Jaws present, derived from modified gill arches; endochondral bone present (see Smith and Hall, 1990); paired limbs usually present; three semicircular canals (and two or more maculae); gills covered with ectoderm and directed externally; gill arches not fused with neurocranium, internal to gill lamellae; gills opening to surface in fishes through slits (opercular opening, when present, may be porelike); myelinized nerve fibers. There are many characters that carry over in the transition from jawless fishes to jawed vertebrates that were subsequently modified. For example, the notochord continues to be present in the various lineages of early gnathostomes but in some it is later replaced with vertebral centra, and a bony exoskeleton is present in early gnathostome fossils but absent in higher lineages.
There are many exciting questions on the origin and evolution of characters in the transition from jawless to jawed vertebrates (thought to be from osteostacans to placoderms). One such question concerns the phylogenetic origin of teeth (dentine based versus the horny teeth of lamprey). Smith and Johanson (2003) and Johanson and Smith (2003) suggested that teeth have originated at least twice, in derived placoderms, the arthrodires, and in the placoderm sister clade. However, Young (2003) reviews past conclusions that placoderms have a primitively jawless mouth and concludes that no placoderm had typical teeth, but rather the tooth-like structures are made up of a special dentine unique to placoderms, called semidentine.
In the fossil record, placoderms appear in at least the Middle Silurian (Jiang and Dineley, 1988; Gardiner, 1993) and acanthodians appear in the Lower Silurian (e.g., Zidek, 1993) and possible chondrichthyan scales and denticles are known from the late Ordovician (see section on Chondrichthyes).
Classically, all jawed vertebrates were recognized in two groups, the jawed fishes and the tetrapods. This was recognized in Nelson (1984) in placing all gnathostomes in either the "Grade Pisces" or the "Grade Tetrapoda." It was well recognized some years earlier that although tetrapods form a mono-phyletic group, the jawed fishes did not. In order to recognize the phyloge-netic relationships as generally accepted, Nelson (1994) did not recognize Pisces as a taxon and placed all jawed vertebrates in three taxa, namely in the grades Placodermiomorphi, Chondrichthiomorphi, and Teleostomi (comprising the acanthodians, sarcopterygians, which contains some fishes, and the actinopterygians). While the former taxon Pisces is not monophylet-ic and no longer recognized in classification, the term "jawed fishes" is still a useful one, even though referring to a paraphyletic group.
The jawed fishes comprise the first two grades and about half of the species of the Teleostomi. In all, there are about 27,869 species of extantjawed fishes and about 54,603 species of extant jawed vertebrates (gnathostomes). This represents an expected disproportional increase in the number ofjawed fishes over tetrapods from Nelson (1994), with an estimated 24,535 species of extant jawed fishes and about 48,100 species of extant jawed vertebrates (gnathostomes) (leaving an estimated increase in the number of described species of extant fishes of 3,334, and of extant tetrapods of 2,199). Many new forms of fishes are known that are thought to represent undescribed species, and when these are described the numbers will be substantially higher.
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