LA REDISTRIBUCIÓN LIMITADA EN LA INSTRUCCIÓN EHE-08

Plants had been greening Earth for as long as 100 million years before the vertebrates finally set foot onto dry land. Prior to the amphibian invasion, freshwater invertebrates and fish had been inhabiting lakes and streams. By the early Carboniferous, the vertebrates had spent more than 160 million years underwater, with only a few short forays onto the land. Relatives of the lungfish lived in freshwater pools that dried out during seasonal droughts, requiring the fish to breathe with primitive lungs as they crawled to the safety of the nearest water hole.

A common misconception is that land animals evolved from fish and developed armlike limbs after flopping onto shore. A distant ancestor of the catfish living 370 million years ago had complex fingerlike bones in the underside of its fin and spent its life entirely in water.The fish grew to 8 feet long and weighed up to 200 pounds. It dated from within a few million years of the beginning of the tetrapods, the first four-legged animals with vertebrae that were the forebears of the amphibians and reptiles.

The fish had complex eight-fingered, handlike limbs, suggesting that some fish developed legs first before venturing out onto land.The fingered fin shows that limb bones could have evolved in fish for use in water long before being useful on land. The fish probably used its walking fins to help it move around and hunt in the shallow, weedy streams, giving it a distinct advantage over other fish. Also, at this time, shallow swamps began to appear. Limbs with digits would have helped the first aquatic tetrapods travel through the plant-choked wetlands. Eventually, the fins evolved into paws that its progeny could use on land.

Amphibious fish (Fig. 108) probably spent little time on shore because

land offered no large animals to eat, and vertebrates had not yet evolved the capacity to consume plants. Therefore, the only food consisted of inverte-brates, including millipedes, centipedes, and the forerunners of insects.

The earliest known tetrapod was Acanthostega (Fig. 109), meaning “spine plate.” It was essentially an aquatic animal with attached hands and feet. The salamander-like body had large eyes on top of a flat head for spotting prey swimming above as it sat buried in the bottom mud. It sported eight toes on the front feet and seven toes on the rear feet, perhaps the most primitive of walking limbs. The digits were sophisticated and multijointed. However, because they were attached to an insubstantial wrist, the legs were virtually useless for walking on the ground. The rest of the skeletal anatomy also sug-gests acanthostega could not have easily walked on land. Instead, it probably crawled around on the bottom of lagoons and used gills for respiration.

One of the earliest known amphibians was an ancient land vertebrate called Ichthyostega, meaning “fish plate,” which lived its life half the time in Figure 108 Amphibious

fish were the first tetrapods.

Figure 109

Acanthostega walked on the bottoms of lagoons with legs having eight toes.

water and half the time on dry land. It was dog sized, with a broad, flat, fish-like head and a tail topped with a small fin, apparently used for swimming. It developed a sturdy rib cage to hold up its internal organs while on land and crawled around on primitive legs with seven toes on the hind limbs. Amphib-ians also possessed six and eight digits on their feet, indicating the evolution of early land vertebrates followed a flexible pattern of development. However, no terrestrial vertebrates evolved a foot with more than five true digits.

Neither acanthostega nor ichthyostega could do much more than wad-dle around on land.Their upper arm bones had a broad, blobby shape ill suited for walking.Their hind limbs splayed out to the side and could not have eas-ily held up the body. The backbones were looser than those of terrestrial tetrapods and were similar to those of fish, which offered less support on land.

A small amphibianlike animal called microsaur was less than 6 inches long, a mere midget compared with its aquarian counterparts. It was among the first four-legged vertebrates to crawl onto the land more than 300 million years ago.The animal had uniquely shaped spine bones and a simplified skull, with just one bone instead of three.The skull was attached to the first verte-bra of the spine with a pair of rodlike bones, which limited head rotation only to up-and-down movements. This feature is also found in the large, more primitive amphibians that predated microsaur.

The weak legs of the early amphibians could hardly keep their squat bodies off the ground, making them slow and ungainly.The amphibian tracks are generally broad with a short stride.The animal walked with a clumsy gait.

Therefor, running to attack prey or escape predators was simply not possible.

In order to succeed as hunters without requiring speed or agility, the amphib-ians developed a unique whiplike tongue that lashed out at insects and flicked them into the mouth.This successful adaption enabled the amphibians to pop-ulate the land rapidly.

Although the amphibians had well-developed legs for walking on dry land, the animals apparently spent most of their time in rivers and swamps.

They depended on accessible sources of water to moisten their skins as well

Amphibian footprints were quite abundant during Carboniferous period but less so in the Permian, owing to the takeover by the reptiles and the amphibians’ preference for life in the water. The increase in the number of reptilian footprints in the Carboniferous and Permian plainly shows the rise of the reptiles at the expense of the amphibians. Possibly one of the major fac-tors leading to the superiority of the reptiles was their more efficient mode of locomotion. The reptiles were also much more suited for living full-time on dry land, whereas the amphibians had to return to the water periodically.

Populations of amphibians continued to fall during the Mesozoic, with all large, flat-headed species going extinct. The group thereafter was repre-sented by the more familiar salamanders, toads, and frogs.The fossil remains of these amphibians are largely fragmentary because vertebrate skeletons are con-structed with a large number of bones that are easily scattered by surface ero-sion. Although the amphibians did not achieve complete dominion over the land, their cousins the reptiles were destined to become the greatest success story the world has ever known.