Untitled Essay Research Paper BODYINTRODUCTION TO EVOLUTION — страница 4

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contain imprints ofjellyfish, tracks of worms, and traces of soft corals and other animals of uncertainnature. Paleozoic waters were dominated by arthropods called trilobites and largescorpionlike forms called eurypterids. Common in all Paleozoic periods (570-230million years ago) were the nautiloid ,which are related to the modern nautilus, andthe brachiopods, or lampshells. The odd graptolites,colonial animals whosecarbonaceous remains resemble pencil marks, attained the peak of theirdevelopment in the Ordovician Period (500-430 million years ago) and thenabruptly declined. In the mid-1980s researchers found fossil animal burrows inrocks of the Ordovician Period; these trace fossils indicate that terrestrialecosystems may have evolved sooner than was once thought. Many of the

Paleozoic marine invertebrate groups either became extinct ordeclined sharply in numbers before the Mesozoic Era (230-65 million years ago).During the Mesozoic, shelled ammonoids flourished in the seas, and insects andreptiles were the predominant land animals. At the close of the Mesozoic the once-successful marine ammonoids perished and the reptilian dynasty collapsed, givingway to birds and mammals. Insects have continued to thrive and have differentiatedinto a staggering number of species. During the course of evolution plant and animal groups have interacted toone another’s advantage. For example, as flowering plants have become lessdependent on wind for pollination, a great variety of insects have emerged asspecialists in transporting pollen. The colors and fragrances of

flowers have evolvedas adaptations to attract insects. Birds, which feed on seeds, fruits, and buds, haveevolved rapidly in intimate association with the flowering plants. The emergence ofherbivorous mammals has coincided with the widespread distribution of grasses,and the herbivorous mammals in turn have contributed to the evolution ofcarnivorous mammals. Fish and Amphibians During the Devonian Period (390-340 million years ago) the vast land areasof the Earth were largely populated by animal life, save for rare creatures likescorpions and millipedes. The seas, however, were crowded with a variety ofinvertebrate animals. The fresh and salt waters also contained cartilaginous andbony Fish. From one of the many groups of fish inhabiting pools and swampsemerged the first land

vertebrates, starting the vertebrates on their conquest of allavailable terrestrial habitats. Among the numerous Devonian aquatic forms were the Crossopterygii,lobe-finned fish that possessed the ability to gulp air when they rose to the surface.These ancient air- breathing fish represent the stock from which the first landvertebrates, the amphibians, were derived. Scientists continue to speculate aboutwhat led to venture onto land. The crossopterygians that migrated onto land wereonly crudely adapted for terrestrial existence, but because they did not encountercompetitors, they survived. Lobe-finned fish did, however, possess certain characteristics that servedthem well in their new environment, including primitive lungs and internal nostrils,both of which are essential for

breathing out of the water.Such characteristics, called preadaptations, did not develop because the others werepreparing to migrate to the land; they were already present by accident and becameselected traits only when they imparted an advantage to the fish on land. The early land-dwelling amphibians were slim-bodied with fishlike tails, butthey had limbs capable of locomotion on land. These limbs probably developedfrom the lateral fins, which contained fleshy lobes that in turn contained bonyelements. The ancient amphibians never became completely adapted for existence onland, however. They spent much of their lives in the water, and their moderndescendants, the salamanders, newts, frogs, and toads–still must return to water todeposit their eggs. The elimination of a

water-dwelling stage, which was achievedby the reptiles, represented a major evolutionary advance. The Reptilian Age Perhaps the most important factor contributing to the becoming of reptilesfrom the amphibians was the development of a shell- covered egg that could be laidon land. This development enabled the reptiles to spread throughout the Earth’slandmasses in one of the most spectacular adaptive radiations in biological history. Like the eggs of birds, which developed later, reptile eggs contain acomplex series of membranes that protect and nourish the embryo and help itbreathe. The space between the embryo and the amnion is filled with an amnioticfluid that resembles seawater; a similar fluid is found in the fetuses of mammals,including humans. This fact has been