Showing posts with label Pterosaurs. Show all posts
Showing posts with label Pterosaurs. Show all posts

Tuesday, October 20, 2009

Darwinopterus, Germanodactylus, and Pterorhynchus


Well, here he is: Darwinopterus modularis, the new "rhamphodactyloid" that defies precise classification from China. This is the skull, which you'll notice is very similar to a certain European pterodactyloid: Germanodactylus (wonder where it's from?). The skulls are indeed quite similar. In fact, I think I remember David Unwin remarking at SVP that if the skull alone was run through a phylogenetic analysis, it came up as being very close to Germanodactylus. Doesn't surprise me in the least: I just drew both of them. I'm a little unsure of how accurate the skull restoration of the German genus is in Wellnhofer's usually excellent tome. I say this because I was a bit perplexed by the shape and positions of some (read: many) bones. But, generally, the shapes are very similar. There are some differences, sure:


That's Germanodactylus on top, Darwinopterus on the bottom. These are not to scale, although both genera were fairly small pterosaurs. I notice many similarities, especially in the snout. Both have roughly the same number of teeth, both have similar separations between the premaxilla and maxilla. The bony portions of the crest begins just cranially to the nasoantorbita fenestrae in both, and runs up to the back of the orbit. The structure of the mandible in both genera is strikingly similar. Of course, differences remain: in Germanodactylus, the nasoantorbital fenestrae are invaded by two distinct prongs of bone (unless that's an artifact of preservation). The shape of the orbit is very different in both animals, as is the basic architecture of the back of the skull.

Chris Bennett reported a soft-tissue crest in Germanodactylus way back in 2002 that grew out from a bony base. This base is figured in Wellnhofer's book. An osteological base for a soft-tissue (perhaps cornified) crest is not unheard of. Some of you may remember Pterorhynchus, a Mongolian taxon described by Czerkas & Ji in 2002. The description is comical in its near complete absence of technical details on the horribly-preserved skeleton: most of the text is used to discuss the animal's soft-tissue crest and hairlike structures surrounding the body. The animal has never been formally described, so I hesitate to even mention it here. However, in the comments of Darren Naish's excellent discussion of Darwinopterus (which I suggest reading over this meager yarn), Jaime Headden suggested that the genus may be synonymous with Pterorhynchus. I scoffed, reminding the readers that Pterorhynchus' skeleton and skull were (as far as I knew) hidden by damage and matrix. I also raised this concern on DinoForum. And lo, the forum gods did descend and offered me this wonderful photograph:


Wow! That's Pterorhynchus, believe it or not. It was re-prepared by the Chinese after Czerkas & Ji were done with it, I guess. Looks incredible, no? Although it's still really beat up, some features immediately stand out. Most notably, the dentition is a lot closer to Rhamphorhynchus than pterodactyloids. However, most interestingly, it appears to have a singular nasoantorbital fenestra. The dentary is oddly shaped, its ventral margin sloping downward from the joint and then suddenly straightening out, leaving a very obvious "corner" about halfway down. I'm going to say that Pterorhynchus is not synonymous with Darwinopterus, but I think it should be re-described (not by Czerkas) and thrown into a phylogeny, as I'd like to see where it falls out.

Anyway, note the impressive soft-tissue crest that is preserved as an impression in the rock. It has its humble beginnings as a small raised bump just in front of the nasoantorbital fenestrae. This has at least some consequence to the crest of Darwinopterus. I asked Dr. Unwin, at SVP, whether the new pterosaur's crest was bone or soft-tissue. He said that as the animals grows, the dorsal margin of the crest becomes jagged, which he takes to mean that it began supporting a large soft-tissue structure. This would be awesome, though I'd like to see more specimens. There are apparantly more than 20, though only two are featured in the description.

So there you have it. I don't think that Pterorhynchus = Darwinopterus based on a purely superficial look at a low-res photo of the re-prepared fossil of the former, and I think the skull of Darwinopterus looks a lot like the skull of Germanodactylus! SCIENCE!

Wednesday, October 14, 2009

Hello, Frank!


I'll have much more to say on this subject once I finish prepping the art, but for now, ya'll should run over to Tetrapod Zoology, Archosaur Musings, and Mark Witton's site for all the info you could possibly want on this awesome new pterosaur. Oh, it's name? Darwinopterus modularis, a.k.a. Frank!

Wednesday, January 07, 2009

Confusing Pterosaur Take-Offs

This confuses me greatly. According to a new paper published in Zitteliana by Mike Habib, pterosaurs launched themselves into the skies quadrapedally with a "leap-frog" motion. I cannot, for the life of me, envision this. I remember seeing this concept as a poster session at last year's SVP, but there, as here, there are no figures showing exactly how this would work. What we really need is an animation. Can somebody explain this one to me?


Ridiculously awesome update: In the comments, Christopher pointed to the launch sequence of a vampire bat. I'm sold, and it would probably work for rhamphorhynchoids, too.

Friday, August 29, 2008

Pterosaur Digits

Folks, a fella named Jim Cunningham email me today, suggesting that pterosaurs lost their thumb (rather than their pinkies), and that the wing finger is actually Digit V. This seems false. From what I've read and have been told, pterosaurs lost their fifth finger, and that they retained fingers II-IV. But after telling Jim this, he responded thusly:

"Pterosaurs have no thumb. It was the one that was lost. The digits are I, II, III, & IV with IV being the pinky finger which is also the wingfinger. If you'd rather not believe me, you might ask Chris Bennett, Kevin Padian, Wann Langston Jr., Peter Wellnhofer, John Conway, Mike Habib, Dave Unwin, or Greg Paul. Conway will be off on his honeymoon this coming week, so he probably won't respond immediately. All the best."

Those are some big names. Have I been wrong all these years?

Thursday, August 21, 2008

Archosauria, I Presume?



Is that an external mandibular fenestra I see? How well-known is the skull of Dimorphodon? I've always questioned the relationship between archosaurs and pterosaurs based on the apparent lack of mandibular fenestrae, but if Dimorphodon actually has such a feature, then I may have to change my tune!

Wednesday, May 28, 2008

Of Naish, Witton, and Giant Freaking Pterosaurs

Congratulations to fellow paleoblogger Darren Naish and fellow paleoartist Mark Witton! The two of them have cooked up a wonderful new paper in PLoS One about azhdarchid pterosaur behavior. Click this link, and be amazed! Darren has already summarized the paper at his blog, and as if that weren't enough, the two of them have set up a brand-new blog which serves as yet another summary of the paper, but with more illustrations. Beautiful, beautiful illustrations. Long-time readers of Tet. Zoo may recognize Darren's point, and indeed, he mused on the subject of azhdarchid feeding habits way back in Tet. Zoo Volume 1.

I cannot stress how excellent the paper is, because Darren actually tests and questions some dogma about pterosaurs! How novel! I won't ruin it for anybody--you'll have to read the paper yourself to get the gist of it, but you'll come away with a new appreciation of Mesozoic landscapes! Theropods weren't the only critters to look out for back then! Oh, I've said too much! Run over the Darren's blog(s) and congratulate him!

Tuesday, April 08, 2008

Raeticodactlyus = Fishing Eagle?


Without giving entirely too much away, I'm preparing a largish post about the new pterosaur, Raeticodactylus filisurensis, and that post includes an attempted life restoration. I have, as usual, run into a snag. What we have here is a rhamphorhynchoid pterosaur which seems close to Eudimorphodon and Austriadactylus, but has different limb proportions (longer), a very unique skull shape, and...most problematic for me, parasagittal hind limbs. Or so they seem. No pelves were found, so the acetabular angle is unknown. The femur, however, has a distinctly perpendicular head (like dinosaurs), which implies that the hindlimbs were directed underneath the body.

Problem? Well, every other rhamphorhynchoid pterosaur had sprawling or semi-sprawling hindlimbs, and those hindlimbs were thusly incorporated into the wing surface. Because the femoral socket was on the same general plane as the humerus socket, the legs could be moved up and down with the flapping action of the arms. But if you throw me a rhamphorhynchoid pterosaur with parasagittal hindlimbs, the picture blurs.

Remember when Microraptor was found and initially restored in a sort of flying squirrel pose? We know such a posture would be impossible for the hindlimbs to take--the femora would dislocate. That's the problem facing Raeticodactylus if it had a parasagittal posture against a sprawling (flying) posture for the forelimbs. So what do I do? Clearly, the hindlimbs must not have been attached directly to the cheiropatagia. Stecher suggests that Raeticodactylus was a fish-eater (and/or shellfish eater), so perhaps...

Perhaps it used its parasagittal, unladen hindlimbs to grab fish out of the water, like eagles do! If that's the case, then the cheiropatagium must have terminated caudally to the pelvis. Stecher also suggests that Raeticodactylus had a typical "hook" toe like other rhamphorhynchoids, but figuring out that problem will be far easier than this one was.

UPDATE: Dave Hone has shot the fishing-grabbing idea down, for the simple reason that pterosaurs weren't doing a lot of grasping with their feet. I still wonder about how the patagium attached to a parasagittal leg, though.

Wednesday, April 02, 2008

Nyctosaurus 3.5


Are you readers getting sick of seeing Nyctosaurus? That's okay, because I'm tired of drawing it. I couldn't even get the whole length of the beak on my paper this time. This version has a ridiculously long crest, the proper pteroid orientation (and length, although you can't see it), and fur. I got rid of the "fleshy pad" at the knuckle of the wing finger, because I honestly couldn't figure out how the pad would compress when the finger was extended.
Aside from some minor tinkering here and there, this will probably be the final version, which will be blown up to the size of my canvas and somehow transferred to it. Unless there's something horribly wrong with it, I am DONE with Nyctosaurus!
And don't worry--everything's been measured, based on John Conway's wonderful skeletal reconstruction.

Sunday, March 30, 2008

Dimorphodon v. 1.5


The preceding Nyctosaurus drawings are but one picture in a series of three pterosaurs in the art show. If all goes as planned, Nyctosaurus, Dimorphodon, and Tapejara will be represented. This is to show the breadth of pterosaur diversity but also some really freaking cool crests will be in there. Dimorphodon has been giving me trouble, just because its proportions are different from other "rhamphorhynchoids." Today, while in Palmer (discussing the art show), I whipped up this sketch, which is the first I've been happy with for this taxon.

On a related note, Nyctosaurus is functionally finished. I just have to shrink the sketch, improve the crest on another sheet of paper, then blow that sketch up and use it for the canvas piece. The Nyctosaurus will be colored like a murre, and the Dimorphodon will get puffin colors. I think another artist (Raven) will be doing the Tapejara, and a tropical pterosaur with a giant crest will be very colorful indeed!

Again, be brutal when it comes to comments about this picture. Head too small? Body too long?

Wednesday, March 12, 2008

Nyctosaurus 2.0


First of all, a thousand thank-you's to everyone who commented on my previous attempt at Nyctosaurus. Your comments on the post itself, as well as your emails (which, in some cases, included the relevant literature) helped to improve my understanding of this strange pterosaur a million times over. Perhaps of most help, however, were the skeletal drawings of Nyctosaurus by Chris Bennett and John Conway, accessable here and here. John Conway is some kind of god among paleo-artists; his measured drawings are, dare I say, authoritative. Bennett's picture is initially comical until you realize the ridiculous length of the wing metacarpal. Here are some things I learned while re-drawing Nyctosaurus based on various sources:

1) I knew that Nyctosaurus had lost its first three fingers, but I didn't realize that it had lost the first three metacarpals. In their place are ossified tendons which (as far as I can tell) ran from the pteroid to the distal tip of the fourth metatarsal.

2) Nyctosaurus has ridiculously short legs, or perhaps ridiculously long arms. Bennett's restoration of a vertically-oriented "naked lizard" is very nearly required for this beastie to shamble about on the land. This was clearly not an animal that spent a lot of time on the ground. Still, Chris' picture cannot be "final," because the amount of weight in front of Nyctosaurus' chest in an upright stance would cause it to constantly tip forward.

3) In attempting to figure out a more traditional quadrapedal posture, I kept the feet on the ground and angled the body upward in such a way that the ridiculously long arms could touch the ground without lifting the hindlimbs off the ground. Even in the resulting position, however, I am stupified as to how the creature moves forward.

4) The possibility exists that Nyctosaurus kept its arms in a sprawling posture while on the ground, so that more distance existed between its knuckles than between its feet, but this would make for a very wobbly structure, especially with that enormous head.

5) Look at the size of that freaking head! It's not enough that the skull is twice the length of the body, but the crest is almost three times the length of the skull! While sketching the whole animal (crest and all), I began to wonder if maybe the crest grew to such an extreme size to counterbalance the enormous head.

6) Nyctosaurus only has three wing phalanges, and the third phalange is strongly bent, like a very wide "V." Intuition tells me that this indicates a narrow cheiropatagium, perhaps terminating at the knee. Conway's illustration has the cheiropatagium terminating at the back of the ribcage, but somehow that just looks "wrong" to me. Personal preference, I admit.

I am still unsure of how to restore the "knuckle." An epidermal pad is still attractive, but wouldn't a tough pad like that interfere with the folding and unfolding of the wing finger? But if Nyctosaurus just had a tough knuckle, wouldn't the forelimbs be unsteady on the ground? And I apologize for the sketchy feel of this picture, but it really is just an inked sketch. I drew it this morning during my meds routine and thought it was good enough to not immediately crumple up. Rough though it may be, I feel it's a marked improvement over the last draft, because it incorporates many more reference materials.

Again, comments (no matter how severe!) are always welcome. And for that crest...I think I'm gonna need a bigger canvas!

Saturday, February 23, 2008

Introducing Nyctosaurus


Well, here it is, ladies and germs: The first sketch I have accepted as "good" from the last two weeks of trying to knock out a Nyctosaurus for the art show. Previous efforts have been cave paintings by comparison. I will be painting Nyctosaurus on a large canvas for The Archosauria, and it will be accompanied by a rhamphorhynchoid of undetermined identity.
The original specimens of Nyctosaurus, described in 1958, lacked any kind of headcrest. Marsh allied Nyctosaurus with Pteranodon, but more complete skeletons, described and drawn by Williston in 1902, suggested that Nyctosaurus belonged in a family of its own. For one thing, the "naked lizard" is significantly smaller than Pteranodon, having a wingspan of less than ten feet. Additionally, Nyctosaurus has just three wing phalanges instead of the usual four. Stranger still, this Lower Cretaceous pterosaur has lost all of its non-wing digits (I-III). Because Nyctosaurus could not have spent its entire life on the wing, I have restored it with a fleshy pad capping the main joint of the wing finger.
The crestless Nyctosaurus existed until 2003, when two new specimens were discovered with giant reindeer antlers sticking out of their noggins. Marsh and Williston's crestless forms were probably sexually immature, and, like so many pterosaurs, the crest likely had a display function. Other possibilities exist, I suppose. Perhaps Nyctosaurus grew the crest annually, like modern deer do, but the lack of a rugose surface on the skullcap of the original Nyctosaurus material scratches that out. It's also possible that, like Pteranodon, Nyctosaurus was sexually dimorphic, and only one sex had the crest. This intruiging possibility cannot be adequately tested do to the scant remains of this "naked lizard."
Nyctosaurus has also been regaled to a more basal place on the pterodactyloid family tree. In their recent description of Nemicolopterus, Wang et al. (2008) placed Nyctosauridae as the basalmost clade within the Dsungaripteroidea.
Nyctosaurus is among my very favorite pterosaurs, and I'd like to do it proud. If there are any criticisms out there (Head too big? Legs too long? Crest too short?), I'd be happy to hear them!

Sunday, February 17, 2008

Pterosaur Wings--As I Understand Them

First of all, I have to thank David Hone, David Unwin, and Michael Skrepnick for helping me to understand this concept, as it was very difficult for me.

I am a pterosaur fan. I think the group is incredibly interesting, and I have devoted several posts to them in the last few years. I try to keep up on my pterosaur literature, but it's tough in Alaska. I try to nab any pterosaur books I become aware of, most recently David Unwin's spiritual sequel to Wellnhofer's Prehistoric Flying Reptiles, Pterosaurs from Deep Time. It is an excellent read, and it brought me up to date on many aspects of pterosaur anatomy that I was only vaguely aware of before reading it.

One of the areas I've constantly struggled with, however, is how the wing folds. More specifically, how the joint between the wing metacarpal and the first wing phalange operates. I've seen plenty of reconstructions, few of which agree with each other, of how the wing folds up while on the ground, and it's just confusing. Without a 3D model, imagining the process is strenuous, but I have put all the lessons together and tried to make an accurate diagram of how the finger joints operate (above).

If you want a human equivalent, here we go:

Put your arms out to your sides, slightly bent at the elbows, palms facing up. Now chop off your pinkie finger--you won't need that. Also, reorient your thumb so that it's no longer offset. Curl your now injured thumb, index finger, and middle finger. Notice how they all curl toward the palm. Great. Now here's the painful part: Dislocate your ring finger, spin it 180 degrees, and pop it back in place. Your ring finger will now curl toward the back of your hand. Ouch, right? Keep your palms up! When you curl that ring finger, notice that it starts pointing toward your elbow.

That's basically the situation in pterosaurs. While in flight, a pterosaur's palm faces forward, and its "ring" finger is bent slightly back. The manual digits--short 'n' scrawny by comparison--were kept in a relaxed position, probably slightly curled inward, toward the palm.

But now the pterosaur lands, and starts walking on all fours. What did the wing look like then?

Don't worry--things don't completely go to hell. You can still use your own arms as examples.

Get down on all fours (knees on the ground), and sprawl your forelimbs out to the sides, so that your elbows and fingers are pointing away from the body. Now rotate your wrists slightly so that your fingers are actually pointing slightly back. Remember that your pinkie finger has been severed (hope you've got some gauze on that wound!) and your ring finger is backwards.

Bend the base of your reversed ring finger "up," and notice how your ring finger now points toward your elbow again. In a pterosaur wing, the individual wing phalanges were virtually unable to move against each other, so aside from the metacarpal/phalange joint, your ring finger is a straight rod. Now imagine that your ring finger is just a bit longer than your entire arm. Whallah! Pterosaur wing!

Of course, if you want to go...you know...all out, you could try and keep your palm off the ground, because of course pterosaurs walked in a digigrade manner for their forelimbs, but plantigrade hindlimbs!

So, please tell me if I've gotten it right, folks! I apologize for using overly violent examples for a human analogue, but all the attempts I made to draw a human analogue ended up looking entirely too disturbing.

Monday, February 11, 2008

Speaking of pterosaurs...

Nemicolopterus crypticus was just announced at the National Geographic website. It's a smallest pterosaur ever found (check out the pictures, including the skeletal restoration) and will be published in the Proceedings of the National Academy of Sciences tomorrow. If the paper is not open-source, could one of you kind readers send me a copy? This is a pterosaur I need to restore!

UPDATE: Thanks to Nick Gardner for sending it to me. If you ever need a paleo-related picture, brother, let me know!

Are Rhamphorhynchoids Real?


I was musing this morning, as I often do, about pterosaurs, when a thought struck me. Is "Rhamphorhyncoidea" a natural group? Supposedly it forms a sister group to the Pterodactyloidea, but what if pterodactyloids are derived directly from rhamphorynchoids? Pterodactylus is still, as far as I know, the earliest known pterodactyloid, and it doesn't show up until the Late Jurassic, whereas rhamphorhynchoids are known since the Late Triassic, and in all likelihood, the group originated well before that. Were pterodactyloids an actual sister group, that would imply a very long ghost lineage leading back to the origins of the Pterosauria.
But then you look at anurognathid pterosaurs, who are "short-tailed rhamphorhynchoids," and wonder where that group perhaps shares a more direct relationship with pterodactyloids. Were that the case, then "Rhamphorhychoidea" would be paraphyletic. I'm not suggesting that some rhamphorhynchoids don't share some common ancestry. Rhamphorhynchus and Dorygnathus are certainly sister taxa. Dimorphodon and Peteinosaurus seem close enough. Certainly the scaphognathines form a monophyletic clade. But how do all these smaller groups relate to one another? What if rhamphorhynchoids form a stepwise progression leading to Pterodactyloidea, much like we know understand Prosauropoda to be a paraphyletic grade of animals leading to Sauropoda?
The name "Rhamphorhynchoidea" would have to be abandoned, unless we start calling Pteranodon a pterodontid ornithocheiroid pterodactyloid rhamphorhyncoid pterosaur. That wouldn't work.
Has any work been done on the interrelationships between the various families of the Rhamphoryhncoidea and that group's relationship to the Pterodactyloidea? And while we're on the subject of pterosaurs, does anybody have a good paper detailing Nyctosaurus' reindeer antler? I'm restoring that monster for the upcoming Archosauria art show.

Wednesday, August 29, 2007

Pterosaurs: What the hell ARE they?

Pterosaurs were, as far as anyone can tell, the first vertebrates to achieve powered flight. These little reptiles developed a unique method of flight via elongation of the fourth manual digit. This is unlike the case in bats, wherein digits II, III, IV, and V are elongated. The pterosaur wing was, by comparison, long and narrow. It must have worked out alright, though, because pterosaurs ruled the skies for the entirety of the Mesozoic. In fact, although the first unquestionably pterosaurian fossils are of Late Jurassic age, those unquestionable specimens are unquestionably advanced. There is no Archaeopteryx among pterosaurs. This means two things: First, pterosaurs were probably around for quite some time before they show up in the fossil record. Second, their ancestry is a matter of some debate. Traditionally, the Pterosauria has been seen as either an outgroup to either the Ornithodira or, in some cases, the Dinosaurmorpha. Other workers see them branching from the more basal archosaurs. Still others see the pterosaurs as closer to protorosaurs or even, dare I say it, the prolacertiforms.
Part of the problem, of course, is that the first pterosaur fossils known (Eudimorphodon, Dimorphodon) are already incredibly derived. Ironically, this quandry is the same as for bats, who just "appear" in their modern form. Molecular evidence points to a Canivora ancestry for bats, but there is no such evidence for pterosaurs. I suppose a good analogy would be whales: If you had to figure out where whales fit into the Mammalia, but the only whale you could look at was a modern baleen whale, you'd have some trouble. This is essentially the situation with pterosaurs, who, even at their earliest known appearance, are so derived and suited for a flying lifestyle that their ancestry is clouded by those very derivations.
One thing we can be sure of is that pterosaurs are reptiles. Better yet, they are diapsid reptiles. Of course, seeing as diapsid reptiles ruled the world for the better part of the Mesozoic, our search for a pterosaur ancestor still crosses a broad spectrum. Like I said before, the traditional model is that pterosaurs are, in some vague way, related to dinosaurs. But I'm hoping you'll see, after reading this post, that this hypothesis is in fact not very well supported.
Ornithodirans?
Behold, Marasuchus, the iconic dinosaurmorph. Fully digigrade and bipedal with a tri-radiate pelvis, shortened forearms, a long, flexible tail, armor scutes, and a unique ornithodiran ankle structure allowing that specialized horizontal posture. Surely, pterosaurs even as "primitive" as Dimorphodon could not have come from as specialized a creature as this. Marasuchus stood on the very threshold of being called a dinosaur. Indeed, Greg Paul has said that "lagosuchids" were early dinosaurs. We know that pterosaurs were not dinosaurs, so perhaps Marasuchus is a bad example. We need something a little more basal, but still among the ornithodirs.

This big-headed cutie is Scleromochlus, a primitive ornithodir discovered in Scotland, and it has been suggested by a few scientists as being a pterosaur ancestor. This seems to be due to its basal nature, as it's usually considered an outgroup of the Ornithodira. So, if we want pterosaurs to be ornithodirs, then maybe Scleromochlus is a kind of ornithodiran "Euparkeria," providing a nice basal form from which several more specialized animals can branch from. However, both Marasuchus and Scleromochlus do nothing more than emphasize the problem with placing the Pterosauria within the Ornithodira. Specifically, the structure of the pelvis is completely different in pterosaurs compared to ornithodirans (I'll get to specific aspects of pterosaur anatomy in a bit). Whereas ornithodiran pelves are built with strut-like elements that point forward and back, and connect in the middle, pterosaur pelvises are shield-like when viewed laterally with no elements connecting in the middle. Moreover, pterosaurs were plantigrade like humans and bears--we walk on the soles of our feet. One of the highlights of being an ornithodir is that you walk on your tip-toes, thus increasing the length of your stride. Finally, ornithodir arms are short while the legs are long. In pterosaurs, the opposite is true. And perhaps most frustratingly, pterosaurs are unquestionably sprawling in their gait (rhamphyrhychids moreso than pterodacyloids), while Marasuchus, Scleromochlus, and all of their cousins were fully bipedal with parasaggital stances. In his 1998 redescription of Scleromochlus, Michael Benton had this to say:

"Padian (1997) named the postulated clade containing Scleromochlus and Pterosauria the Pterosauromorpha. The characters proposed to link Scleromochlus with either pterosaurs or dinosaurs are not wholly convincing. Sereno (1991, pp. 36-37) listed the following as potential synapomophies of Sclermochlus and Pterosauria: (33) Skull more than 50 percent presacral column length; (34) Length of scapula is less than 75 percent of humeral length; (35) Fourth trochanter absent; (36) Metatarsal 1 length is at least 85 percent of metatarsal 3 length.
"The first three of these could relate simply to the small size of the animals. Character (33) is true also for phytosaurs and many crocodylomorphs (although because of snout elongation). Character (34) is probably spurious because it compares what is genuinely a relatively short scapula in Scleromochlus with the relatively elongated forelimbs of pterosaurs (Bennet 1996, p. 279). Character (36) is present also in some basal crocodylomorphs, so it can evidently arise convergently."

So what, pray tell, is keeping pterosaurs rooted so firmly in the Ornithodira in the first place? Quite simply, the structure of the ankles share features with ornithodirans and dinosaurs which no other diapsid group (heck, reptile group) shares. This could, of course, be due to convergence. And I'm tempted to think so. After all, when the number of features that would keep an organism out of a particular group so heavily outnumbers the quota for keeping that animal in, something's up.

Archosauriforms, perhaps?

If Scleromochlus does not provide a basal enough form, than why not just start climbing down the diapsid family tree until we get somebody who fits the bill? Pterosaurs are almost always thought of as archosaurs based mainly on a single, perhaps unambiguous character: the antorbital fenestrae, that big skull window in front of the eyes shared by all archosauriforms (and secondarily lost in many crurotarsians). However, practicality puts a dent in even this idea. If it were true that pterosaurs, those most lightweight and long-armed of organisms, would sit among fat, heavy, short-limbed crocodile-like critters. Also of note is that pterosaurs lack a mandibular fenestrae, a hole in the jaw that is also diagnostic of the Archosauriformes. If pterosaurs are archosauriforms, then that means that they, for whatever reason, secondarily lost the mandibular fenestrae. This makes no sense at all, given that pterosaurs excavated their skeletons like crazy in an effort to become more lightweight.

Sharovipteryx cousins?

There are few gliding vertebrates stranger than Sharovipteryx. Aside from its strange anatomy, Sharovipteryx is unique because it was found in the same area (Kyrgyzastan) in the same year (1965) by the same person (Alexander Sharov) as the infamous Longisquama, the animal that because Alan Feduccia's posterchild for a non-dinosaurian avian ancestor. At any rate, Sharovipteryx has often been suggested as a pterosaur ancestor, and it does have one big thing going for it: gliding membranes. This little prolacertiform, who is also related to Tanystropheus, was clearly arboreal. The physical remnants of Sharovipteryx, however, provide little more than trace evidence for the creature. It's clear that the legs were extremely long, as was the tail, and the spine and parts of the skull are preserved. Aside from some questionable humeri shapes, the arms are unknown. Sharovipteryx's fossil, however, does preserve impressions of a skin membrane stretching from the toes of the feet to the tail. Sadly, the state of preservation above the pelvis is terrible, so whether Sharovipteryx actually had gliding membranes above the waist is unknown.

However, several studies involving paper and computer models have suggested that, in order to control aerial stability and extend the gliding period, Sharovipteryx must have had a sort of canard membrane originating on the forelimbs. Whether this membrane stretched from the front of the arm and attached to the neck, or the back of the arm to the body, or both, is unknown. A 2006 study by Dyke, et al. suggested that Sharovipteryx had a forelimb membrane like the one above, making it the first "Delta-wing glider," like a Vulcan bomber.

Tempting though Sharovipteryx is as a pterosaur ancestor, there are several problems even with it. Despite its aerodynamic capabilities, Sharovipteryx has extremely long legs and ridiculously small forelimbs (from what's known). Its arms are also simply unknown, and although Sharov suggested in 1971 that the animal's fourth finger may have been elongated, but there is no evidence of the animal's hands. So, although the gliding membranes provide some indirect evidence for a pterosaurian relationship, Sharovipteryx is simply too poorly preserved to answer our question.

So what ARE they?

What makes pterosaurs so hard to place, like I said before, is their level of derivation. And whether certain features are actually primitive or derived is hard to say. For example, pterosaurs were plantigrade--they walked on the soles of their feet like humans, bears, lizards, and crurotarsians. Pterosaur trackways and a well-preserved 3D fossil of a Dimorphodon foot have secured pterosaurs as plantigrade. However, is the plantigrade posture retained from an ancestor or was that ancestor digigrade, only to switch over to plantigrade due to the physical constraints of the connecting wing membrane? In discussing Dimorphodon weintraubi's plantigrade posture to the competing bipedal pterosaur model, Clark et al had this to say:

"Other features of digits I-IV of the D. weintraubi foot indicate a capacity for grasping that is consistent with an ability to climb but is unexpected in an obligate cursor. The claws are moderately curved (nearly as strongly as the claws of the manus); all phalanges except the most proximal have well developed flexor tubercles for the insertion of digital flexors (Fig. 2); and all of the IP joints allow for extensive flexion of the digits (as exhibited by digit IV; Fig. 2). Furthermore, the phalangeal proportions of the digits of Dimorphodon and other basal pterosaurs are similar to those of birds with grasping feet (that is, perching, climbing, and raptorial species) and unlike those of primarily ground-living birds, bipedal dinosaurs and the primitive dinosauromorphs Lagerpeton and Marasuchus."

Pterosaurs, or at least rhamphorhychoids, were scansorial and probably rarely ventured to the ground. In fact, according to David Unwin, no confirmed rhamphorhychoid trackways have ever been found. Perhaps, given the extremely limiting connections of the rhamphorhychoid wing membranes, being grounded meant getting eaten.

Pterosaurs pelves are also a bit of a mystery. Again, the old bipedal model depends on the structure and orientation of the femur socket, and a bipedal posture was proposed by Padian and Bennett at different times (Padian preferring a horizontal posture while Bennett suggested a rather comical human-like construction). For a dinosaurian bipedal posture to work, however, the legs would have to be able to be held underneath the body. This picture, as well as the next one, are from Wellnhofer's excellent study of an Anhanguera pelvis from Brazil. While this particular pterosaur is fairly large pterodactyloid, it is comparable to rhamphorhychoid pelves and so serves as a great model for pterosaurs in general. The first thing I notice upon looking at this is that the acetabula is completely closed. In ornithodirans, the acetabula is at least partially open, and in early dinosaurs it is mostly open if not completely so. The construction of the iliac blades are also wildly divergent from a supposed ornithodiran (or even archosaurian) ancestor in that they are thin, not flat. The pubis is wide, as is the ischium. In rhamphyrhynchoids, the pubis and ischium actually fuse to form a sort of D-shaped bone. Pterosaurs also developed "prepubes." A prepube looks a bit like a traditional theropod pubis, but chopped off right above the boot. The prepubes attach to the front of the pubis and connect to the belly ribs. They essentially anchor the pelvis to the sternum, making the body even more immobile than birds. Prepubes are "new" bones which, in addition to the pteroid wrist bones, are entirely unique to pterosaurs.


As you can see from a front view of Anhanguera's pelvis, the pubis and ischium do not connect in the middle. Rather, each side forms a sort of "pelvic shield" which widens and strengthens the body. In addition, the position of the acetabula prohibit a parasaggital posture. Notice that the femur sockets actually point out and up. Wellnhofer reflects on the diagram above:

"Putting the femur in its proper place and allowing some space for cartilage in the acetabulum, the most comfortable setting for the head would result in a horizontal position of the shaft (Figure 6, left). Then, the axis of the collum femoris coincides with the axis of the plane of the acetabular rim directed 35 degrees upward. A higher lifting of the femur was certainly possible. In the opposite direction, the femur could not be adducted too much (Figure 6, right). A maximal angle of the shaft 150 degrees downward would have been achieved at best, because the ventral rim of the acetabulum formed by the pubis was a natural stop. On the other hand, the head could no longer find support in the hip socket without being in danger of luxation. Therefore, the orientation of the femora during terrestrial locomotion was probably less extreme than shown here."

In other words, pterosaurs who tried a parasagittal posture would have dislocated their hips. More to the point this time, Wellnhofer continues:

"In any case, a parasagittal swing of the hind legs was absolutely impossible. The femora of pterosaurs were splayed out, their stance and gait was semi-erect. Consequently they could not have been bipedal animals. Furthermore, the morphology of the foot skeleton suggests that they were not digigrade."

That last comment makes me chuckle, because Wellnhofer realized that pterosaurs were plantigrade in 1988, ten years before the foot of Dimorphodon weintraubi was described.

Again, however, we can't be sure whether the strangely-directed acetabulum is a primitive or derived characteristic among pterosaurs. Perhaps the hip socket was directed upwards as a necessity for a smooth wing surface. When a pterosaur flew, it moved its legs into a full lateral position, which straightened the patagium but also moved the foot into a position perpendicular to the wing surface. According to Unwin, the toes of pterosaurs were webbed, not to aid in aquatic habits, but to control yaw during flight. Had the acetabula of pterosaurs been directed fully to the side, it is doubtful that the legs could have been of much use during flight. Thus, it is entirely possible that the seemingly primitive sprawling condition of the hind legs is a derivational necessity of the flight aparatus.

I cannot pretend to advance my own theory of pterosaur origins here, but I often find myself questioning the traditional viewpoints regarding their branching-off points. It seems obvious to me that pterosaur cannot be ornithodirs, and perhaps just as unlikely that they are archosaurs proper. A more basal archosauriform is more likely, in my opinion. Certainly the possibility of a relationship with Sharovipteryx and the greater Prolacertiformes cannot be ruled out. Like I find myself saying at the end of almost every paleo-related post, we need more specimens! And better ones! It would be fantastic to find the arms of Sharovipteryx, for example, or better yet, a pterosaur who could only glide instead of fly. That would make my day. Pterosaurs are my favorite flying vertebrates by far, and I hope you've enjoyed reading about their murky origins as much as I've enjoyed writing about them.

P.S. Much as I'd love to keep writing about how pterosaurs are incredibly unique--for example, I didn't even go into their bizarre forelimb anatomy--I've got to force myself to stop at some point, because otherwise, I'll write a book. And nobody wants that.

References:

Unwin, D. M. (2006). The Pterosaurs from Deep Time. Pi Press, New York, NY.

Wellnhofer, P. (1996). The Illustrated Encyclopedia of Prehistoric Flying Reptiles. Barnes & Noble Books, London, UK

Wellnhofer, P. (1988). Terrestrial locomotion in pterosaurs. Historical Biology(1): 3-16.

Clark, J. M. et al (1998). Foot posture in a primitive pterosaur. Nature(391): 886-889.

Benton, M. J. (1999). Scleromochlus taylori and the origin of dinosaurs and pterosaurs. Phil. Trans. R. Soc. Lond. B(354): 1423-1446.

Dyke, G. J. et al (2006). Flight of Sharovipteryx mirabilis: the world's first delta-winged glider. Journal of Evolutionary Biology (published online).