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The Origin and Early Evolution of the Mammals

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Title: The Origin and Early Evolution of the Mammals


1
The Origin and Early Evolution of the Mammals
  • Class Mammalia 4800 extant species (extremely
    diverse in size, form, and lifestyle) exhibit
    hair, mammary and sweat glands, three middle-ear
    bones and fleshy external ears, skull with two
    occipital condyles, secondary palate (allows
    breathing during suckling), turbinate bones in
    nose (hold mucus membranes that retain heat),
    fused pelvic bones, muscular diaphragm, teeth
    diphyodont and heterodont enlarged cerebral
    cortex
  • Early Evolution of Mammals evolved from
    therapsids (Paleozoic synapsids)
  • Cynodonts the only therapsids to survive the
    Permian Extinction exhibited erect gait with
    ventral versus lateral limbs, increased jaw
    musculature, heterodont teeth, turbinate bones,
    secondary palate, loss of lumbar ribs (related to
    the evolution of the diaphragm)
  • Earliest true mammals (Triassic fossils) mouse-
    or shrew-sized, with diphyodont teeth, three
    middle-ear bones (stapes homologous to columella,
    malleus and incus derived from articular and
    quadrate, respectively ? mammalian jaw joint
    between dentary and squamosal bones) likely laid
    amniotic eggs and nursed immature young (like
    extant monotremes)
  • Early mammals existed 150 million years with
    little diversification (likely nocturnal)
    greatest radiation of the mammals not until
    early Cenozoic (after the extinction of the
    dinosaurs likely related to newly available
    niches)

2
Fig. 28.2
3
Fig. 28.3
4
Fig. 28.4 and Fig. 28.5
5
Class Mammalia Diversity and Taxonomy
  • Subclass Prototheria, Order Monotremata
    (Monotremes) lay an amniotic egg, examples
    include platypus (with electroreceptive bill and
    venomous spine) and echidnas of Australia and
    New Guinea
  • Subclass Theria
  • Infraclass Metatheria (Marsupials) immature
    young nursed in abdominal pouch (marsupium)
    include American opossums, possums, koalas,
    wombats, wallabies, kangaroos, and Tasmanian
    devils (mostly Australian region)
  • Infraclass Eutheria (Placental Mammals) 18
    orders (12 shown below)

Insectivora shrews, hedgehogs, moles 440 species Rodentia squirrels, woodchucks, rats, mice, beavers, porcupines 2052 species
Chiroptera bats 977 species Sirenia sea cows and manatees 4 species
Primates prosimians, monkeys, apes, humans 279 species Perrisodactyla odd-toed hoofs (horses, asses, zebras, tapirs, rhinos) 17 species
Xenarthra anteaters, armadillos, sloths 29 species Artiodactyla even-toed hoofs (swine, camels, deer, hippos, giraffes, cattle, sheep) 221 species
Proboscidea elephants 2 species Carnivora dogs, wolves, cats, bears, weasels, pinnipeds (seals and sea lions) 280 species
Lagomorpha rabbits, hares, pikas 81 species Cetacea whales, dolphins, porpoises 78 species
6
Fig. 28.41
7
Mammals Integument, Teeth, and Feeding
  • Integument and Derivatives
  • Hair composed of dead, keratin-packed epidermal
    cells follicles of epidermal origin, but extend
    into dermis underhair for insulation, guard
    hairs protect against wear and provide
    coloration of pelage (fur coat) hair
    continuously replaced, but most mammals molt
    entire coat periodically (summer coats often
    brown, thin vs. white, thick winter coats)
    vibrissae sensory hairs on snout (whiskers)
    quills modified hairs for defense (ex.
    porcupines)
  • Horns and Antlers true horns (keratin) with bony
    core, not shed, grow continuously (ex. sheep and
    cattle) antlers (deer family) branched and
    composed of bone covered by soft skin (velvet),
    shed after breeding season rhino horns not
    attached to skull, composed of cemented filaments
    of keratin
  • Glands sweat glands cool body (eccrine glands)
    and signal reproductive stage (apocrine glands)
    scent glands secrete pheromones used to attract
    opposite sex and mark territories sebaceous
    glands secrete sebum, which lubricates hair
    mammary glands secrete milk via nipples (excl.
    monotremes)
  • Teeth and Feeding heterodont teeth (incisors,
    canines, premolars, molars)
  • Specializations insectivores herbivores
    (grazers, gnawers) feed continuously, with long
    digestive tracts and fermentation chambers
    (endosymbiotic bacteria digest cellulose)
    carnivores with shorter GI tract, piercing and
    slicing teeth (incl. carnassial teeth)
    omnivores include bears, pigs, raccoons,
    primates rodents store food in caches rabbits
    coprophagous
  • Metabolic rates decline with increasing body size

8
Fig. 28.6
9
Fig. 28.10
10
Fig. 28.11
11
Fig. 28.12
12
Fig. 28.15
13
Mammals Reproduction, Behavioral Ecology,
Populations, and Endangered Species
  • Reproduction female fertility limited by estrous
    cycles monotremes lay eggs in a burrow, young
    drink milk after hatching marsupials with short
    gestation, embryonic diapause, and development
    of young in external pouch (marsupium)
    placentals with long gestation, fecundity highly
    variable (usually higher for small mammals)
  • Territory and Home Range territories (defended
    home ranges) commonly marked by scents home
    ranges typically foraging areas (can overlap)
    social mammals include prairie dogs
  • Migration more rare than in birds and fishes
    caribou winter in taiga, calve in tundra
    cetaceans and pinnipeds with longest migrations
    (ex. gray whales migrate between Alaska and
    calving lagoons in Baja California)
  • Flight and Echolocation bats only true flying
    mammals (gliding in some rodents and
    marsupials) bats and toothed whales use
    echolocation to find food and navigate bats
    collect echoes with ears, toothed whales create
    sound in sinuses, collect echoes with lower jaws
  • Mammalian Populations and Endangered Species
    small mammals often with highly fluctuating
    populations top predators with naturally low
    populations in 2006, 510 mammals listed as
    endangered (imminent risk of extinction),
    including most cetaceans (baiji recently
    declared extinct), wild cats, and non-human
    primates, plus many more considered threatened
    (forseeable risk of extinction)

14
Fig. 28.22
15
Fig. 28.23
16
Fig. 28.25
17
Fig. 28.17
18
Fig. 28.18
19
Fig. 28.20
20
Fig. 28.28
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