TAILIEUCHUNG - Linzey - Vertebrate Biology - Chapter 4
C H A P T E R 4 Early Chordates and Jawless Fishes There are many hypotheses concerning the evolution of vertebrates. These hypotheses are continually being changed and refined as new studies uncover additional evidence of evolutionary relationships and force reassessments. | Linzey Vertebrate Biology 4. Early Chordates and I Text The McGraw-Hill Companies 2003 Jawless Fishes CHAPTER 4 Early Chordates and Jawless Fishes INTRODUCTION There are many hypotheses concerning the evolution of vertebrates. These hypotheses are continually being changed and refined as new studies uncover additional evidence of evolutionary relationships and force reassessments of some earlier ideas about vertebrate evolution Fig. . New fossil evidence morphological studies and comparative studies of DNA and RNA are gradually filling gaps in our knowledge and providing a more complete understanding of the relationships among vertebrates. Evolution takes place on many scales of time. Gingerich 1993 noted that field and laboratory experiments usually are designed to study morphological and ecological changes on short time scales in contrast fossils provide the most direct and best information about evolution on long time scales. The principal problem with the fossil record is that the time scales involved typically millions of years are so long that they are difficult to relate to the time scales of our lifetimes and those of other organisms. Many biologists have difficulty understanding evolution on a geological scale of time and many paleontologists have difficulty understanding evolution on a biological scale of time. One reason for this is that we have almost no record of changes on intermediate scales of time scales of hundreds or thousands of years that would permit evolution on a laboratory scale of time to be related to evolution on a geological scale. No living protochordate tunicate and lancelet is regarded as being ancestral to the vertebrates but their common ancestry is evident. In 1928 Garstang proposed a hypothesis by which larval tunicates could have given rise to cephalochordates and vertebrates Fig. . Garstang suggested that the sessile adult tunicate was the ancestral stock and that the tadpolelike larvae evolved as an adaptation for .
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