Re: do scientists believe all dinosaurs were cold-blooded?

David:

The short answer to your question is no. Back in the 1970's Dr. Ralph
Bakker and others began to question the accepted idea that dinosaurs were
cold-blooded. Modern reptiles are poikilotherms (meaning they cannot
regulate their body temperature by altering their metabolism) and so it had
been assumed that all ancient reptiles were poikilotherms as well.
Poikilotherms CAN regulate their body temperature using behavior, which is
why reptiles sun themselves on cool sunny days - they are warming up.
However, dinosaurs were different from modern reptiles, and similar to
modern mammals and birds, in ways that suggested they might have been 
homeotherms, or animals that use metabolism as well as behavior to regulate body
temperature. First of all, dinosaur fossils are found in rocks that were
near the poles when the dinosaurs were alive, but other reptiles that lived
during the time of the dinosaurs are not preserved in these deposits. Among
large animals, only homeotherms can live near the poles. Second, mammals and 
birds have a different kind of bone than modern reptiles; they have spongy areas
in their bones where blood cells were made, but modern reptile bone is
denser. Dinosaur bone has these spongy areas. Third, poikilotherms live at
a slower pace than homeotherms; they don't use as much energy. Small
mammals and small birds eat a lot--they have to just to keep their fast
metabolism burning. Small reptiles don't eat a lot; they can go weeks
without food. In an ecology with poikilothermic predators, the predators
can be relatively common, because they don't eat much. Large homeothermic
predators are rare, because they would eat everything in sight keeping
their hot metabolisms going if there were too many of them.  If you look at
dinosaur fossil faunas, the large predators are as rare as you would expect
if they were mammals or birds. By contrast, ancient large non-dinosaur reptile
predators were as common as you would expect if they were poikilotherms
like modern reptile predators. These are just three of the pieces of
evidence that led scientists studying the ecology of dinosaurs to think
that at least some groups of dinosaurs were warm-blooded.

If you want more information about this, there is a good 4-part series of
videos published by PBS called "Dinosaurs!" Each video is an hour long, and
they are accurate and fun. Another good source of information is the
National Museum of Natural History, which is part of the Smithsonian
Institution:
 http://www.nmnh.si.edu/
departments/paleo.html/

David Kopaska-Merkel
Geological Survey of Alabama
P.O. Box 869999
Tuscaloosa AL 35486-6999
(205) 349-2852
fax (205) 349-2861
www.gsa.state.al.us