INTRODUCCIÓN AL MÉTODO DE LAS BIELAS Y LOS TIRANTES

The Devonian has been popularly named the “age of fish.” The widespread distribution of primitive fish fossils throughout the world suggests a long ver-tebrate record early in the Paleozoic.The fossil record reveals so many and var-ied kinds of fish that paleontologists have a difficult time classifying them all.

Every major class of fish alive today had ancestors in the Devonian. However, not all Devonian fish species continued to the present, having gone extinct in the intervening time.

The rise of fish in the Devonian seas contributed to the decline of their less mobile invertebrate competitors. This culminated in an extinction that eliminated many tropical marine groups at the end of the period.When a mass extinction occurs, those individuals that evolve into a better adaptive form are Figure 91 Marine fauna

and flora of the middle Devonian.

(Courtesy Field Museum of Natural History)

selected for survival, which is why certain species survive one major extinc-tion after another. This is particularly true for marine species such as sharks, which originated in the Devonian around 400 million years ago and have sur-vived every mass extinction since.

Fish comprise more than half the species of vertebrates, both living and extinct.They include the jawless fish (lampreys and hagfish), the cartilaginous fish (sharks, skates, rays, and ratfish), and the bony fish (salmon, swordfish, pick-erel, and bass). The ray-finned fish are by far the largest group of living fish species. Fish progressed from rough scales, asymmetrical tails, and cartilage in their skeletons to flexible scales, powerful advanced fins and tails, and all-bone skeletons, much like they are today.

The jawless fish, which first appeared in the Ordovician, are the earliest known vertebrates, having been in existence for 470 million years. They had a flexible rod similar to cartilage, which functioned as a spine along the back.

They were probably poor swimmers, however, and restricted to shallow water.

Bony plates surrounded the head for protection from invertebrate predators.

However, the additional weight forced the fish to live mostly on the seafloor, where they sifted bottom sediments for food particles.

The development of jaws about 460 million years ago revolutionized predation. Giant jawed vertebrates, some of which were monsters in their day, climbed to the very top of the food chain. The extinct placoderms (Fig. 92), which reached 30 or more feet in length, were ferocious giants that preyed on smaller fish.They lived in shallow, freshwater streams and lakes.They had cam-ouflage of red scales that helped them blend in well with their reddish brown habitat. They had well-developed articulated jaws and thick armor plating around the head that extended over and behind the jaws. One of these groups gave rise to land animals, emphasizing the great importance jaws played in ver-tebrate evolution.

Figure 92 The extinct placoderms were giants measuring 30 feet in length.

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) were thought 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 the Figure 93 The

coelacanth lives in the deep waters of the Indian Ocean.

rear appendage on the opposite side. Such an adaptation would have eased the transition from sea to land, making the coelacanth the most direct ancestor of higher terrestrial animals.

A possible link between fish and terrestrial vertebrates were the Devon-ian crossopterygDevon-ians and lungfish, another living fossil still in existence today.

The crossopterygians were lobe finned, meaning that the bones in their fins were attached to the skeleton and arranged into primitive elements of a walk-ing limb.They breathed by takwalk-ing air into primitive nostrils and lungs as well as by using gills. This placed them into the direct line of evolution from fish to land-living vertebrates that gave rise to amphibians and reptiles (Fig. 94).

The sharks were highly successful from the Devonian to the present. An ancient freshwater shark called Xenacanthus had a back fin that stretched from head to tail, allowing it to slither through the water like an aquatic snake. Closely related to the sharks are the rays, with flattened bodies, pectoral fins enlarged into wings up to 20 feet across, and a tail reduced to a thin, whiplike appendage.The rays literally fly through the sea as they scoop up plankton into their mouths.

Figure 94 The rough evolution from

crossopterygians (top) to the amphibious fish (middle) to the amphibians (bottom).

Sharks breathe by drawing water in through the mouth, passing it over the gills, and expelling it through distinctive slits behind the head.The body of the shark is heavier than water, requiring it to swim constantly or else sink to the bot-tom. Instead of skeletons composed of bone as with most fish, shark skeletons are made of cartilage, a much more elastic and lighter material. However, cartilage does not fossilize well. About the only common remains of ancient sharks are teeth, found in marine rocks of Devonian age onward (Fig. 95).