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Causality and dinosaur energetics
This whole discussion on RTs and endothermy reminds me of the old adage
of three blind men trying to describe an elephant by holding onto three
First, drawing conclusions of causality. True, RTs are important in
modern endotherms, and can distinguish between them and modern ectotherms.
The key word here, though, is *modern*, and anything outside of this is
extrapolation, nothing more. Once you go into extrapolation, you cannot
simply ignore the rest of the evidence.
Further, there are other important factors that cannot be explained away
nor ignored. They have looked at four skulls that do not have soft parts
preserved such as cartileganeous RTs if they existed, nor can they guarantee
that the skulls were complete enough to preserve (or not have prepared away)
bony RTs. They state that those four dinosaurs skulls had passageways too
narrow for RTs ... but are they not talking about MODERN RTs? Could they
have had, in fact, very small, primitive RTs? Can they guarantee that the
problem of water and heat loss wasn't solved differently? Perhaps, because
of the climate, they could be endothermic without having these problems.
Or, perhaps, they were endothermic and just had to live with water and heat
Plus, can they prove beyond a shadow of a doubt that RTs are *required*
for endothermy? This whole argument is based on the requirement of RTs for
endothermy. How is this conclusion arrived at? *How* does one prove that
they are *required* for endothermy? All modern tetrapods have five digits.
Are five digits *required* for tetrapodality? How and why did RTs evolve?
Which came first, RTs or endothermy? Where is THAT evidence? I don't think
you CAN prove causality here. (As an aside, we had a long discussion about
linear regression in this thread ... I remind everybody that linear
regression shows CORRELATION. Correlation and causation are NOT synonyms.
My graduate stats professor gave the example of a high correlation between
the number of priests ordained in the US and the amount of Irish Whiskey
produced in europe. Do you really think there's causation there?)
The bottom line is, RTs *may* be the be all and end all of MODERN
endothermy. It *cannot* be used as the end all and be all of ancient
endothermy because of the problems stated above. You have to look at it as
part of the overall picture.
(just out of curiosity, do PTEROSAUR skulls show evidence of RTs?)
At 12:36 PM 12/19/96 -0500, Terry D. Jones writes
>List all features of modern birds and mammals that are directly linked
>(i.e., functionally and causally linked) to endothermy (therefore this
>list could not include: ilium length, posture, etc). This list may
>include: insulation (not just flight feathers, but real insulation),
Providing scrutiny does not prove otherwise, you have _Archaeopteryx_,
"Sinosauropteryx" and _Pelicanimimus_ with insulation.
>four-chambered heart (not functionally four-chambered as in crocs),
Impossible to know.
>high respiratory surface area: lung vol., high resting rates of O2
>consumption, high lung ventilation rates,
Impossible to know, although evidence from the skeleton suggests large
lungs and a bird-like respiratory system.
>I am confident that none of the things in your endotherm list will
>you be able to find evidence for in dinosaurs.
Even if it does IT DOES NOT PROVE ECTOTHERMY IN DINOSAURS. It does NOT
disprove endothermy in dinosaurs. It is not even strong evidence either way
... it is only strong evidence for modern animals. Absence of evidence is
not necessarily evidence of absence. There are far too many factors
involved, and you have to look at them all.
I also want to revist something you said earlier about obligate
bipedalism not suggesting endothermy. I think you're absolutely correct.
Both Dr. Thomas Hetherington and Dr. Robert Bakker write that muscles
only work optimally in a certain range of temperatures. Once you get
outside of that range on the low side, problems occur. Indeed, I had
problems typing the first part of this message as I had just come in from
outside and couldn't get my fingers to move properly. Bipedalism requires
fine muscle control ... it has often been likened to living constantly on
the edge of disaster. The obligate bipeds must have been able to maintain a
body temperature within a certain range in order not to flop over. In other
words, they had to be homeothermic.
Now, homeothermy does not require endothermy. HOWEVER.....
We now turn to more of that nagging evidence. There were polar
dinosaurs. The poles at that time were not as cold as today, but did see
frost. They were in darkness 6 months of the year. HOW did they keep their
body temperatures in that range? Basking on rocks? Nope, darkness half the
year. Perhaps they migrated? Northern dinosaurs could, southern couldn't,
at least not in the Cretaceous. Perhaps they found a unique way to solve
this problem, neither traditional ectothermy nor endothermy. However, if
you use modern animals as you wish to, then the question is how many
ectotherms live in regions that are around freezing much of the year, and
how many endotherms? Polar living, especially combined with obligate
bipedality, certainly suggests endothermy in at least the polar dinosaurs.
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