The evolution of jaws also improved fish respiration by supporting the
gills.After a fish draws water into its mouth, it squeezes the gill arches to force
the water over the gills at the back of the mouth. Blood vessels in the gills
exchange oxygen and carbon dioxide as the water flows out the gill slits. The
jaws had the advantage of clamping down on significantly large prey, allowing
fish to become fierce predators. Primitive jawed fish might have even caused
the demise of the trilobites,once spectacularly successful in the Cambrian seas.
The coelacanths (Fig. 93) werethought to have gone extinct along with
the dinosaurs 65 million years ago. However, in 1938, fishermen caught a 5-
foot coelacanth in the deep, cold waters of the Indian Ocean off the Comoro
Islands near Madagascar. The fish looked ancient, a castaway from the distant
past. It had a fleshy tail, a large set of forward fins behind the gills, powerful
square toothy jaws, and heavily armored scales. The most remarkable aspect
about this fish was it had not changed significantly from its primitive ances-
tors, which evolved in the Devonian seas some 400 million years earlier.
Because of this, the coelacanth has been given the title of “living fossil.”
The coelacanth’s head contained a small organ that is thought to detect
faint electric fields. Sharks have similar sensors to home in on weak electric
fields generated by the moving muscles of smaller fish upon which they prey.
The coelacanth would perform a number of acrobatic feats, including head-
stands, swimming backward, or flipping upside down to pinpoint the electric
tracks of prey.
The coelacanth came from the same evolutionary branch in direct line
to land-dwelling vertebrates. Stout fins on the fish’s underside enabled it to
crawl along the deep ocean floor.The fins were precursors of amphibian limbs
and were coordinated in a manner not seen in most fish but common in four-
legged terrestrial animals. The fins moved similarly to the legs of a crawling
lizard, with the forward appendage on each side advancing in concert with theFigure 93The
coelacanth lives in the
deep waters of the Indian
Ocean.
Historical Geology