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

Re: pterosaur femora sprawl

MHabib wrote:

> Absolutely, molecular trees have their own problems.  I'm just 
> cautioning against firm statements where they are not yet warranted - 
> you originally stated that civets and bats are close relatives, as if 
> it were a certain relationship.  What you meant, of course, is that 
> your tree places them there, specifically.  You might be right on 
> target, but I'd also suggest being more open to the uncertainty of the 
> situation.  That's just par the course for phylogenetics.

Tested against _all_ prior contenders, civets/Ptilocercus/and a fossil rostrum 
(can't recall name) come closest, so the uncertainty is reduced. If you saw 
what I saw in the skeletons, you'd be on board.
> > If you're using your hands pressed against the substrate to locomote, 
> > your hands have no opportunity to become oversized, foldable flying 
> > organs.
> Why is that?  After all, the rules of macroevolutionary trends are not 
> the same as individual daily-use.  Seeing as how bats and pterosaurs 
> have/had dual-use forelimbs, how can we be certain that the limbs 
> didn't evolve that way in the first place?

"Why" questions are difficult to answer. But the dual use of pterosaur 
forelimbs was secondary, as demonstrated by Sharovipteryx and Longisquama. The 
dual use of bird forelimbs (think Archaeopteryx and Hoatzin) was secondary as 
demonstrated by Coelophysis and kin. With bat origins and how they stopped 
using their forelimbs clouded in mystery,I don't know why or how they suddenly 
stopped using their forelimbs to locomote, but instead developed the ability to 
hyperflex their metacarpals. Animals generally don't stop locomoting palms down 
without going through some sort of intervening phase. Apes were suspending 
themselves from branches before becoming knuckle-walkers, so that's in the same 
league. Chalicotheres and anteaters, also knuckle walkers, may have just one 
day decided to "save their nails."  I don't know about how they changed 
patterns. Were some small and arboreal?
> >> True, a small pterosaur could facultatively launch bipedally.  
> >> However,
> >> I see no reason to expect that this was their normal mode of launch.
> >
> > If primitive pterosaurs were small and launched bipedally, that, by 
> > definition is the normal mode. Anything deviating from that norm is 
> > derived.
> Ah, but I said that they could *facultatively* launch bipedally.  That 
> is, quad launch was probably the norm for small, primitive pterosaurs, 
> but they were not limited to using only a quad launch simply because 
> their required launch speeds were not very high.  I was referring to 
> the "normal" mode of launch in the sense of day to day activity.  That 
> is, a given small-bodied, early pterosaur would normally launch with 
> all four limbs, but might sometimes use only the hindlimbs if need be.

Until I see an example of the kinematics of quadrupedal launching from a 
horizontal surface, I can't envision it. Not when pterosaur ancestors and the 
first pterosaurs were incapable of touching the ground with their forelimbs 
(while balancing glenoids over toes) -- and they had big thighs with a pelvis 
1/3 the torso length.

> Yes pterosaurs have reasonably large thigh muscles, but they have much 
> larger muscles associated with the forelimb and pectoral girdle.  The 
> bone strengths are also vastly different in the forelimb and hindlimb, 
> which further supports this trend.  The ratio of bone strength between 
> the humerus and femur (strength in bending, that is) for Anhanguera is 
> 56 times the same ratio for an albatross.  If pterosaurs were bipedal 
> launchers, like birds, then the ratio should actually be smaller in 
> Anhanguera (given it is larger than an albatross).  Remember, the 
> hindlimbs are still involved in the quad launch, they just generate 
> less power than the forelimbs, and the forelimbs leave the ground last.

Certainly Anhanguera had a different launch mechanism, with the largest 
forelimbs and smallest hind limbs and feet in pterosaurland. But you're 
bringing up a _very_derived taxon. This is a taxon that, like an albatross, 
rarely landed. Not sure (can't envision) that a quadrupedal launch would work 
here either. A stiff breeze might have been all that was necessary. And for 
that: a bipedal platform.
> Also, vampires have reasonably large thighs and strong femora relative 
> to other bats.  Their femora are not the strongest among bats, which 
> indicates that hindlimb strength is not the only critical factor in 
> terrestrial mobility for bats (see Riskin), but several studies 
> (including my own dataset) show that vampires actually have elevated 
> strength in the femora as well as their humeri.  However, the humeri 
> are still much stronger than the femora - pterosaurs match this trend 
> (they have good, strong hindlimbs, but much stronger forelimbs).
> In terms of parsimony, quad launching does not require pterosaurs to 
> change gaits for launch - we know they walked quadrupedally, 

The beachcombing taxa, yes. No prints yet for the soarers, skimmers, 
insect-eaters, basal forms.

> so switching to a bipedal launch dynamic is actually less parimonious. 

False paradigm based on beachcoming taxa: those with short fingers, small 
unguals, plantigrade pedes, small fifth toes. Not basal. Not the majority.

> Phylogenetically, it's still uncertain - there might have been an early 
> shift to quad walking (and, as Jim and I suggest, quad launching), 
> assuming the ancestors were bipeds, or quad gaits might be basal for 
> the group.  

Why do you take the odd conviction that quad gaits were basal for the group 
when the best candidates for pterosaur outgroups were bipedal (Sharovipteryx, 
basal dinosaurs, Scleromochlus)? That's an irony that you quad guys are 

Either way, it's a single, fairly simple shift on the tree, 
> so phylogenetic parsimony is probably not the best way to judge the 
> likelihood of the proposed launch system.  

Never discount phylogenetics. It's the key you're trying to ignore because it 
doesn't fit your paradigm. Shake off all prejudices and let the phylogenetic 
evidence guide you. It's real. It's not made up by engineers.

It's really a mechanics 
> problem - we can detect what the animal's were capable of (and their 
> likely locomotor behavior) using structural mechanics.  Of course, we 
> need the phylogenies to determine things like timing and convergence.

I agree that mechanics can give answers, but workers who have provided us with 
robo-pterosaurs have not included options exploring other possibilities (and in 
the rare instances where that has happened, the options were sabotaged with 
poor mirroring of nature).
> >> Nyctosaurus too?
> I suspect that Nyctosaurus quad launched, yes, though with rather less 
> power and speed than critters like azhdarchids (which had power-house 
> builds).

Just as in the pteroid orientation question, whatever you come up with has to 
work (albeit modified) for all pterosaurs. I brought up short-legged, 
long-handed Nyctosaurus because it would seem it was operating with oversize 
"crutches" that would not have been able to be angled at a posterior vector for 
a quad launch that sent the pterosaur forward. 
> > Please send some drawings whenever you can. Jim has had the same 
> > request for awhile. And what would it take to get you to be convinced 
> > that there are no known closer relatives to pterosaurs than 
> > Sharovipteryx and kin?
> There will be illustrations made available.  In terms of pterosaur 
> sister-taxa, I'd be convinced by the typical means - if the proposed 
> sister group (Sharovipteryx and allies) repeatedly falls out next to 
> pterosaurs within the analyses of various authors using a variety of 
> character sets.  Quantitative support systems also help, of course.  
> And, of course, the characters in question can make a difference, but 
> this can be very group specific - synapomorphies like feathers can be 
> unusually compelling, for example.  Hard to say what this would be for 
> pterosaurs, but it's a secondary issue (I don't require any sort of 
> silver bullet, nor expect one).

Silver bullets: Pedal digit V. Sternal complex. Aktinofibrils. Antorbital 
fenestra without a fossa. Large manual digit IV, short V. Extended ilium. 
Prepubis. Chevrons parallel to centra. All these are found nowhere else (except 
some characters are found in Proterosuchus, easily convergent).
> > Bats? or Pterosaurs? In either case I've used heuristic and bootstrap 
> > algorithms with high 90s and 80s except where taxa are known by skulls 
> > only or without skulls. I employ various decay analyses, deleting 
> > taxa, characters, randomly, non randomly all with confirming results.
> Sounds great; I look forward to the upcoming publication(s).  Out of 
> curiosity, what are your strongest and weakest clades in terms of 
> bootstrap and decay analyses?

Sorry, answering from the office today. But I can tell you, no major clades are 
weak. Only individual sister taxa, such as Elanodactylus w/Eopteranodon, 
Eoazhdarcho (all incomplete) break down into polytomies under initial decay.
> >  Unlike prior workers, I get a single tree. One in which sisters look 
> > alike. Speaking of which, no prior pterosaur analyses discuss the 
> > issues you raise.
> Finding a single tree is convenient, but actually doesn't say much 
> about accuracy one way or the other.  Sisters should "look alike" from 
> a character standpoint by definition - I assume you mean in a broad, 
> qualitative sense.  That's interesting, as it suggests low levels of 
> predicted convergence, but it also doesn't say much about accuracy, per 
> se.  It could mean, for example, that some homoplasy is being missed.

Finding a single tree in a cladogram with 25,000 matrix boxes indicates that 
lots of possibilities were looked at and only one solution was indicated. BTW, 
convergence occurs in 90% of my characters and often more than twice. Thank 
goodness for computers.

Spirited discussion. 


> Cheers,
> --Mike
> Michael Habib, M.S.
> PhD. Candidate
> Center for Functional Anatomy and Evolution
> Johns Hopkins School of Medicine
> 1830 E. Monument Street
> Baltimore, MD 21205
> (443) 280 0181
> habib@jhmi.edu