Chapter 20 (pgs
Chapter 20 (pgs BIO 102
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This 2 page Class Notes was uploaded by rachel potts on Friday February 6, 2015. The Class Notes belongs to BIO 102 at Washington State University taught by Strofer in Fall. Since its upload, it has received 33 views. For similar materials see Biology in Biology at Washington State University.
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Date Created: 02/06/15
l The Origin and Adaptive Radiation of Mammals A Fossil records show that mammals evolved from reptiles B Living mammals differ from living reptiles 1 Ex the way they move the nature of their teeth the structure of their jaws C It is difficult to draw a line between mammals and reptiles in the fossil record though II The mammalian jaw and teeth evolved from reptilian forms in three stages A The nature of their teeth 1 Mammals TeethJaw 81 b C d Mammals have complex teeth and jaws compared to reptiles They differ from one portion of the jaw to another 1 Ex Some teeth are used for tearing incisors others for hunting or defensecanines or for grindingmolars The hinge has moved forward and is controlled by strong cheek muscles including some positioned in front of the hinge Their jaws can be both more powerful and precisely controlled than the simple snapshut jaws of reptiles Reptiles TeethJaw 8 b Reptilian teeth change little in form or function from one position along the jaw to another Has a hinge at the back for the attachment of muscles that simply snap the top and bottom of the jaw together B How did these differences in the teeth and jaws of mammals and reptiles arise 1 The fossil record shows that these changes arose gradually over the course of about 80 million years C There are three key steps in the transition from reptile to mammal 1 A group of reptiles evolved to have an opening in the bones behind the eye called the temporal fenestra a A muscle passed through this opening and increased the power with which the jaw could be closed The therapsids group of reptiles evolved a larger temporal fenestra and therefore more powerful jaw muscles and their teeth showed the first signs of specialization 3 Jaws very similar to the mammalian jaws arose in one subgroup of the therapsids the early cynodonts with teeth more specialized and the hinge of the jaws moved forward using a different set of bones from those used in other reptiles D Over time mammals lost the reptilian hinge the bones of which evolved to become bones of the inner ear 1 Ancestral reptile gt Therapsid gt Early cynodont gt Cynodont gt Advanced cynodont gt Early mammal gt Tree shrew lll Mammals increased in size after the extinction of the dinosaurs A Mammallike reptiles and dinosaurs 1 There were many species of mammallike reptiles in the early Triassic period 245 MYA By 200 MYA the mammallike reptiles population declined when the dinosaurs came to dominate the Earth Mammallike reptiles became extinct but they left behind the first mammals as their descendants a The earliest mammals were small rodentsized organisms that evolved about 220 MYA not long after the first dinosaurs arose about 230 MYA Most mammals remained small throughout the reign of the dinosaurs a Many appeared to be nocturnal because they had large eye sockets b Mammals that were nocturnal were hard to notice and too small to eat Most of the major radiations within groups of mammals did not occur until after the dinosaurs went extinct 65 MYA a After they were gone the mammals radiated greatly to include many new forms that were large and active by day Many land mammals reached enormous sizes 1 Ex The extinct Beast of Baluchistan was over three times as large as an elephant b Different groups of mammals became specialized for different things 1 Whales life in water 2 Bats flight and hunting at night 3 Primates life in trees and evolved large brains
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