Re: Pterosaur size (Was: Great in the air, not so good underwater)

[Michael Habib <mhabib5@jhmi.edu>]

> Modern birds nowhere near that size have a lot of trouble.
>
> -----This is true.  It appears to be related to two things., the first 
> being the launch techniques available to birds, and the second being 
> that membrane wings are capable of maximum steady-state lift 
> coefficients about a third greater than the maximum that birds achieve 
> (bat wings do not do quite as well as birds).

There is one more bit I would add in which is to consider the ecology 
of many of the largest birds.  Because birds are very good cursors, it 
is not uncommon for birds to sacrifice some flight abilities for 
terrestrial performance.  Several of the cursorial taxa fly very well, 
but others have reduced take-off rates which we see as "having 
trouble".  This is why, for example, using bustards as a metric of near 
maximum sizes for volancy in birds is problematic.  At the other 
extreme, some birds have jettisoned hind limb function for particular 
selective flight advantages; because the hind limbs are the primary 
launch power in birds, these species often need gravity assistance to 
launch.  Such taxa (like Phaethon, Fregata, Apus, and Chordeiles) are 
clumsy on the ground but this has nothing to do with scaling (in fact, 
Apus and Chordeiles are quite small).

> I think the mega-volants needed different conditions to find giantism 
> advantageous,
>
> ------This may be true.

I agree this may be true; I also think such conditions may be more 
widespread than we give credit for.  Mega-volants persisted for much of 
the Cretaceous, saw a short hiatus at the K/T (or K/Pg) extinction due 
to the loss of pterosaurs, and then re-emerged as a morphotype amongst 
birds via pseudodontorns in the Paleocene.  Pseudodontorns then did 
well world wide until the Late Pliocene, so we've really only been 
without some kind of mega-volant for a short period of time.  Given the 
advantages of large size for pelagic, soaring-adapted species 
(especially dynamic soarers), this isn't really surprising.  It appears 
that pterosaurs ended up huge more regularly than birds (and became 
much large-bodied overall) because their membranous wings and launch 
system made large size especially viable.

> Here we have in chronological order the flying vertebrates; pteros
> (biggest)=> birds (smaller)=> and bats (smallest). Bats are the
> most recent, and (correct me if I am wrong) the largest known bats are
> extant.

The fact that the largest known bats are extant is probably an 
artifact.  The largest microbats (Megadermatids, Rhinolophids) have 
been around for a very long time.  The modern genus Megaderma may be go 
back to 25 mya (Megaderma lopezae, see Sevilla, 1990).  Other 
megadermatids are known from 36-37 mya, and are not small, either.  
Rhinolophus, another modern genus, probably dates to 41 mya (and again, 
is roughly in the same size range at that time).  What we're missing is 
a good megabat record, but ghost lineages demonstrate that they must be 
relatively old.  Recent phylogenies tend to either recover a basal 
split (thereby placing the first pteropids in the Paleocene to early 
Eocene), or they recover pteropids within Rhinolophoids (placing their 
origin in the middle to late Eocene).  The older pteropids could be 
smaller, but they could just as easily have been larger.  We don't 
really have enough material to say.  As far as I know, the indet. 
pteropids from La Colombiere are roughly within the modern megabat size 
range (at least for the known elements).  Propotto is only a scrap, but 
it was not tiny, either (nor huge).  Archaeopteropus transiens is 
reasonably small, if I recall correctly, but it's probably not a 
megabat, as it turns out.

> Like I say, this is suggestive, given the consistency of the 
> terrestrial record and the contrast with the aquatic record.
>
> -----Not really.  It can be more parsimoniously related to differences 
> in wing mechanics between the three.

Agreed.  In fact, if one looks at a combination of the wing mechanics 
and ancestry of each group (which are interconnected factors), the 
relative sizes and dominant morphotypes of all three groups of 
vertebrate flyers is rather predictable (though hindsight is 20/20).  
There are a few morphs that one might expect to see from bats that we 
haven't, but time constraints and competitive exclusion probably play a 
role.

Cheers,

--Mike H.