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Re: [Re: Insulation does not = "Warm-blooded"]
"David Marjanovic" <email@example.com> wrote:
> > I believe that L.C. endothermy didn't show up in birds until the
> > enantiornithines (i.e. it probably showed up in some > >
enantiornithines). I don't think all enantiornithines were L.C. > >
endotherms though. Of course since enantiornithines were the sister > > group
to neornithines it doesn't help much with extant avians. I do > > think all
Neornithines were L.C. endotherms, but I don't know enough > > about them to
say much more. I will say, though, that I don't believe > > that
archaeopteryigiformes like _Archaeopteryx_ and _Rahonavis_ were > > L.C.
endotherms. Everything after them gets confusing.
> You believe, you think... why?
I base it, right now, off of two controversial pieces of evidence and one
piece of evidence that I have yet to hear anyone use.
1) Respiratory Turbinates are not found in any birds prior to 70 mya.
Personally I don't think RTs are all that important and we already have
evidence that L.C. endotherms exists without RTs. In fact it wouldn't surprise
me at all if we found a few non L.C. endotherms with RTs as well.
2) Enantiornithines and non-neornithines have LAGs in their bones. This holds
less water for me with each new study on bone histology that I've seen. Right
now these bones only seem to indicate degree of activity and not L.C.
3) _Archaeopteryx_ and _Rahonavis_ (do we know of any other
archaeopterygiformes?) are all long bodied animals. Long bodies are a problem
for L.C. endotherms because there is that much more surface area to allow for
heat escape. That is why most mammals and all birds are short bodied. The only
exception to this rule is in weasels (and otters, which I think are weasel
relatives) and they probably have higher metabolisms to handle the extra
length. Meanwhile we have plenty of long bodied non-L.C. endotherms. Without a
complete reliance on internal power for heat production, longer bodies are
possible. Of course we also have short bodied ectotherms as well, so it is
more like one sided evidence (i.e. usually only non-L.C. endotherms can
support long bodies).
Anyhoo, by themselves they aren't that supportive, but altogether they make
for a stronger argument.
> > > Does bradymetabolism imply low rates of
> > > activity, stereotypically "reptilian" activity levels (notice I said
> > > "stereotypically")?
> > You mean like sprawling, swamp dwelling sauropods and scavenging
> > theropods? I think that in popular imagination it does, but in > >
reality a bradymetabolism doesn't hinder any of the proposed > >
lifestyles of dinosaurs, so I don't see why the stereotypical > >
reptilian activity levels should ever come into play.
> Why doesn't it hinder any of the proposed lifestyles of dinosaurs, such > as
migrating from Alberta to Alaska and back, galloping at 50 km/h, > running
bipedally at even higher speeds, ... according to P&L even > growing to
over 1 tonne in the first place?
Because we have modern and/or fossil examples of bradymetabolic (or probably
bradymetabolic) creatures that do these things. Going all the way from the
Galapagos lowlands up a mountain and into the caldera of an extinct volcano is
quite a feat for a lizard that is only 4ft long. So is going from the volcanic
highlands to the flooded lowlands for food when one is a 550 lb tortoise.
_Crocodylus johnstonii_ has been clocked at galloping speeds of 17 km/hr and
they grow to only 10-12ft. That's pretty fast for an animal that isn't even
built for this type of lifestyle. Just imagine how fast a pristichampsid could
have been capable of?
Bipedalism (both facultative and obligate) has been observed in dozens of
lizard species, and we have non-dinosaurian fossil reptiles that were bipedal
As for the 1 tonne thing, that all depends on your measurements. As I
mentioned before, _Testudo atlas_ has been estimated at 4 1/2 tonnes.
HP Tom Holtz brought up a good point in a separate post in this thread that
bodyplan might have an effect on size limit. It is quite possible that the
only reason why varanids didn't exceed one tonne was because sprawling limbs
can only hold so much weight. This could certainly explain why crocodylians,
with their variable gait, grow and grew to huge sizes (both in and out of the
water). It also might explain why tortoises can get so big, since they tend to
have "more erect" stances than most lizards (even Komodos hold their legs a
little more under than other lizards).
Large size does seem to be a good reason for erect stance selection. Certainly
better than any others that I've heard of.
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