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Lecture.21 - Mesozoic Mammals Mammal origins Mammals are...

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Mesozoic Mammals
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Mammal origins Mammals are synapsids; unlike the dinosaurs and other “reptiles”, which are diapsids http://content.answers.com/main/content/wp/en-commons/ thumb/1/17/200px-Skull_synapsida_1.png Synapsid http://upload.wikimedia.org/wikipedia/commons/thumb/f/fd/Skull_diapsida_1 .png/240px-Skull_diapsida_1.png Diapsid
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Synapsids were a very diverse group during the Permian Period Some familiar ones, which are often mistaken as dinosaurs, include Dimetrodon and Edaphosaurus http://images.google.com/imgres?imgurl=http://usuarios.lycos.es/dinosaurios/imagenes/dimetrodon/ dimetrodon31.jpg&imgrefurl=http://usuarios.lycos.es/dinosaurios/dimetrodon.htm&h=367&w=510&s z=35&hl=en&start=18&um=1&tbnid=SBVuCE5FG2ZyPM:&tbnh=94&tbnw=131&prev=/images%3Fq% 3Ddimetrodon%26svnum%3D50%26um%3D1%26hl%3Den%26client%3Dfirefox-a%26rls%3Dorg.mozilla: en-US:official%26hs%3D3Z8%26sa%3DX http://www.skulls-skeletons.com/catalog/images /Dimetrodon%20skull%20(black).JPG http://www.kheper.net/evolution/pelycosauria/Edaphosaurus.JPG
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Mammal-like synapsids, such as the cynodonts, are found in the fossil record by the Late Permian The cynodonts show a variety of mammal festures in the skull, most of which are chewing adaptations Carroll (1988), fig. 17.43 Zygomatic arch Saggital crest Tall coronoid process
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The post-cranial skeleton also shows a number of mammal-like characteristics, including the division of the vertebrae into thoracic and lumbar vertebrae and a more upright posture of the limbs Benton (2005), fig. 10.2
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Two subgroups of cynodonts that are very close to mammals are the tritylodonts and the trithelodonts However, neither shows one of the major mammalian characteristics, the jaw joint formed by only the squamosal and dentary bones Benton (2005), fig. 10.4
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Jaw joints, middle ear bones, and the definition of mammals
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