GEOL 104 Dinosaurs: A Natural History
Fall Semester 2000
Eat or be Eaten: Dinosaur Paleoecology
Ecology is NOT what most people think it is!
It is not environmental activism. Instead:
- Ecology is the scientific study of the factors which control the abundance and
distribution of organisms.
Therefore, ecology isn't about saving the whales, but it WILL tell us something about HOW to
save the whales...
Paleoecology is attempting to reconstruct the ecology of extinct forms.
Some aspects about paleoecology:
- Ecological niche: the "way of life" of a particular taxon
- Species ranges: how large an area did a particular species occupy at a given
point in time (very difficult for most dinosaurs, as they are known from very few specimens)
- Trophic relationships: aka "who eats who". Often restored as a food web
- Can get some good idea about carnivores vs. herbivores, but difficult to establish
exactly which carnivores ate which herbivores, and which herbivores ate which plants.
Bakker used his interpretations of trophic relationships to try and determine the
thermophysiology of dinosaurs and other extinct forms. His technique:
- In modern endothermic communities very few predators compared to many herbivores
(tachymetabolic predators require a lot of food, so only a few can survive in a given
- Bradymetabolic predators require a lot less food, so same amount of potential food
can support many bradymetabolic predators.
- In order to calculate P/P ratios, Bakker had to consider the different sizes of the
various populations. Used biomass (# kgs or tons of flesh) rather than number of
- Found that modern populations had P/P ratios of 0.5 – 4 %
- Looking at fossil record, found:
- 1st Radiation (Basal synapsids): 25 – 30%, much higher than modern populations. Most
paleontologists have accepted this as a cold-blooded community
- 2nd Radiation (Therapsids): 10 – 20%, seemingly between endo- and ectothermic
- 3rd Radiation (Pseudosuchians): 10 – 20%, as in 2nd
- 4th Radiation (Dinosaurs): 0.5 – 3.5%, as in modern endotherms!
- 5th Radiation (Mammals): 0.5 – 4.5%, known endotherms
- How do you know the dinosaur mass estimates are correct?
- How do you know that the numbers accurately sample fossil populations?
- At least 1st and 5th Radiation populations seem to match expectations (but new finds
from Germany may show almost mammal-like levels for upland 1st Radiation communities!)
- How do you know which dinosaurs ate which?
- At best can only give the thermal physiology of predators!
- For herbivores would need a Herbivore Biomass – Plant Productivity ratio
- Some preliminary evidence suggests that MORE herbivores per acreage in dinosaur
populations than in modern or fossil mammal populations
So, P/P ratios are problematic, at best.
Some ways in which dinosaurs are distinctly different from modern mammalian communities:
So, what is the answer to dinosaur physiology & ecology? We still don't know.
- 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
- 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
- All groups of dinosaurs show upright stance and other adaptations suggesting active
- 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
- 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
- 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.
To Next Lecture.
To Previous Lecture.