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Lecture 14 Reading

Lecture 14 Reading - 1 T HE ORIGIN AND E VOLUTI ON OF...

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1 THE ORIGIN AND EVOLUTION OF MAMMALS LECTURE OUTLINE 1) What are mammals and how are they distinguished from reptiles? 2) When did mammals evolve and who were their ancestors? 3) Examine three key innovations in the origin of mammals: the ear, endothermy, and limb posture. I. CLASS MAMMALIA A. Includes two extant subclasses: 1) Prototheria : monotremes 2) Theria : marsupials and placentals B. What distinguishes mammals from reptiles? SOFT TISSUE CHARACTERS: Hair Mammary glands / lactation Endothermy OSTEOLOGICAL CHARACTERS 1) Jaw joint composed of dentary and squamosal bones (as opposed to quadrate and articular in reptiles and birds) 2) Three middle ear bones (stapes, incus, malleus) (as opposed to one [stapes] in reptiles and birds) 3) respiratory turbinate bones in nasal cavity 4) teeth that are replaced only once (as opposed to continuous replacement in reptiles) 5) teeth with two or more roots (as opposed to single rooted teeth as is typical of reptiles) C. Soft tissue characters cannot be observed in the record, but both endothermy and lactation can be inferred from osteological evidence. Each of the five osteological characters relates to either one or both of these fundamental features of mammals: (#4 => lactation, and #'1-3, 5 => endothermy and associated increased nutritional needs) II. ORIGIN OF MAMMALS
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2 A. Vertebrates moved onto land about 365 million years ago, and within 60 million years or so (Late Carboniferous), we find fossils of the first amniotes, that is, completely terrestrial tetrapods known as "reptiles". These early "reptiles" already had diverged into two distinct lineages, one ancestral to dinosaurs, birds, and living reptiles (called sauropsids) and a second group ancestral to mammals This second group of "reptiles" is known as the synapsids or mammal-like reptiles. Thus the split between reptiles and mammals occurred very early. B. The history of mammal-like reptiles (MLR's) consists of three phases 1) pelycosaurs, 2) therapsids, and 3) cynodonts. 1) PELYCOSAURS: Most pelycosaurs were large bodied (2-3 meters) carnivores and herbivores with a sprawling posture. Most famous pelycosaur is Dimetrodon -–the sail-backed pelycosaur. Function of the sail may have been thermoregulatory. Pelycosaurs went extinct by the middle of the late Permian. 2) THERAPSIDS: Therapsids appear in the late Permian and show several advances over pelycosaurs. They have larger jaw muscles, bigger upper canine teeth, and less of a sprawling posture. One of the herbivorous forms of the late Permian and Triassic was the pig-sized Lystrosaurus , an animal famous for its role in establishing continental drift. 3) CYNODONTS: Cynodonts appear first in the late Permian, diversified in the Triassic, and then dwindled to go extinct in the Jurassic(except of course for those who went on to become mammals).
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  • Winter '10
  • jaw joint, cynodonts, Evolution of mammalian auditory ossicles

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Lecture 14 Reading - 1 T HE ORIGIN AND E VOLUTI ON OF...

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