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Re: pterosaur femora sprawl



"Why" questions are difficult to answer. But the dual use of pterosaur forelimbs was secondary, as demonstrated by Sharovipteryx and Longisquama.

Assuming those are the outgroups, then yes, it could be that quad gait is secondary in pterosaurs. However, it would be a secondary gait that shows up right at the basal node.


The dual use of bird forelimbs (think Archaeopteryx and Hoatzin) was secondary as demonstrated by Coelophysis and kin.

Hoatzins are derived neornithine birds, so they're not really informative for the basal braches of Aves (though they are very interesting functionally). Archaeopteryx doesn't actually tell us the ordering of the forelimb functional characters - dual use in feeding and climbing may have predated use in flight. Or, the forelimbs might have been feeding/flying implements first, and climbing was rather unimportant (or became useful later). We don't really know. In all likelihood, the forelimbs of basal birds were, as you suggest, secondarily dual-use as least in terms of flight. However, birds may be the exception rather than the rule.


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.

I don't know of any pterosaurs that were incapable of touching the ground with their forelimbs. Not a single pterosaur specimen has longer hindlimbs than the walking portion of the forelimb to the best of my knowledge. Even if we prefer the walking position that you're describing (and I prefer a slight different one, as do many others), and bring the forelimbs up off the ground in a few basal taxa, the animals can still rock forward to reach the ground for quad launch. Sure, they have good sized thighs - they still have bigger chests/forelimbs.


As for being able to envision the launch kinematic, that's really just an issue with visualization, and is one reason that I am happy to be an illustrator on the side (and happen to know other illustrators superior to myself). So, I'll work up the images as time permits. That said, remember that similar "envisioning" issues could be just as easily raised for bipedal launch. Bipedal launching is not the "default state", nor the null model. There is no null model for launch in vertebrates - we have to examine the mechanics of the animals in detail to determine which launch mechanisms are most likely. The fact that bipedal launch is the standard in the literature is simple a by-product of thinking that pterosaur should operate like birds, when pterosaurs are, in reality, much more unique. I have a hard time "envisioning" bipedal launch because many pterosaur species can't get off the ground that way without special conditions.

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.

Nope, not really. The specific kinematic is a bit different, sure, but it's still a quad launch. Incidentally, while I used Anhanguera as my example, similar ratios are found in azhdarchids and dsungeripterids, as well as pteranodontids. Thus, it's basal to at least most pterodactyloids. I'm working on rhamph data now, and the trend holds - the ratios are not as extreme in the smaller bodied animals, just as one would expect, but they still don't follow an avian trend.


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.

Again, why is a bipedal launch easier to "envision"? If it's just a matter of what we're used to seeing, then that's not really a good test of the model, is it? And yes, gusts are handy - quad launchers can use gusts, too. Being reliant on special wind conditions all the time, however, is a poor way to make a living. You seem to associate bipedal platforms with some kind of launch advantage. In reality, that is the primary cost of bipedal launch systems. Birds are much more constrained in many ways, especially with regards to size, than pterosaurs ever were. However, birds get one big advantage: bipedal platforms provide decoupled locomotor modules, so limb use disparity can be very extensive and functional diversity is very plastic. Thus, birds have produced things like loons and grebes, that dive with the hindlimbs, as well as long-legged cursors like secretary birds, all the way to amphibious flyers like puffins. There is huge range of functional diversity in birds: divers, perchers, clingers, runners, soarers and hypercarnivores. Pterosaurs, by contrast, were more like "variations on a theme".


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

The supposed beachcombing track-ways could just as easily be from soarers and skimmers. They walk around on beaches as well. The fact is that a range of pterosaurs from small to large definitely walked around on the ground quadrupedally. That means that an individual pterosaur would have to shift gait to a bipedal stance to launch using the hindlimbs only, at the cost of losing most of its launch power potential, and gaining virtually nothing in return.


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

But long fingers, large unguals, and a more digitigrade stance would hardly negatively affect quad launching. And, quite frankly, given the pterodactyloid diversity, we could consider it the majority.


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 overlooking.

I'm not overlooking it, I'm arguing that, if the ancestry was indeed bipedal, then pterosaurs were secondarily quadrupedal. It's either basal to the group or a secondary adaptation - the fact that the outgroup might be bipedal does not turn pterosaurs into bipeds any more than the possibility of basal dinosauromorphs being quadrupeds makes basal dinosaurs have to be quadrupedal. We know that the most basal dinosaurs on record were bipeds - if the outgroup turns out to have been quadrupedal, then that just means there's a secondary gait acquisition near the root. Same thing for pterosaurs.


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.

I never do discount phylogenetics. In fact, my primary training is in comparative methods. However, some questions simply are not answered phylogenetically. In particular, phlylogenetic brackets don't predict derived states - they can give you ancestral reconstructions to polarize what *is* derived, but you seem to be arguing that we can reconstruct pterosaur locomotion using the state at the root node. That is not an effective methodology in this particular case.


I should also add that the phylogenies jive just fine with the "paradigm" I am proposing: pterosaurs could be secondarily quadrupedal animals that evolved from a bipedal ancestry (in which case the hindlimb was expanded very little, but the forelimb was expanded a lot) or they may have inherited quad gaits from their ancestors, in which case the hindlimbs were expanded slightly more, and the forelimbs were expanded slightly less (relative to the outgroup). Those working on the trees (like yourself) hold the keys to answering those questions of sequence and ancestry. That's not the same as reconstructing the specific, derived mechanical abilities of the ingroup, however, which is a different sort of problem.

Oh, and the jab at engineers really isn't appropriate - they aren't making up the mechanical relationships. Colin Pennycuick once noted (in one of his excellent books on flight) that many biologists think of aerodynamics and mechanics as very theoretical. In reality, they are constrained by known rules of physics, and thus are much *less* uncertain than things like paleoecological inference. The biological conclusions are not as constrained or certain, but the mechanics are quantitative. Pterosaurs were simply better built for quad launch than bipedal launch - that doesn't make it certain that all pterosaurs used quad launches, but I see no reason at present to assume a bipedal launch for any them (except as an occasional, facultative dynamic used by smaller species).

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).

Well, I'm not trying to build any robots; I just calculated the load potentials for the skeletal spars. I've layered on some more qualitative analysis of things like mechanical bracing and muscle fractions. In all cases, quad launch is supported.


Just as in the pteroid orientation question, whatever you come up with has to work (albeit modified) for all pterosaurs.

Well, technically speaking, a proposed dynamic doesn't have to work for them all (there can be diversity in behavior). However, in this case the model does work for all known taxa (at least so far).


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.

They can be; it's not in a passively stable position, of course, but that's fine. The quad launch dynamic includes a phase of pre-load, with the animal in a transient 'unbalanced' phase (just as many animals are when leaping). Think of the early phase as more of a vault, and less of a posterior push. A launching pterosaur would launch by first pushing onto the forelimbs, essentially vaulting over them into preload, then unloading the forelimbs before the vault was completed.


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).

Very interesting; I look forward to seeing the full datasets and topology. Incidentally, if they are easily convergent in at least one group, then I wouldn't call them "silver bullets". Compelling, perhaps, but not a slam dunk. Still, sounds like a neat character list.


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.

True, but it doesn't say that the single tree recovered is any more correct than the 500 MPT's that someone else retrieves. After all, you can't possibly have *the* tree, because we don't have all the taxa (and probably never will).


BTW, convergence occurs in 90% of my characters and often more than twice.

Very interesting - what characters are most often convergent?

Thank goodness for computers.

No kidding; I'll second that.

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