TAILIEUCHUNG - Linzey - Vertebrate Biology - Chapter 9

C H A P T E R 9 Mammals Descendants of synapsid reptiles, mammals are vertebrates with hair and mammary glands. Additional characteristics distinguishing mammals from other vertebrates include a lower jaw composed solely of a dentary bone articulating with the squamosal bone. | Linzey Vertebrate Biology 9. Mammals I Text The McGraw-Hill Companies 2003 CHAPTER 9 Mammals INTRODUCTION Descendants of synapsid reptiles mammals are vertebrates with hair and mammary glands. Additional characteristics distinguishing mammals from other vertebrates include a lower jaw composed solely of a dentary bone articulating with the squamosal bone two sets of teeth deciduous and permanent three middle ear bones ossicles a pinna to funnel sound waves into the ear canal marrow within the bones loss of the right fourth aortic arch nonnucleated red blood cells and a muscular diaphragm separating the thoracic and abdominal cavities. In addition most mammals have sweat glands heterodont dentition and extensive development of the cerebral cortex. Approximately 4 600 species of mammals currently inhabit the world. EVOLUTION Fossil evidence indicates that mammals arose from a synapsid reptilian ancestor Fig. . The subclass Synapsida appeared during the Lower Pennsylvanian over 300 million years ago and became extinct about the end of the Triassic period some 190 million years ago. The earliest synapsid Archaeothyris was a pelycosaur found in Nova Scotia Reisz 1972 . The climate of Nova Scotia some 300 million years ago was warm and moist and much of the land was covered by forests dominated by giant lycopods. A cladogram of the synapsids emphasizing mammalian characteristics is presented in Fig. . Synapsids were quadrupedal reptiles Fig. that possessed a single temporal fossa whose upper border was formed by the postorbital and squamosal bones see Fig. . Some researchers feel that a chain of small bones articular quadrate angular that formed the hinge attaching jaw and skull in mammal ancestors began moving back along the skull in synapsids. These bones were beginning to do double duty hinging the jaw and likely picking up higher frequency sounds perhaps made by insects . They also were destined to join with the columella stapes already in the ear to .

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