Chapter 14 Outline

CONCEPT OF "WARM-BLOODED" DINOSAURS

• BOB BAKKER POPULARIZED THE CONCEPT
  • THERAPSID-DINOSAUR TRANSITION
    • Therapsids dominated the Early & Middle Triassic terrestrial vertebrate fauna & appeared in position to continue that domination
    • Dinosaurs, which evolved at the same time as mammals in the Late Triassic, instead, displaced the synapsid line in dominating terrestrial land faunas for the next 160 million years
    • Bakker suggested that dinosaurs out-competed mammals & hence must have been "warm-blooded"
    • It has been suggested, however, that dinosaurs & mammals may not have directly competed
      • EARLIEST MAMMALS WERE SMALL INSECTIVORES THAT WERE PROBABLY ARBOREAL AND NOCTURNAL
      • EARLIEST DINOSAURS WERE LARGER DIURNAL CARNIVORES
    • Furthermore it is not clear that "warm-bloodedness" is inherently superior to "cold-bloodedness"
      • MODERN "WARM-BLOODED" VERTEBRATES ARE GEOGRAPHICALLY DISTRIBUTED FROM POLE TO POLE, WHILE MODERN "COLD-BLOODED" VERTEBRATES ARE FOUND ONLY WITHIN 45^o OF THE EQUATOR
      • ALSO MODERN "COLD-BLOODED" VERTEBRATES ARE SMALL, AT LEAST IN PART TO INCREASE THEIR SURFACE AREA TO VOLUME RATIO (LARGE "COLD-BLOODED" VERTEBRATES, LIKE TURTLES & CROCODILES TEND TO BE AQUATIC)
      • HOWEVER, MOST LIVING VERTEBRATES ARE "COLD-BLOODED"

ENDOTHERMY AND ECTOTHERMY

• TEMPERATURE REGULATION AMONG VERTEBRATES
  • TERMS
    • Endothermy = "warm-bloodedness"
      • ENDOTHERMS REGULATE THEIR TEMPERATURES INTERNALLY
      • NOT RESTRICTED TO BIRDS & MAMMALS
    • Extothermy = "cold-bloodedness"
      • ECTOTHERMS REGULATE THEIR TEMPERATURES USING EXTERNAL SOURCES OF HEAT
    • Homeothermy = maintainance of a constant internal temperature
    • Poikilothermy = internal temperature fluctuates
      • HUMANS ARE ENDOTHERMIC HOMEOTHERMS
      • LIZARDS ARE ECTOTHERMIC POIKILOTHERMS
      • THERE ARE ENDOTHERMIC POIKILOTHERMS, HOWEVER, LIKE BATS AND HUMMINGBIRDS, AS WELL AS HIBERNATING SPECIES, LIKE BEARS
    • Endothermy & ectothermy are two biochemically different methods of obtaining heat, while homeothermy & poikilothermy are endpoints on a spectrum where many animals cluster at the endpoints, but many do not
• ENDOTHERMIC AND ECTOTHERMIC METABOLISM
  • ENDOTHEMIC TETRAPODS ARE CAPABLE OF HIGER LEVELS OF ACTIVITY SUSTAINED OVER LONGER PERIODS OF TIME THAN ARE ECTOTHERMIC TETRAPODS
  • ENDOTHERMIC TETRAPODS REQUIRE LARGER (10-30 TIMES) AMOUNTS OF ENERGY TO MAINTAIN AN ENDOTHERMIC METABOLISM THAN ECTOTHERMIC TETRAPODS REQUIRE TO MAINTAIN AN ECTOTHERMIC METABOLISM
  • ALL THINGS BEING EQUAL, AN ACRE OF LAND IS CAPABLE OF SUPPORTING FEWER ENDOTHERMS THAN ECTOTHERMS

DINOSAUR ENDOTHERMY: THE EVIDENCE

• ANATOMY
  • POSTURE
    • Dinosaurs maintained a fully erect stance, which among living vertebrates occurs only in mammals & birds, both of which are endothermic
      • ALSO LONG-LEGGEDNESS IS CHARACTERISTIC OF LIVING ENDOTHERMS
      • ALSO THE ONLY LIVING TETRAPODS THAT ARE BIPEDS ARE ENDOTHERMS
    • A fully erect stance allows lungs to fill to capacity because the torso isn't flexed
  • SPEEDS
    • Small bipedal dinosaurs ran at speeds of up to 40 kph
      • ORNITHOMIMOSAURS MAY HAVE SPRINTED AT SPEEDS UP TO 60 KPH
    • Larger theropods, ornithopods & ceratopsians probably averaged about 20 kph
    • Sauropods moved about 15 kph, and stegosaurs & ankylosaurs at about half that speed
    • Note: human sprint & run champions exceed 23 kph for distances up to 5000 m (men) or 1500 m (women), with a maximum in excess of 35 kph for the 100 m sprint
  • PROCESSING LARGE VOLUMES OF FOOD
    • Ceratopsians & hadrosaurids had chewing mechanisms as efficient as modern herbivorous mammals
    • Hadrosaurids & ankylosaurs had secondary palates that allowed simultaneous breathing & chewing
    • Birds don't chew or have secondary palates, so it is not clear the significance of these features in terms of endothermy
  • BRAINS
    • Brain size may reflect the total level of activity of an animal and therefore its metabolism
    • Small theropods had EQs equivalent to living mammals & birds
    • Large theropods & ornithopods had EQs less than living mammals & birds, but significantly higher than living reptiles
    • All other dinosaurs had EQs equivalent to living reptiles
• BONE HISTOLOGY
  • HAVERSIAN BONE
    • Densely vascularized Haversian bone is found only in endotherms among living vertebrates
    • It is also found dinosaurs (and pterosaurs & advanced therapsids) suggesting that these animals were endothermic as well
  • GROWTH RATES & LAGS
    • Densely vascularized Haversian bone forms from fast growth rates & suggests that many dinosaurs grew at rates comparable to living birds & mammals
    • LAGs (lines of arrested growth) form in modern ectotherms when bone stops growing due to seasonal temperature fluctuations; they are poorly-developed in living endotherms
    • Many dinosaurs also have LAGs, suggesting that they had a metabolism intermediate between living ectotherms & endotherms
      • SOME MESOZOIC BIRDS ALSO EXHIBIT LAGS
• ECOLOGY
  • PREDATOR-PREY RATIOS
    • Endothermic predators require an order of magnitude more prey animals on an annual basis than ectothermic predators
    • Bakker estimated endothermic predator:prey ratios (biomass of both) to be 1-3% & ectothermic predator:prey ratios to be 40%
    • Dinosaurs had predator-prey ratios of 2-4% based on museum specimens, suggesting dinosaurs were endothermic
    • Problems with how representative museum specimens are to the true living assemblage
• ZOOGEOGRAPHY
  • HIGH LATITUDES
    • Living ectotherms do not live at high latitudes
    • Dinosaurs did live at high latitudes in the Late Cretaceous, and so were endothermic
  • MIGRATION
    • Dinosaurs living at high latitudes in the Late Cretaceous probably migrated seasonally, and so were endothermic
      • LEATHERBACK TURTLES, LARGE ECTOTHERMS, ALSO MIGRATE LARGE DISTANCES
• PHYLOGENY
  • INSULATION
    • All small - to medium- sized living endotherms have fur or feathers
      • HAIR ON PTEROSAURS
    • Non-avian coelurosaurs with feathers have now been found
  • FOUR-CHAMBERED HEARTS
• GEOCHEMISTRY
  • OXYGEN ISOTOPES VARY WITH TEMPERATURE
    • Ectotherms should have large variations in oxygen isotopes, while endotherms should have smaller variations
    • Dinosaurs generally have relatively small variations
• NOSES
  • RESPIRATORY TURBINATES EXTRACT MOISTURE FROM OUTGOING BREATHS OF LIVING ENDOTHERMS
    • Dinosaurs do not seem to have repiratory turbinates
      

TOWARD A CONSENSUS?

• MASS HOMEOTHERMY & GIGANTOTHERMY
  • LARGE SIZE ALLOWS ECTOTHERMIC ANIMALS TO MAINTAIN A HIGH CONTSTANT INTERNAL TEMPERATURE
    • Applies only to large dinosaurs
• ENDOTHERMY
  • PROBABLY APPLIES TO SMALL DINOSAURS
• COMBINATION
  • PROBABLY APPLIES TO MEDIUM-SIZED DINOSAURS
    • Endothemic when young
    • Switched to more gigantothermy as adults
• DINOSAURS HAD THEIR OWN UNIQUE STYLE
  
  
  
  
  
  

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