Jaws remade vertebrates and now define nearly every living one
Jawed vertebrates, or gnathostomes, hold roughly sixty thousand species—about ninety-nine percent of living vertebrates. Cartilaginous and bony fishes, plus every land tetrapod, sit inside this clade. Fossils and anatomy suggest jaws began as bent gill supports that first pumped water, then learned to bite.
The name blends Greek for jaw and mouth. Beyond jaws, many gnathostomes keep true teeth, a stomach, and paired fins or limbs. Extra innovations include elastin, a horizontal semicircular canal, myelinated nerves, and adaptive immunity with spleen and thymus that builds receptors by V(D)J recombination. Jawless Osteostraci are usually treated as their closest sister group.
Jaws likely formed when the first gill-arch pair flexed around the mouth, sucking water across the gills by buccal pumping; repeated use later enabled biting. The customary homology story still maps jaws to those arches, with the second arch's upper piece becoming the hyomandibula that links jaw to skull. Ancestors are thought to have already carried two pairs of paired fins; some placoderms even show a third pair turned into male claspers.
A four-hundred-nineteen-million-year-old placoderm, Entelognathus, mixes bony-mouth anatomy with traits shared by cartilaginous and bony fishes, hinting that cartilaginous fish lost a bony skeleton secondarily. Juvenile placoderms grew true teeth on jaw surfaces without roots, so teeth seem to have arrived with, or soon after, jaws. Unambiguous gnathostome fossils appear by the early Silurian; Qianodus and Fanjingshania from Guizhou, about four hundred thirty-nine million years ago, currently mark the oldest clear evidence. Two whole-genome duplications shaped the lineage—one before the split from cyclostomes, one after via hybridization. Most jawless fishes vanished by the Triassic, leaving hagfish and lampreys as sparse survivors of a remodelled skull story still hard to reconstruct.
Source: Gnathostomata
