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Re: New refs (long)
In a message dated 2/2/2003 5:54:36 PM Eastern Standard Time,
<The small size and cursorial habits of hypsilophodontids may have favored a
greater emphasis on endothermy than their bulkier relatives (e.g. hadrosaurs),
who could rely more on their sheer body mass to retain heat.>
This is one of those false truisms that permeates dinosaur paleontology.
It usually goes something like this: "Since dinosaurs displayed such a striking
range of size, they probably displayed a similar range of metabolic
adaptations." There is absolutely no support for this amongst extant animal
groups. Placental mammals show a larger size range (from a few grams to 200
metric tonnes) without anywhere near the variation in metabolic rate that is
being implied for dinosaurs.
Usually the response to this is: "That doesn't mean that extinct animals
where the same." Which is true, except that the burden of proof is on those
who wish to apply a novel metabolic distribution to an extinct group.
Dinosaurs show less morphological variation and less size variation than extant
placentals, period. Supposing a larger range of metabolic rates directly
contradicts the analysis by analogy method, and so requires positive evidence
from the fossil record (or other extant groups).
<Among modern mammals, small mammals can be regarded as "more" endothermic than
big mammals, since their high surface area to volume ratios renders them more
prone to loss of body heat.>
This depends on what you are refering to when you say "more endothermic."
A large endotermic organism's mass specific resting metabolic rate (= RMR,
which is the oxygen consumption per unit mass, divided by mass) is indeed lower
than small endothermic animals'. But wait, the mass specific RMR of large
endotherms is lower than that of small ectotherms. Does that mean there is no
real difference between them? Of course not. RMR can be calculated as
follows: X kcal/ mass of the animal in kg ^.75
The .75 exponent describes the falling mass specific RMR of larger
organisms that Tim referred to. But that first value (x kcal) shows true
metabolic difference. Typical RMR for extant reptiles is 5-10 kcal / kg^.75.
The typical value for placental mammals is 70 kcal / kg^.75. Both large and
small placentals maintain that 70 kcal constant (or close to it). So big
mammals are not "less" endothermic, that is, they have the same scaler. As
Greg Paul details in DOTA, when non-placentals are included, mammals have a
much larger range of RMR, approaching the highest reptilian levels (~15 kcal /
kg^.75. But Mammalia shows a much wider range of morphological diversity than
do dinosaurs, so such variation is not suprising.
There is a final falacy that Seebacher's paper falls for; The supposed
importance of body temperature. High (and more importantly, constant) body
temperatures are advantageous to enzymatic processes, as well as a host of
other metabolic pathways. But without an elevated RMR, your body can't take
advantage of it. Large crocs (in the wild) do not have significantly higher
metabolic scalers. It is possible to stimulate an increaes in RMR in ectoterms
in a laboratory setting, when they are provided with lots of food, kept at a
high temperature, and excersized regularly. In the wild, the inability of
ectotherms to thermoregulate as effectively, and the inabiliy to gather enough
food (due to lower aerobic excersize capabilities) prevents them from enjoying
most of the benefits of a higher body temperature.
This is confirmed by growth series of Deinosuchus. Undoubtadely at some
point in their growth the large crocs must have aquired "gigantothermic" fairly
constant body temperature. Despite this, they never show the fast growth rate
(i.e. fibrolemellar bone deposition) that all dinosaurs show, regardeless of
All dinsoaurs would have needed to have had RMRs above the extant
reptillian range in order for them to have grown like they did. I'm not imply
that all, or even any, non-avian dinosaurs were at the placental level (though
I suspect many were) but they had to be elevated at least into the low