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

Re: Great blue heron - Pterosaurs




don ohmes wrote:

>  Is the fossil material good enough to determine if sexual dimorphism
> exists in ptero's/ptero
> crests?

In some, it seems to be compatible with dimorphism.  I don't think it is
proven yet, but I personally buy into it.

> Or a large skim-fishing ptero might find an inertial counterweight
> useful.............

It could, but the crests generally don't have enough inertia to be very
significant in that regard.

> I'd be real surprised if that's a new idea, ..... Has it been
> discounted or rejected?

I mentioned it a few years back.  I doubt that I was the first either.
I don't think it has been rejected by consensus.

> Or discussed?

Not in detail, nor quantified.  Again, the inertia generally isn't
substantial in comparison to the rest of the skull.

> ...my usual default explanation for bizarre protuberances,
> thermoregulatory function.

Thermoregulatory function is certainly a possibility.  However with
normal atmospheric temperature drop as altitude increases, heat loss
would be a significant problem when flying at altitudes greater than a
couple of thousand feet, so an ability to shut down much of the blood
flow to the crest would probably be mandatory.  When flying at altitude,
retaining heat is the problem; not losing it.

> Aerodynamic advantage-- How would that work? Were crests really large
> enough to generate useful
> aerodynamic force at the slow speeds involved in surface effect
> soaring (I'm thinking Great Albatross
> here)?

Yes, they were.  Some of the larger crests had as much surface area as
the same animal's wing semispan (50% of the total wing area).  So the
crest aerodynamic forces could be about half that of the wings if
generating similar lift coefficients.  Equal to the wings if generating
twice the lift coefficient (possible under some sailing scenarios).  The
speeds involved in surface effect soaring aren't slow; they are normal
flight speeds.  It is the speed made good across the water that is slow,
not the speed made good through the apparent wind.  The sailglide
skimming technique works best when sailing close to the wind (upwind)
and in broad reaches (across the wind).  It is not effective downwind.
And Wandering albatrosses aren't slow either.  They are one of the most
heavily loaded of all birds, consequently quite fast, and fly at similar
lift coefficients to pterosaurs with similar aspect ratios (optimal lift
coefficient is related to aspect ratio).

> Of course, big as they were, they may have had difficulty banking or
> using their wings for turning
> when close to the surface.

No more so than an albatross.  And albatrosses will bank 90 degrees and
more to the horizontal when lee shear soaring at wavetop height.

> Sort of a front rudder or canard-type effect?

They tend to minimize that effect with increasing neck length, by making
the crest smaller.  Big lever arm leads to small crest.  Normally crests
are very destabilizing, like putting the feathers at the wrong end of an
arrow.  At altitude, the animals would have needed to spend more time
minimizing the side forces generated by the crest during flight than
than exploiting them.  That said, some animals that may have had
extraordinarily large crests (Thalassodromeus and perhaps the KJ1 & KJ2
specimens of Nyctosaurus species -- if that bony crest supported a
membrane) would also have carried them in positions where the center of
lift of the crest would have been far enough aft compared to the center
of lift of the wings to be stabilizing instead of destabilizing.  But
not acting as a front rudder, per se.

Jim