Gnathostomata

The gnathostomes or gnathostomes (Gnathostomata) are a clade of vertebrates characterized by having articulated jaws (from the Greek γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth").

The group is considered an infraphylum and includes most of today's vertebrates, such as fish, amphibians, reptiles, birds, and mammals. The group is opposed to that of the agnaths (Agnatha) or vertebrates without jaws.

Other distinguishing features of gnathostomes are myelin sheaths around neurons and an adaptive immune system.

The traditional classes of Gnathostomata are:

• Placodermi †
• Chondrichthyes - cartilizing fish.
• Acanthodii †
• Osteichthyes - bone fish
  • Actinopterygii - oil fish of radiated fins.
  • Sarcopterygii - bone fish of lobed fins.
• Amphibia - amphibians
• Sauropsida - reptiles and birds
• Mammalia - mammals

On the other hand, there is a paraphyletic clade, Pisces, which includes all non-gnathostome fish, including hagfish and lampreys.

Features

The most important autapomorphy (characteristic feature) of the mandibular mouths is the reinforcement of the mouth margins by means of articulated and interconnected cartilaginous or bony hooks. This gave rise to a mostly toothed jaw, which allows the animals to grasp, hold, and crush food. This opened up entirely new feeding possibilities for jawed animals.

Training the jaw

According to the classical model of mandible evolution, the mandibular arch formed from a branchial arch, a supporting skeletal element between the gill pouches. It is not known with certainty which branchial arch it is. One hypothesis assumes that it is the third and that the two anterior praemandibular arches have receded. Cartilage (labial cartilage) in the skull of Neoselachii (modern sharks and rays) may be remnants of the two reduced anterior gill arches. The mandibular arch of jawed mammals is formed solely by the two central elements of the arch, which are joined by the primary mandibular joint, in mammals by the secondary mandibular joint. The strong adductor muscles serve to close the jaws.

The branchial arch that follows the one that has become the mandibular arch becomes the hyal arch (hyal arch, hyoid arch), the remaining branchial arches, with few exceptions usually five, remain in the basic gnathostoma pattern as carriers of the branchial apparatus. The branchial arches each have four skeletal bars (pharyngo, epi, cerato, and hypobranchial). Between the mandibular arch and the hyoid arch in the basic Gnathostoma pattern there is a splash hole in each case, through which respiratory water can be aspirated. The teeth are made up of bone flakes and may be situated at the margins. of the jaws, in the oral cavity and in the pharynx.

An alternative hypothesis considers the mandibles homologous to the cartilaginous parts of the velum of lamprey larvae (Ammocoetes larvae). The velum sits in the pharynx of these filter feeders and, together with the musculature of the gills, produces the flow of water from the mouth opening to the gills. According to this hypothesis, the jaws thus evolved from a structure that has always served for food intake.

Postcranial skeleton

The axial skeleton is initially formed by the dorsal chord. The vertebrae formed several times independently to reinforce the cord and therefore do not belong to the autopomorphies of mandibular mouths. Of the unpaired fins, the dorsal fin and caudal fin were inherited from jawless ancestors, and the anal fin is a neologism not yet found in the most primitive Gnathostoma, the Placodermis.

European tree frog: The four extremities are another characteristic of frogs with jaw

The caudal fin was originally heterocerk, meaning that the end of the spine bends upward and supports the larger, upper part of the caudal fin.

Another autapomorphy is paired pectoral fins and pelvic or ventral fins supported by skeletal elements, which mainly enhanced the maneuverability of Gnathostoma and from which the fore and hind legs of terrestrial vertebrates evolved. The extinct jawless osteostraci also had paired pectoral fins, but they were skeletal and supported only by muscle.

The pectoral fins are joined by the pectoral girdle, the pelvics by the pelvic girdle. The paired flippers are moved by muscle groups called elevators (elevators, extensors) and depressors (depressors, flexors).

Sensory Organs

The eyes of maxillipeds have a true cornea. The position and innervation of the six external eye muscles differ from those of the jawless. Through the development of the internal muscles of the eye, the eye acquires the ability to visualize an object at any distance between the near point and the far point of the fovea of the retina of the eye. both eyes, so that a clear visual impression (accommodation) is produced. Two nostrils develop as an olfactory organ. The organ of balance develops three arcades, one for each plane of three-dimensional space. A lateral line organ as well as electroreceptors are probably also part of the basic pattern of mandibular mouths.

Internal Organs

The brain of jawed mouths is divided into three parts, the prosencephalon (forebrain), midbrain (midbrain), and rhombencephalon (rhombic brain). The neural tube leaves a spinal nerve for each muscle segment, which divides into a dorsal and a ventral branch. Glial cells in the axons of peripheral nerves allow rapid transmission of information.

In the digestive tract, the stomach has evolved as a storage organ. In the primitive gnathostoma, the surface of the intestine is enlarged by a spiral fold. The Liver and Pancreas are the largest intestinal appendages and have formed from the Entoderm. The digestive tract, kidneys, and reproductive system (oviduct and vas deferens) end in a cloaca.

Placodermo Circle Dunkleosteus'

Phylogeny

The clades grouped in Gnathostomata are the following: