Re: Ornithodira, breathing with long necks

["David Marjanovic" <david.marjanovic@gmx.at>]

>    The clade Ornithodira is characterized by an s-shaped neck.    How did
they breath with the additional air way dead space?  The longer neck seems
to imply that they would have had to compensate for the additional used air
trapped in their throats while breathing.  Obviously, ostriches, camels, and
giraffes can do it.  I however, put a snorkel on and I have to reduce my
activity level.  If the snorkel is too long, I cannot even clear water from
it, let alone breath fresh air.

You can decrease the diameter of the trachea for reducing dead space.
Giraffes do it. However, I can't imagine *Sauroposeidon* or even
*Apatosaurus* doing likewise.
Several SVP abstracts of the last 3 meetings say that in many dinosaurs the
energy costs of breathing surpassed the energy production of metabolism,
means that those dinosaurs couldn't breathe, at least not through their
nostrils, because of the dead space in the long trachea.

>    As an engineer, I would design an air exchange device as follows.
[...]
> 3.  Adjust temperature, moisture for optimal gas exchange (moist
surfaces).

Adjust temperature? Seems to call for homeothermy?

> 7.  Moisture and heat recovery. (Moist surfaces)

Respiratory turbinates or analogs? Are said to be a prerequisite for both
homeo- and endothermy...

>    They can not warm cold air into their lungs (unless warm themselves
–

Computer incompatibility?

> warm water absorbs more gas. Does it improve gas exchange?).

I was taught gases are better soluble in cold water...

>    Birds [...] with long necks still have extra dead air they must breathe
again.  The relatively small lung is compensated by the counter current air
exchange.  Air sacs in bones are possibly a lightening feature that
prevented hollow bone collapse during flight due to pressure differentials.
Bird bones needed to be hollow for the most bending resistance to weight (I
can expand if needed).

At the SVP meeting 2000 it was proposed that sauropods held their necks
stiff not with long, superstrong ligaments (no evidence for these can be
found), but with air sacs between and in the vertebrae, just like birds do
it. The abstract sounds very convincing.

>    The bipedal gait combined with the long neck IMHO probably went with
the flow through (one way) lung.

This is AFAIK the only solution proposed so far that can explain why
sauropods were able to evolve long necks in the first place -- the air sac
system is a synapomorphy of Ornithodira, and lung + air sac volume was big
enough to allow the evolution of 15-m-long tracheae.

Is there any idea as to how air sacs evolved? As the strangest form of lung
cancer ever?

>    When combined with a four-chamber heart, flight and aerobic hunting
could possibly have occurred before homothermy.

A large, powerful heart requires endo- and hom(o)eothermy.

>    The additional steps towards homothermy in theropods and Pterosaurs
could have included:
> 1.  Heat released during elevated exertion levels would allow exertion
after external air started cooling.
> 2.  Insulation on extremities would extend the late exertion time by
reducing heat losses in early evenings.  This insulation on arms/wings could
be wrapped around the non-insulated body surfaces and eggs at the end of the
day to help retain heat, increasing growth time.

If you say that the present knowledge of the fossil record is not enough to
test this, I probably have to agree; however, all known feathered dinosaurs
have (proto)feathers all over their bodies, and at present pure parsimony
says that wing feathers arose after short proto-/contour feathers. I'd also
say that it's easier to evolve short feathers all over the body than long
ones (short ones would have no effect) just on the arms. Simpler
explanations for the evolution of wings, and IMHO even for that of feathers,
have been proposed onlist, among other places.

> 3.  Non-insulated surfaces could still be exposed to sunlight to warm up
the body.

Do bats do this? (AFAIK bats are considered heterothermic like hummingbirds,
which means they fall into low-temperature torpor when they sleep.)

> 4.  Increased metabolism, especially when small or young would push for
more insulation.

Again, the fossil record is patchy, but all known basal ornithodirans are
very small when adult. (Not as small as contemporary basal mammals though.)

> 5.  Freed hands due to the bipedal gait, with skin or feathers, would lead
to flight.

?!?
Freed hands may facilitate the evolution of flight, even though bats never
were bipedal as far as is known... On the other hand, many bipedal animals
(kangoroos? humans? *Oreopithecus*?) have never evolved flight (but reasons
are easy to find here). For pterosaurs, the debate whether they had bi- or
quadrupedal ancestors continues (I favor the former).

> Small size for flight would push for a higher metabolism [...]

Small size may facilitate or even be a prerequisite for the evolution of
endothermy, and it surely facilitates the evolution of flight, while flight
in animals above insect size seems to require endothermy (some insects are
endothermic...), but the above sentence looks a bit jumbled to me.

> 8.  A rapid metabolism, would allow flying animals to grow fast.  Their
need for light-weight would work to prevent continuous growth.

Does anyone know where in or before the bird lineage determinate growth
evolved?

>    Herbivores that did not become armored or large, would feel
evolutionary pressure for a higher metabolism (not necessarily homothermy)
and aerobic exertion levels from the theropods.

... or would benefit from endo- and homeothermy (including aerobic exertion
levels) inherited from ancestral ornithodirans? *Lesothosaurus* doesn't look
less warm-blooded to me than *Hypsilophodon*, say (Disclaimer: I don't know
histological studies on these or comparable dinosaurs).

>    Sauropods became so large that, except while young, a high metabolism
would not be needed (but may have occurred in the ancestral or youth
stages).  However, they would need exertion levels adequate to move to new
feeding areas and for protection from theropods.  I wonder if their necks
brought air in the throats in one tube and out the neck air sacs back to the
throat near the neck to prevent breathing as much used air.

HP Gregory S. Paul is onlist, so if he hasn't already done this today (my
mail server is overloaded), he will surely tear any argument that sauropods
didn't need high metabolism to very, very small pieces. I'm sure he has done
this years ago onlist, and anyway he has published a paper in the Dinofest
1994 volume with Guy D. Leahy:
Terramegathermy in the Time of the Titans: Restoring the Metabolics of
Colossal Dinosaurs, p. 177 -- 198.

I hope I didn't sound too annoying!

David,
getting withdrawal symptoms from waiting for the "Notheonychus" paper :-)