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)

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Fig. 28.2
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Fig. 28.3
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Fig. 28.4 and Fig. 28.5
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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
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Fig. 28.41
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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

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Fig. 28.6
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Fig. 28.10
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Fig. 28.11
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Fig. 28.12
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Fig. 28.15
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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)

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Fig. 28.22
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Fig. 28.23
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Fig. 28.25
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Fig. 28.17
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Fig. 28.18
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Fig. 28.20
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Fig. 28.28