erect posture and its implications

["Colette H. Adams" <cadams@hh.gpz.org>]

I reiterate that terrestrial ectotherms walk at ectothermic speeds and
terrestrial endotherms walk at endothermic speeds.  If a sauropod doesn't
walk at <1 kph, why should we think it's an ectotherm?  And if it doesn't
walk that slowly, WHY NOT?  The question demands an answer.  I am not
impressed by answers such as Ruben et al.'s that the trackway evidence is
"at best, equivocal."  To give such a vast body of important evidence such
a nonchalant dismissal betrays a severe lack of objectivity.

I reiterate that every correlation implies a causal link.  If one gives the
matter a little serious consideration, rather than dismissing it out of
hand (having had the mantra drilled into us by our professors as if it were
some religious doctrine), one will realize that variables do not simply
correlate out of thin air.  The causal link may end up being of no great
interest and quite unexpected.  But two variables covary for a reason.  The
matter can be made clearer by realizing that even in a controlled
experiment, it is a correlation that is of interest.  The difference here
is that all variables are held constant (or random) except the ones of
interest.  This eliminates all other causal links, direct and indirect, and
forces a direct expression of the effect of one variable on another.  The
result is either a correlation or no, the presence of which is evidence
that A causes B.  That is all causation is; B consistently follows A, all
other things being equal.  The absence of a correlation in such an
experiment only means that direct causation is eliminated.  Indirect
effects are still possible.  These hypothetical variables that correlate
yet have nothing to do with each other simply do not exist in reality.  If
anyone can cite me just one example I will be only too happy to eat my words.

I am indeed hypothesizing that a fully erect posture necessitates a higher
walking speed than can be sustained by an ectothermic metabolism.  I agree
wholeheartedly with Greg's "pendulum" biomechanical remarks.  Therefore, a
fully erect posture "forces" endothermy, according to my view.  But I would
not be surprised if the causal link were less direct than this.  It may be
that a fully erect posture, an endothermic metabolic rate, a high-surface
area respiratory system, a high performance cardiovascular system, and
physiological temperature regulation all tend to evolve simultaneously in
response to selection for increased aerobic capacity.  A partially erect
posture and 4-chambered heart may preadapt an animal for this.

It is interesting to see how different people have slightly different views
of what should be considered "fully erect" and how this applies to
dinosaurs.  To me it reflects the fact that we are really picking nits (not
that I have anything against the fine details, they are interesting too).
When it comes to dinosaurs, the posture issue in my mind was more or less
resolved a hundred years ago.  The fact is that as a very good
generalization, dinosaurs were fully erect.  

I should point out that of course dinosaurs would have demonstrated some
degree of variation within the endothermic theme.  As with living
endotherms, metabolic rate probably varied as a function of diet and other
factors.  Some species may have had tenrec-like temperature regulation
while resting at warm temperatures.  The presence of growth rings in some
dinosaur bone suggests that some species may have had winter torpor states
or even an ectothermic type of metabolic response below certain
temperatures.  All of this is nevertheless within the overarching theme of
endothermy, just as the ability of pythons to elevate their temperature
above ambient for months at a time is within the theme of ectothermy.  

The suggestion that ectothermic resting metabolic rates reflect a common
cellular machinery and universal selection for energy efficiency is
interesting, and I believe this is pretty much on the mark.  The question
remains why is there this tremendous gap in metabolic rates between extant
endotherms and ectotherms, and why birds and mammals have such similar rates.

Best regards,

Dave