[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]


The discussion on dinosaur energetics becomes more interesting. 

Jones asserts that aerodynamics require tachymetabolic animals to breath
through broad nasal passages. This is one of those good ideas that, as
sensible as it seems in theory, those darn animals refuse to live by.
Literally. (Analogy, it makes no physical sense for giants to live on land
rather than in the water, so in theory sauropods should have been aquatic.
But not only were those silly sauropods primarily terrestrial, they were the
most successful land megaherbivores ever, having lasted ~130 million years.)
Jones, and others in their comments, seem to think that I have a problem with
the statistical analysis  involved in the supposed four fold difference
between main nasal passage diameter in endotherms versus ectotherms, but this
is not really so. The primary problem with Ruben et al using so few data
points (measured along just one part of the nasal passage that contains
nonolfactory conchae) is that they failed to capture the full variation of
nasal passage cross-sectional area in endotherms, which is in fact very
extreme. The assertion by Jones that "look at an endotherm and ectotherm of
the same mass--you'll see that the endotherm always has a large nasal
passage" - and that this must be so because of the laws of physics - is an
unsubstantiated statement of faith that stems from his failure to measure
enough bird and mammals to find out otherwise! Or, how would he know without
sampling a largfe numbers of animals to see if the hypothesis holds up? I
have sampled enough animals to show that it does not. (As for Jones going on
about the need to scale nasal passage size in relation to body mass, of
course I did that no brainer.) It is a fact that at least one bird and at
least one mammal have very long, EXTREMELY slender nasal tubes that plot well
below the reptile line calculated by Ruben et al. (Let's have some fun all
you dinofans. Guess what these two animals are. Hint. You have very probably
seen both of them at the zoo, although one is somewhat rare. Be the first and
win a fabulous trip to Bora Bora, accomodations provided*.) Other birds have
conchae containing anterior nasal passages no broader than those of theropods
and reptiles. Jones implies that I have to explain how tachymetablolic
animals can breath through such remarkably slender nasal passages. Not so,
the mere existence of such creatures proves that it works. (Nor am I sure
whether I can directly address in my paper Jone's argument about Poiseuille's
law as applied to nasal passages, in that his hypothesis has not yet been
published.) Besides, some birds have entirely closed nasal passages, and they
do not suffocate either. 

There are lots of 3-D preserved ceratopsid skulls, the notion that ALL of
them have been badly prepared internally seems as convenient as it is hard to
swallow. They have huge nostrils, entering into a very big snout that
contains a capacious nasal cavity that seems to be unobstructed by extensive
sinuses. MAYBE these passages were filled with soft tissues, but we have not
the means to tell.

The assessment by Jones of the diagnostic value of RT versus limb posture,
ilium length etc is rather skewed. He asserts that RT are causally linked to
metabolic rates. However, RT have nothing to do with the production of
metabolism, nor are they the direct result of power production (as are
walking speeds recorded by trackways). At best they are a mere secondary
adaptation for coping with the consequences of having a high resting
metabolism. However, even that link is tenuous - as has been noted by Nagy,
and as proven by primates (including savanna baboons, as well as people) that
have poorly developed RT. 

In contrast, the size of the ilium strongly influences the size of the thigh
musculature, which is one of the primary centers of aerobic power production
in locomoting tetrapods. The longer the ilium, the larger the thigh muscles,
the more oxygen burned by the muscles. Now THERE is a straightforward causal
link between metabolism and a skeletal element that is frequently preserved
(unlike those oh so delicate and often cartilagenous RT, which are virtually
never preserved in fossils). 

As for limb posture, I only claim that fully erect legs are diagnostic for
aerobic capacity above the reptile level. In doing so I do not assert that
erect limbs are REQUIRED for endothermy. In principle an endotherm can have
any leg type. However, erect legs probably FORCE the elevation of aerobic
exercise capacity via the high energy expensive walking speeds they tend to
create. Another causal link. If so, then erect legs compel at certain
metabolic minimum, not the reverse. Fully sprawling legs may also be
diagnostic for low metabolic rate (am not sure whether there are any truly
sprawling land mammals). Semi-erect legs lack diagnostic value because both
brady- and tachyaerobic animals have them (including some fairly large Early
Paleogene mammals). Jones said twice that there are bradyaerobic animals with
erect legs, but I am not aware of any living today. Jones also tries to use
early, erect legged crocodilians as examples of bradyaerobes with erect
legs. However, animals can be used to test a metabolic character only if their
metabolic status is unambiguous, and Jones cannot show that the extinct
gracile crocodilians definitely were bradyaerobic. In fact, some lines of
evidence suggest early crocs were more aerobically capable than modern
reptiles (albeit probably not by much). In this view modern crocodilians have
reverted to full bradymetabolism as a consequence of becoming aquatic ambush
predators where not having to breath and being able to go without food for
long periods is a good thing (manatees also have much lower metabolic rates
than their terrestrial ancestors). This may explain why modern crocodilians
have unusually sophisticated respiro-circulatory systems despite having lower
aerobic capacity than some lizards.

The interesting comments by Jones concerning activity levels in komodo
monitors versus dinosaurs seem to reveal a failure to appreciate how very low
the activity potential of reptiles inherently is. As I mentioned in my recent
posting on oras, watching videos of the big herps is usually like watching
motion pictures of predatory mammals slowed down two or three fold. The
suggestion by Jones that dinosaurs could have been energy efficient (I
therefore presume he means they had the pathetically low aerobic exercise
capacity characteristic of reptiles too) ectothermic homeotherms, yet more
active than lions, is so implausible that it boggles the mind of those who
understand the principles of biothermodynamics. It is true that lions sleep a
lot, but when they are up and about doing what lions do, their high aerobic
exercise capacity allows them to regularly walk and trot long distances at
speeds many times higher than any reptile can sustain. As I have discussed
earlier, trackways show that predatory dinosaurs walked as fast as lions. And
tigers. And bears. 

Oh, my theory is that dinosaurs therefore must have had elevated aerobic
capacity. Because moving about at high walking speeds and otherwise being
highly active burns lots of oxygen, they could not have been as energy
efficient as reptiles. This is especially true if vertebrates with high
aerobic capacity must also have high resting metabolic rates for mind numbing
and poorly understood anyway reasons we shall not go into here (the Aerobic
Capacity Hypothesis proposed by Bennett & Ruben). Jones seems to subscribe to
notion that dinosaurs fit what I call the "ideal fantasy land vertebrate"
(IFLV), the one that can be both energy efficient, yet highly active at the
same time. If so, why are all energy efficient tetrapods living today unable
to sustain high levels of activity, and why are all highly active tetrapods
energy inefficient? Because being active requires the use of energy, and
being efficient requires the NON use of energy. You cannot have your
metabolic cake and eat it too.   

Oras are not, as Jones implies, examples of truly "successful" reptilian
predators. They are limited to a few islands that are free of large mammalian
predators (the DiscChann Fangs program noted that oras may have evolved to
hunt the dwarf elephants that lived on the islands til fairly recently. Way
cool! Nowadays they have to make do with deer, oxen, and the odd kid. How sad
- for the oras that is.) Its bigger Australian relative likewise evolved on
an isolated continent with few big mammalian predators. Nor was the
super-monitor Megalania particularly successful, judging by the scarcity of
its remains. 

It is interesting how some whom disfavor dinosaurs being energetically very
nonreptilian seem to be in a tizzy that others could have the audacity to
think otherwise, as though we have formed some sort of cult. That is of
course the most absurd idea tha.................OK, we admit it! Those of us
who worship the Infinitely Cool Gods of Dinosaur Endothermy & Tachyaerobiosis
demand that all others conform to our paleotheology regardless of the
rational evidence for or against! Those sinners who do not believe shall be
sent (via our miraculous time machine) back to Paleozoic Hell, when those
slimy ectotherms slithered through ancient muds. Those who believe, Believe,
BELIEVE, please send your generous checks to moi, so that we may continue our
good works. 

All hail the late great Carl Sagan.


(*Proverbial small print: Prize awarded only if GSP wins the multi-million MD
state lottery. Unlikely since GSP never plays same.)