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Unformatted text preview: 624 Endothermy, the ability to generate and maintain elevated body temperatures, has arisen several times in the evolu-tionary history of animals. It goes hand in hand with the ca-pacity to produce heat through metabolism, and therefore activity levels. Most modern birds and mammals have high metabolic rates and are able to maintain their body tem-peratures well above ambient temperature, often within narrow thermal windows. While both are perceived as higher vertebrates, birds and mammals arose from sep-arate reptilian ancestors. Thus, endothermy arose inde-pendently at least twice. However, fossil evidence suggests that other extinct reptiles may also have been endotherms. The fossil record of the animals in the paleontological pe-riod from 200 to 65 million years ago is particularly clear, showing definitive examples of the transitions from rep-tiles to mammals and birds. The first mammals appeared approximately 200 mil-lion years ago, evolving from small, nocturnal reptiles that were only distantly related to the dinosaurs that would dominate Earth in later years. Fossils dating back to this period reveal the existence of several distinct mammalian-like reptilian lineages. These animals differed from other reptiles by the morphology of the skull and the organiza-tion of the teeth. Although most of these lineages disap-peared, one group of reptiles called cynodonts gave rise to true mammals. The earliest mammals retained the reptil-ian trait of egg laying, like the modern monotremes, echidna and platypus. By the early Cretaceous period (144 million years ago), mammals had diversified into several lineages of marsupials and insectivores. When the di-nosaurs disappeared about 65 million years ago, at the end of the Cretaceous period, there was an explosion of mam-malian diversification. New species of mammals began to occupy the environmental niches vacated by the dinosaurs. It cannot be said for certain when endothermy arose in the transition from mammalian-like reptiles to true mam-mals. However, it is likely that the cynodont reptiles were already endothermic. Unlike most other reptiles of the day, Thermal Physiology C H A P T E R 1 3 8140606_CH13_p624-661.qxd 10/11/08 9:36 PM Page 624 P e a r s o n L i g S l u t N F R O D b 625 cynodonts possessed a bony, secondary palate in the roof of the mouth that would have allowed them to breathe while chewing. This anatomical arrangement is a charac-teristic of endotherms because they must maintain unin-terrupted respiration to sustain high metabolic rates. Cynodonts also appear to have possessed hair, which could have helped insulate their bodies. Birds, the other group of modern endotherms, also arose from reptiles, although much later than mammals and from different reptilian ancestors. Around the time di-nosaurs were declining, several reptilian lineages had al-ready evolved featherlike body coverings. In one group, the theropod dinosaurs such as Archaeopteryx , the feathers were similar in structure to those of modern birds. Their were similar in structure to those of modern birds....
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This note was uploaded on 10/05/2011 for the course BIO 203, CH taught by Professor Lacey,simmerling,deng,hanson during the Fall '10 term at SUNY Stony Brook.
- Fall '10