Most gnathostomes have retained ancestral traits like true teeth, a stomach,[2] and paired appendages (pectoral and pelvic fins, arms, legs, wings, etc.).

[3] Other traits are elastin,[4] a horizontal semicircular canal of the inner ear, myelin sheaths of neurons, and an adaptive immune system which has discrete lymphoid organs (spleen and thymus),[5] and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.

This development would help push water into the mouth by the movement of the jaw, so that it would pass over the gills for gas exchange.

[14][15] Late Ordovician-aged microfossils of what have been identified as scales of either acanthodians[16] or "shark-like fishes",[17] may mark Gnathostomata's first appearance in the fossil record.

Undeniably unambiguous gnathostome fossils, mostly of primitive acanthodians, begin appearing by the early Silurian, and become abundant by the start of the Devonian.

However studies of the cyclostomes, the jawless hagfishes and lampreys that did survive, have yielded little insight into the deep remodelling of the vertebrate skull that must have taken place as early jaws evolved.

[33] While potentially older Ordovician records are known, the oldest unambigious evidence of jawed vertebrates are Qianodus and Fanjingshania from the early Silurian (Aeronian) of Guizhou, China around 439 million years ago, which are placed as acanthodian-grade stem-chondrichthyans.