Jaws may have begun as better gill pumps, not better bites
Jaws are paired, hinged structures at the mouth for seizing and handling food. Arthropod jaws are made of chitin and close from the sides; vertebrate jaws are bone or cartilage and close up and down. The vertebrate version probably appeared in Silurian fish, and may first have helped them breathe rather than bite.
In insects, spiders and their relatives the jaws are mandibles or chelicerae, often built from many mouthparts, and many of these parts, like the pedipalps, began as modified legs. Vertebrate jaws come from the front two of the arches that once held up the gills: one became the jaw and the next the hyoid arch, which hangs the jaw from the braincase and lets it swing freely. There is no direct fossil proof, but the count fits, since living jawed vertebrates have seven arches and jawless ones have nine.
Jaws first turn up in placoderm fishes of the Silurian, which spread widely in the Devonian. Their original advantage may have been better breathing, because the jaws drive the buccal pump that pushes water over fish gills or air into amphibian lungs; feeding took over as the main job later. Many bony fishes now have jaws of dozens of bones that shoot forward to suck in prey.
Land vertebrates simplified the design. Most upper jaw bones fused to the braincase and the lower ones merged into a mandible, hinged between the quadrate and articular bones, though some species keep movable joints; snakes have the most flexible skulls of all, letting them swallow large prey. In mammals the jaw is just mandible and maxilla, and two old jaw bones, the articular and quadrate, shrank and moved into the ear. Our lower jaw meets the temporal bone at the temporomandibular joint, where a common disorder causes pain, clicking and restricted movement.
A comparison of two salamanders found that the aquatic Necturus had bigger jaw muscles and a stronger bite than the land-dwelling Salamandra, suggesting habitat alone does not explain skull strength. Sea urchins go their own way, with a five-part jaw called Aristotle’s lantern, each part carrying a single ever-growing tooth of crystalline calcium carbonate.
Source: Jaw
