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Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. Most gnathostomes have retained ancestral traits like true teeth , a stomach , [ 2 ] and paired appendages (pectoral and pelvic fins, arms, legs, wings, etc.). [ 3 ]
Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes).They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras.
The mouth is located just behind the head, after a rostrum, on the underside of the body. It has a pair of cuticular jaws, supplied by strong muscles, and often bearing minute teeth. A "basal plate" on the lower surface that bears a comb-like structure is also present.
Agnatha (/ ˈ æ ɡ n ə θ ə, æ ɡ ˈ n eɪ θ ə /; [3] from Ancient Greek ἀ-(a-) 'without' and γνάθος (gnáthos) 'jaws') is a paraphyletic infraphylum [4] of non-gnathostome vertebrates, or jawless fish, in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct (conodonts, anaspids, and ostracoderms, among others).
Conodont elements consist of mineralised teeth-like structures of varying morphology and complexity. The evolution of mineralized tissues has been puzzling for more than a century. It has been hypothesized that the first mechanism of chordate tissue mineralization began either in the oral skeleton of conodonts or the dermal skeleton of early ...
The labium is attached at the rear end of the structure called cibarium, and its broad basal portion is divided into regions called the submentum, which is the proximal part, the mentum in the middle, and the prementum, which is the distal section, and furthest anterior.
Many evolutionary changes occurred at this stage: eyelids and tear glands evolved to keep the eyes wet out of water and the eyes became connected to the pharynx for draining the liquid; the hyomandibula (now called columella) shrank into the spiracle, which now also connected to the inner ear at one side and the pharynx at another, becoming the ...
The teeth may be homodont (i.e. similar in form across a row), heterodont (otherwise), or ctenodont (comb-like). [102] Their height, width and number of cusps is variable between species. [102] The pattern of teeth repeats, but each row may not be identical to the last; in the octopus, for instance, the sequence repeats every five rows. [102]: 79