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Mammalian Evolution and Early Primate Evolution

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The geological time scale divides the earth's history into eras, periods, ... Reptiles have homodont dentition, whereas mammals have heterodont dentition that ... – PowerPoint PPT presentation

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Title: Mammalian Evolution and Early Primate Evolution


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Mammalian Evolution and Early Primate Evolution
2
Vertebrate Evolutionary History A Brief Summary
  • The geological time scale divides the earths
    history into eras, periods, and epochs.
  • The first vertebrates appear in the fossil record
    in the early Paleozoic (500 m.y.a.).
  • Mammal-like reptiles diversified at the close of
    the Paleozoic (250 m.y.a.).
  • These forms are probably ancestors of modern
    mammals.

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  • Continental drift had profound evolutionary
    effects during the Paleozoic and the Mesozoic
    groups of animals were separated from each other
    and mammals and reptiles were redistributed
    around the globe.
  • During the late Paleozoic, the continents came
    together to form Pangaea.
  • Gondwanaland, consisting of South America,
    Africa, Antarctica, Australia, and India, began
    to split off from Pangaea during the early
    Mesozoic.
  • The other continents (North America, Greenland,
    Europe, and Asia) formed Laurasia.
  • The continents continued to move and began to
    assume their present positions by the end of the
    Mesozoic (c. 65 m.y.a.).

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  • The dominant form of land vertebrates during the
    Mesozoic was the dinosaur.
  • Most went extinct at the end of the Mesozoic due
    to collisions of asteroids or comets with the
    earth (K/Tr).
  • The earliest mammals also existed during the
    early Mesozoic.
  • The first placental mammals appeared
  • about 70 m.y.a. and were extremely
  • successful during the Cenozoic.
  • The Cenozoic is divided into seven epochs the
    Paleocene, Eocene, Oligocene, Miocene, Pliocene,
    Pleistocene, and Holocene.

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Mammalian Evolution
  • The Cenozoic is known as the Age of Mammals.
  • The diversification of the mammals followed the
    extinction of the dinosaurs, since new ecological
    niches became available.
  • Mammals were successful because they evolved a
    complex brain and had flexible behaviors.
  • Mammals are viviparous.
  • Reptiles have homodont dentition, whereas mammals
    have heterodont dentition that allows them to
    process a wide variety of foods.
  • The primitive mammalian dental formula is
    3.1.4.3.
  • Mammals (and birds) maintain a constant internal
    body temperature (they are endothermic).

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The Emergence of Major Mammalian Groups
  • The three major subgroups of mammals are
    monotremes, marsupials, and placental mammals.
  • The monotremes are the most primitive mammals
    because they lay eggs.
  • In marsupials, the young are born extremely
    immature and complete their development in the
    mothers pouch.
  • Placental mammals nourish the fetus with a
    specialized tissue, the placenta.
  • The placenta permits a longer gestation period
    resulting in a more completely developed central
    nervous system in the fetus.

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Processes of Macroevolution
  • An adaptive radiation is the rapid expansion and
    diversification of new life forms into new
    ecological niches.
  • A species will diverge into as many variations as
    allowed by its adaptive potential and the
    adaptive opportunities of the ecosystem.
  • For example, the evolution of the reptilian egg
    caused the adaptive radiation of the reptiles by
    opening new adaptive niches on land.
  • The diversity of the living and sub-fossil lemurs
    of Madagascar are a result of an adaptive
    radiation that began about 50 m.y.a.

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Generalized and Specialized Characteristics
  • An adaptive radiation involves the transformation
    of generalized characteristics into specialized
    characteristics.
  • Traits that are adapted for many functions are
    generalized.
  • Generalized and specialized can also refer to the
    entire organism.
  • Species that are generalized have the greatest
    adaptive potential.

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Proto-primates
  • The first primate-like mammals, or
    proto-primates, were roughly similar to squirrels
    and tree shrews in size and appearance. 
  • The existing, very fragmentary fossil evidence
    (mostly from North Africa) suggests that they
    were adapted to an arboreal way of life in warm,
    moist climates.
  • They probably were equipped with relatively good
    eyesight as well as hands and feet with pads and
    claws for climbing.
  • These proto-primates will remain rather shadowy
    creatures for us until more fossil data becomes
    available.

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Early Prosimians
  • The first primates may have evolved during the
    last part of the Paleocene Epoch.
  • Their bones have been found in 60 myo geological
    deposits in Morocco, but they probably lived in
    other areas at this time as well.
  • They looked different from the primates today.
  • They were still somewhat squirrel-like in size
    and appearance, but apparently they had grasping
    hands and feet that were increasingly more
    efficient in manipulating objects and climbing
    trees.
  • It is likely that they were developing effective
    stereoscopic vision.

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  • The beginning of the Eocene Epoch coincides with
    the appearance of early forms of most of the
    placental mammal orders that are present today.
  • Among them were primate species that somewhat
    resemble modern prosimians such as lemurs,
    lorises, and possibly tarsiers.
  • This was the epoch of maximum prosimian adaptive
    radiation.
  • There were at least 60 genera of them that were
    mostly in two families--the Adapidae (similar to
    lemurs and lorises) and the Omomyidae (possibly
    like galagos and tarsiers).

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  • This is nearly four times greater prosimian
    diversity than today.
  • Eocene prosimians lived in North America, Europe,
    Africa, and Asia.
  • It was during this epoch that they reached the
    island of Madagascar.
  • The great diversity of Eocene prosimians was
    probably a consequence of the fact that they did
    not have competition from monkeys and apes since
    these latter more advanced primates had not yet
    evolved.
  • However, by the end of the Eocene Epoch, many of
    the prosimian species had become extinct. 
  • This may be connected with cooler temperatures
    and the appearance of the first monkeys during
    the transition to the next geologic epoch, the
    Oligocene (about 35.4 million years ago).

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  • Major evolutionary changes were beginning in some
    of the Eocene prosimians that foreshadow species
    yet to come.
  • Their brains and eyes were becoming larger, while
    their snouts were getting smaller.
  • At the base of a skull, there is a hole through
    which the spinal cord passes. This opening is
    the foramen magnum.
  • During the Eocene, the foramen magnum in some
    primate species was beginning to move from the
    back of the skull towards the center.
  • This suggests that they were beginning to hold
    their bodies erect while hopping and sitting,
    like modern lemurs, galagos, and tarsiers.

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