1. The class Osteichthyes includes all bony fishes. Like all fishes, Osteichthyes are cold-blooded vertebrates that breathe through gills and use fins for swimming. Bony fishes share several distinguishing features: a skeleton of bone, scales, paired fins, one pair of gill openings, jaws, and paired nostrils.
2. The class Osteichthyes includes the largest number of living species of all scientific classes of vertebrates, more than 23,500 species.
3. Osteichthyes account for about 96% of all fish species. Fishes not included in the Osteichthyes are the Chondrichthyes (sharks and their relatives), the Myxini (hagfishes), and the Cephalaspidomorphi (lampreys).
Living Osteichthyes are divided into three subclasses: Dipnoi, Crossopterygii, and Actinopterygii.
1 . The subclass Dipnoi (lungfishes) is characterized by an upper jaw fused to the braincase, fused teeth, and the presence of an air-breathing organ that opens to the esophagus. A lungfish's caudal fin is continuous with its dorsal and anal fins. Its pelvic and pectoral fins are long and tubular.
2. The subclass Crossopterygii (coelacanths) is characterized by cosmoid scales, two dorsal fins, and fleshy paired fins that contain skeletal elements. (a description of cosmoid scales.) Scientists used to think that this entire subclass of fishes was extinct. Then, in 1938, a living coelacanth (Latimeria chalumnae) was discovered off the coast of Southeast Africa. Several specimens have since been collected.
3. The subclass Actinopterygii includes all other living bony fishes. Actinopterygians are characterized by rayed fins.
1 . All orders of bony fishes end in the suffix "iformes."
2. Most taxonomists recognize 45 orders and 435 families of bony fishes. For a list of orders and number of families in each order, see the appendix.
There are probably more than 23,500 species of bony fishes (Lagler, 1962 and Thurman, 1981). It's likely that many species, including deep-sea species, have yet to be identified.
Bony fishes are classified based on comparative anatomy, embryology, genetics, molecular biology, and the fossil record.
1. Primitive fishes date back to the Cambrian period, about 550 million years ago. These jawless fishes lived relatively unchanged over the following 1 00 million years.
2. Bony fishes and cartilaginous fishes (such as sharks and rays) arose from a common ancestor. . .
3. The Devonian period, about 360 to 400 million years ago, is known as the "Age of Fishes," because of the abundance and diversity of fishes that appear during this period.
a. In the Devonian, fishes began to develop jaws and paired fins. All four living classes of fishes and the three major groups of Osteichthyes were established by the mid-Devonian.
b. Many of the groups of fishes that developed during the Devonian are now extinct.
(1) Until 1938, scientists thought that all Crossopterygians were extinct. In 1938, however, a living member of this group, the coelacanth Latimeria chalumnae was discovered off the coast of Southeast Africa.
(2) Experts theorize that extinct ancestors of the coelacanth gave rise to amphibians, which appear in the fossil record at the end of the Devonian.
In 1938 a living coelacanth was discovered off the coast of southeast Africa.
4. Bony fishes continued to evolve after the Devonian period.
a. Most orders of bony fishes probably evolved during the Triassic period, about 200 million years ago.
b. Today, the Actinopterygians are the dominant vertebrates in the oceans and in freshwater systems.
c. The most recently evolved orders of bony fishes include the Pleuronectiformes (flatfishes) and Tetraodontiformes (triggerfishes, pufferfishes, and molas).
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