Carbon Amphibia Class Notes

CARBON AMPHIBIA The chapter examines the evolution of amphibians in the great coal swamps of the Carboniferous period and follows the construction of the great supercontinent Pangaea. Carboniferous, from 345 to 280 million years ago, was named for coal-bearing rocks in Wales, Great Britain. It is further divided into the Mississippi and Pennsylvania periods of North America. The flora that appears in the Devonian is abundant and varied during the Carboniferous. Large coal forests, ferns, seeds, and true trees with seeds and logs are scattered in Gondwana and Laurasia in the lower Carboniferous. All forms of marine fauna present in the lower Paleozoic thrived in the Carboniferous except brachiopods, which declined in number and type. Fusulinids appeared for the first time in the Carboniferous. They are large, complex protozoa that resemble wheat grains and range in size from microscopic to 3 inches in length. Primitive amphibians inhabited swampy forests, which were filled with hundreds of types of insects, including huge cockroaches and giant dragonflies. As the climate grew colder and widespread glaciation enveloped the southern continent by the end of the period, the first reptiles appeared and replaced amphibians as the dominant land vertebrates. AMPHIBIOUS ERA Plants had greened the Earth for 100 million years before vertebrates finally set foot on dry land. Before the invasion of amphibians, freshwater invertebrates and fish had inhabited lakes and rivers. By the early Carboniferous, vertebrates had spent more than 160 million years underwater, with only a few brief forays to land. Lung fish relatives live in freshwater ponds that dry up during seasonal droughts, requiring fish to breathe primitive lungs as they crawl to safety in nearby waterholes. A common misconception is that land animals evolved from fish and developed arms-like limbs after falling ashore. The distant ancestor of the catfish that lived 370 million years ago had complex finger-like bones at the bottom of its fins and spent its life entirely in water. The fish grew up to 8 feet in length and weighed up to 200 pounds. It originated within a few million years of the beginning of tetrapods, the first four-legged animals with a spine that were the ancestors of amphibians and reptiles. The fish had complex eightfingered, hand-like limbs, suggesting that some fish developed legs first before venturing ashore. Fingered fins suggest that limb bones could have evolved in fish for use in water long before they were useful on land. The fish probably used its road fins to help it move and hunt in shallow, thin rivers, giving it a distinct advantage over other fish. Also, at this time, shallow swamps began to appear. Limbs with numbers will help the first aquatic tetrapods to travel through clogged wetlands. Eventually, fins evolved claws that their offspring could use on land. Amphibious fish (Figure 108) probably spend little time on shore because their primitive legs cannot support their weight for very long, requiring them to return to the water. Finally, as their limbs strengthen, the amphibious fish wandered further inland, where crustaceans and insects abounded. In the middle Devonian, they began to dominate the land and were mainly attracted to large Carboniferous swamps. About 335 million years ago, amphibian fish evolved into the earliest amphibians. Tetrapods branch out into two groups, with one line leading to amphibians and the other to reptiles, dinosaurs, birds, and mammals. Some species have powerful, toothed jaws and resemble giant salamanders, reaching 3 to 5 feet in length. A 2-foot-long amphibian with a plate-like armadillo rooted in the ground for worms and snails. During the early Carboniferous, soils did not offer large animals to eat, and vertebrates had not yet developed the capacity to consume plants. Therefore, the only food consists of invertebrates, including millipedes, centipedes, and forerunners of insects. The earliest known tetrapod is Acanthostega (Figure 109), meaning "spinal plate." It was basically an aquatic animal with attached hands and feet. The salamander-like body has large eyes on a flat head to see prey swimming above while sitting buried in the bottom mud. He wears eight toes on the front foot and seven toes on the back foot, perhaps the most primitive of the walking limbs. The digits are sophisticated and multijointed. However, because they are attached to the insubstantial wrist, the foot is almost useless for walking on the ground. The rest of the skeletal anatomy also suggests Acanthostega could not easily walk on land. Instead, it may crawl at the bottom of the lagoon and use 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 water and half the time on dry land. It was dog-sized, with a wide, flat, fish-like head and an upper tail with small fins, apparently used for swimming. It developed sturdy ribs to hold its internal organs while on land and crawled on primitive feet with seven toes on the hind limbs. Amphibians also have six and eight digits on their feet, suggesting the evolution of early land vertebrates followed a flexible pattern of development. However, no land vertebrate has evolved a leg with more than five digits in fact. Neither acanthostega nor ichthyostega can do much more than roam the land. Their upper arm bones have the shape of extensive clots and are not suitable for walking. Their hind limbs stretched out to the side and could not easily lift the body. Its backbone is looser than that of terrestrial tetrapods and similar to that of fish, which offers less support on land. A small amphibian-like animal called a microsaur is less than 6 inches long, just a midget compared to its aquarium counterparts. It was one of the first four-legged vertebrates to crawl to the ground more than 300 million years ago. The animal has a uniquely shaped spine and a simplified skull, with only one bone instead of three. The skull was attached to the first vertebra of the spine with a pair of rod-like bones, which limited the rotation of the head only to up-and-down movements. This feature is also found in large, more primitive amphibians that predate microsaurs. The weak legs of early amphibians could hardly keep their squat bodies off the ground, making them slow and clumsy. Amphibious paths are generally wide with short steps. The animal walks with an awkward gait. Therefore, running to attack prey or escape from predators is absolutely impossible. To succeed as hunters without the need for speed or agility, amphibians develop unique whip-like tongues that attack insects and flick them into the mouth. This successful adaptation allowed amphibians to quickly fill the land. Although amphibians have well-developed legs for walking on dry land, the animals seem to spend most of their time in rivers and swamps. They depend on accessible water sources to moisturize their skin as well as for respiration and reproduction. They reproduce like fish, laying small eggs, without shells. After hatching, the juveniles live aquatic, like fish, breathing with gills. When they mature, young amphibians metamorphose into air-breathing four-legged adults. Early amphibians that lived during the late Devonian, when vertebrates first made the transition from sea to land, spent most of their time in water. The need to lead a semiaquatic lifestyle, however, led to the downfall of amphibians when large swamps began to dry out towards the end of the Paleozoic. The void left by amphibians is quickly filled by their cousins, reptiles, which are better suited for life completely out of the water. Amphibian footprints were fairly abundant during the Carboniferous period but less so in the Permian, due to takeover by reptiles and amphibians' preference for living in water. The increase in the number of reptile footprints in the Carboniferous and Permian clearly indicates the emergence of reptiles at the expense of amphibians. Perhaps one of the main factors that led to the superiority of reptiles was their more efficient way of locomotion. Reptiles are also much better suited to living full-time on dry land, whereas amphibians must return to the water periodically. Amphibian populations continued to decline during the Mesozoic, with all large, flat-headed species going extinct. The group was later represented by the more familiar salamanders, toads, and frogs. The fossil remains of these amphibians are largely fragmentary because vertebrate skeletons are built with large amounts of bone that are easily dispersed by surface erosion. Although amphibians did not achieve complete power over the land, their reptilian cousins were destined to become the greatest success stories the world has ever known.