Chordate Zoology - Textbook - Y. V. Tsaryk - 2013

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Chapter 4. DIVISION GNATHOSTOMATA, or ECTOBRANCHIATA, GNATHOSTOMATA, seu ECTОBRANCHIATA. SUPERCLASS PISCES

The division Gnathostomata comprises all vertebrates except cyclostomes. A defining feature of Gnathostomes is The Development of jaws in the visceral region of the Skull. In most gnathostomes, the notochord persists only during the embryonic and larval stages. The skull—either cartilaginous or bony—completely encloses the Brain. Jaws are formed through the modification of gill arches. The olfactory sacs are paired, and the Inner ear capsule always contains three semicircular canals, among other features.

No fossil remains of primitive gnathostomes have been found so far. They likely diverged from primitive Jawless vertebrates of the class Pteraspidomorphi. This divergence was completed during the Silurian period, resulting in The Emergence of the classes of armored fish Placodermi and aphetohyoids Aphetohyoidea. Further evolution led to the rise of two more fish classes: cartilaginous Fishes Chondrichthyes and bony fishes Osteichthyes, followed by tetrapods: amphibians Amphibia, reptiles Reptilia, birds Aves, and mammals Mammalia.

The superclass Pisces represents the most ancient, primarily aquatic jawed vertebrates, which are adapted to live exclusively in Water. They breathe through gills; some species inhabiting oxygen-depleted water bodies develop accessory Organs for breathing atmospheric air. Typically, There is a single circulatory loop, though lungfishes possess an additional, secondary circuit.

According to J. Nelson (2006), the modern fauna includes 27,977 species of fish inhabiting almost all aquatic biotopes. Fish exhibit A wide variety of body shapes, with lengths ranging from 1 cm to 20 m and weights from 0.7 g to 20 t.

THE ORIGIN OF modern fishes is believed to be linked to the aphetohyoids Aphetohyoidea, or Acanthodii, which inhabited fast-flowing rivers. Cartilaginous fishes Chondrichthyes diverged from primitive aphetohyoids at the beginning of the Devonian, while bony fishes Osteichthyes branched off at the end of the Silurian. Bony fishes immediately split into two subclasses: lobe-finned fishes Sarcopterygii and ray-finned fishes Actinopterygii.

Lobe-finned fishes were represented by superorders of crossopterygians. In the mid-Devonian, tetrapods—amphibians—evolved from Representatives of the order Rhipidistia; the modern fauna has preserved coelacanths Latimeria chalumnae from the order Coelacanthiformes and lungfishes Dipneustomorpha.

The first representatives of the subclass of ray-finned fishes to emerge were the superorders of ganoids and palaeoniscids Palaeonisci.

The recommended textbook by N. P. Naumov and N. N. Kartashev (1979) presents the fish Classification system of T. S. Rass and G. U. Lindberg (1971). Today, the vast majority of scientists use the Taxonomy of J. Nelson (2006).

Fishes are The earliest representatives of the division Gnathostomata, characterized by the following features: a jaw apparatus derived from visceral arches; the presence of paired fins; three semicircular canals in the inner ear; and paired nostrils.