Some ways in which dinosaurs are distinctly different from modern mammalian communities:

• Much higher rate or production: dozen eggs per year, independant of size vs. litter size and gestation period scaled to body size
  • From this, dinosaur populations could absorb many more fatalities and survive than equivalent-sized mammals
• Also, dinosaurs occupied many more niches in their lifetime than a mammal, because all dinosaurs begin very small

Current status, and some scenarios:
We know that:

• Living dinosaurs (birds) are all endothermic tachymetabolic homeotherms
• The living sister group to dinosaurs, crocs, are all ectothermic bradymetabolic heterotherms
• All groups of dinosaurs show upright stance and other adaptations suggesting active lifestyles
• All large dinosaurs, and many small ones, show signs of having high breathing rates
• If P/P ratios are real, dinosaur ecologies were more similar to mammals than to basal synapsids
• Many dinosaurs (particularly Late Cretaceous forms) show very sophisticated feeding, locomotory, and social adaptations

Scenario I: Bakker or "Hot-Blooded Dinosaurs" model
Dinosauria (and probably Ornithodira) were endothermic tachymetabolic homeotherms; therapsids and pseudosuchians had intermediate rates (crocs would thus be a reversal).

Scenario II: Ruben or "Good Reptile" model
No dinosaur was warm-blooded, but at least some had means of rapidly oxygenating their blood to be "turbo-charged" and thus function temporarily as highly active animals. True endothermic tachymetabolic homeothermy doesn't appear until after Archaeopteryx.

Scenario III: an intermediate model ("Damn Good Reptile" model)
All dinosaurs had some degree of endothermic tachymetabolic homeothermy while young; small dinosaurs retained this into adulthood. Large dinosaurs experienced a slow down in metabolic rate, but still higher than any cold-blooded animal (~ 2/3 the rate of mammals of same size). Efficient oxygenation of blood and gigantothermy allowed these dinosaurs to be as active as mammals without the same energy costs.

• Benefit of model:
  • Explains large amount of "meat on the hoof" in Mesozoic: a 4 ton hadrosaur with 2/3 mammal metabolism would only need as much food as a 800 kg bison.
  • Thus, the higher capacity for dinosaur population growth would be realized
• Variations:
  • Development of insulation in coelurosaurs suggest that they were fully endothermic
  • Birds retain primitive condition of warm-blooded juveniles into adulthood
  • Progressive variation: as Mesozoic continued, different groups of dinosaurs (hadrosaurids, ceratopsids, coelurosaurs, maybe even titanosaurs and ankylosaurids) independently developed full endothermy from the original Scenario III condition

Still much work to be done.

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Last modified: 14 July 2006