. The chon-drosteans flourished from the Carboniferous to the Triassic and subsequently declined in numbers. Living chondrosteans include sturgeons and paddlefish. The holosteans radiated in the Triassic and Jurassic, and subsequently declined. There are only eight living species, including gars and bowfins. The teleostean radiation began in the Jurassic and is the most diverse group of living aquatic vertebrates. The most common species offish belong to this group, including the salmon, carps, herring, and coral-reef species.

Lobe-fm fish are characterized by fms with muscles supported by a bony skeleton that can acquire a variety of postures. Lobe fms are evolutionary precursors of the legs of terrestrial vertebrates. These fish flourished during the Paleozoic and Mesozoic but lost substantial diversity during the Cenozoic. Early species showed evidence of a unique tooth structure, called labyrinthine, characterized by the folding of the enamel (the external layer). Labyrinthine teeth are also characteristic of early terrestrial vertebrates. There are two groups of living lobe-fm fish, lungfish (Dipnoi), freshwater species living in southern continents, and the two known species of coelacanths (Actinistia). Coelacanths were believed extinct until a specimen of Latimeria chalumnae was unexpectedly fished out of the Indian Ocean, near the Comoro Islands, in 1938 (Figure 2.10). The morphology ofthis animal is so conservative that

Figure 2.10 The coelacanth Latimeria chalumnae, a living fossil photographed at a depth of 200 m off the Comoro Islands, in the Indian Ocean.

Source: Photo by Jürgen Schauer, courtesy of Karen Hissmann.

it has become the paradigm of a living fossil. A second species, L. menadoensis, was discovered near Indonesia (Fricke, Hissmann, Schauer, Erdmann, Moosa, & Plante, 2000). Coe-lacanths are equipped with sharp teeth, suggesting they evolved predominantly as predators.