Wednesday, July 28, 2010

Tracy Responds

Hey, remember that critique I did on Ford & Martin's semi-aquatic Psittacosaurus paper awhile back? Seriously, just scroll down if you haven't seen it. Tracy was good enough to write a rebuttal, but it was too long to be in that post's comments, so I offered to post it as a separate post. I'm rather star-struck that he bothered to write a rebuttal at all, actually! Ah, people do read my humble blog! At any rate, here's how this is going down: Tracy basically wrote a response to me, point-by-point, referencing specific sections of my post. I'm going to reproduce that here, then add MY OWN commentary after each of his points in a different color. So, to be clear, his stuff is in black, mine is in...uh...red. So here we go! The next words you read will be from the pen of Tracy Ford.

First off I have to say that I am the principle writer, not Martin. If anyone has a problem understanding it, it rests on my shoulders, not Larry’s. I’m going to quote Ricky from I love Lucy: “Lucy, you have some ‘splaining to do.”

I won’t go into how the idea for the article came about, for that you can see the recent issue of Prehistoric Times. The editor/peer reviewer didn’t think it was relevant. In fact several things I wanted to put in the article he told me to take out (including some illustrations). I will be addressing Zach’s comments with a Z for Zach and a TLF for me.

Z) Ford, T. L. & Martin, L. D. (2010). A Semi-Aquatic Life Habit for Psittacosaurus.

TLF) Hey, somebody read the article!

Of course I did! It was lovely, really. Despite what I considered shortfalls, I did enjoy it.

Z) The fact that Larry Martin's name was attached to the paper instantly sent multiple red flags up in my mind. His moniker is the kind of warning label one usually associates with "Dougal Dixon" and "Alan Fedducia" (who, I'm told, can't even get ornithology right). Still, I tried to repress my angst and read on, determined to see this theory through. Larry can't get bird origins right to save his life, but maybe he's on the ball when it comes to ceratopsians. Maybe they'll make a slam-dunk case.

TLF) I like Larry. I’ve known him for a few decades now and he’s always been on the up and up with me and he’s the kind of person who will tell you how it is, and will correct anything that he’s said before.

I would hope so. He seems pretty dead-set on this whole BAND thing, but that's another post for another day. I should launch into a giant argument about rhetoric, inductive reasoning, and Aristotle's concept of ethos, but that's yet another post for another day (I am schooled in the art of rhetorical argument, after all).

Z) Readers, they do not.

TLF) Hopefully I can rectify that.

Avast, ye!

Z) The authors draw on six features of Psittacosaurus to make the case for a semi-aquatic lifestyle.

TLF) Yes, but Zach neglects to comment that even though we site 6 reasons, we aren’t the first to comment on an aquatic life style for Psittacosaurus, which is important. Rozhdestvensky (1955), Suslov, 1983, Currie (pers.com. 1997), and Averianov et al. (pers. com. 2006). This will become important as I continue to comment on his comments.

I should mention here that, when writing the original post, I did not cite the Ford & Martin's citations, mainly because I'm incredibly lazy, but also because I believe that if you cite an authors' work in the affirmative, you are basically advocating that position. Therefore, their position becomes your position, too. Just sayin'.

Z) First thing's first, though: I should point out that a modern analogue doesn't really exist, and the authors don't point to one. They never say something like, "Psittacosaurs were Mesozoic hippos!" or "Psittacosaurus lived like a crocodile!"

TLF) AH, DUHH!!! They don’t look like hippos or crocodiles. And we do compare them to beaver-like lifestyle, so this statement is false.

I'm gonna stick to my guns here. Psittacosaurus exists as a sort of mash-up of various semi-aquatic vertebrates in the paper, and even the final beaver analogue is more a comparison of its choice of environment than behavior.

Z) No, instead, their vision of everyone's favorite parrot lizard is a polytomy of various semi (or fully) aquatic modern animals as the authors struggle to come up with "semi-aquatic" reasons for Psittacosaurus' anatomy. Let's dive right in, shall we?

TLF) Again, we aren’t the first.

That's not an argument. Bad Tracy! Argumentum ad...uh...something! Something Greek.

Z) The first point the authors make is that Psittacosaurus is often found lying on its belly, hindlimbs akimbo in a "sprawling" position, and sometimes hyperflexed. This is apparently evidence of a semi-aquatic lifestyle. That seems like a non-starter to me, though. I can name plenty of modern animals with sprawling limbs that are NOT semi-aquatic, and even some fossil animals with sprawling limbs that have never been considered semi-aquatic. It's also worth noting that many modern semi-aquatic animals have parasagittal postures. The authors also do a poor job of explaining how an offset femur head (which Psittacosaurus apparently has) equals semi-aquatic lifestyle. They suggest it has something to do with a "swimmer's kick," because from what I gather, no animal can swim without splaying its hindlimbs and using a scissor, or breast-stroke, kick. Frogs do, after all. Crocodiles don't (they swim with their tail). Birds don't. Mammals don't. But one branch of Lissamphibia does.

TLF) Ah, this is totally mine. Now I take it he accepts the resting life pose that I propose then? Good, but I’d like to say that I’ve seen several different dinosaurs that have been found in a resting pose. Not just dromaeosaurs, and psittacosaurs, Coelophysis, other theropods, ornithischians, and even sauropods! Yes, there are several modern animals that sprawl. But 98% of dinosaurs couldn’t--they physically could not. And if I’m saying that they do (I’ll be using me from now on since I did write it), then I have to explain why they could.

Ah, I see. Other dinosaurs are found in resting postures that are NOT sprawling (like Mei or Saurornitholestes or Coelophysis), so the fact that Psittacosaurus is found in a sprawling posture is reason for pause. I agree!

Z) So, according to Ford & Martin, the animal most closely resembling the alleged swimming mode of Psittacosaurus is a non-amniote. Real good. It's not the only amphibian analogue the authors will make.

TLF) Not sure I understand this statement.

The language of the paper makes it sound like you're suggesting a swimming kick that's analogous to a frog, with splayed limbs making a scissor kick. If that's not actually the point you're trying to make, it's poorly worded in the paper itself.

Z) The femur thing is also inconsistent because in Figure 23.3 of their paper (on page 332), they show a rousing series of genasaur femora. The first two are psittacosaurs. Figure A (P. xinjiangensis) does seem to have an offset femur head. Figure B (P. sibiricus) really doesn't. Figure F (P. xinjiangensis) is a picture of P. xinjiangensis' femur abducted to a comical degree, without any consideration for muscle and cartilage. It's worth noting that plenty of non-aquatic animals with parasagittal hindlimbs have somewhat offset femur heads. I have a sheep femur with an offset head. Tyrannosaurus rex has a somewhat offset head. Animals with truly sprawling postures have very offset femur heads. No living animal can move between a completely sprawling and completely parasagittal posture. But I guess Psittacosaurus could.

TLF) Ah, I couldn’t illustrate a lot of femur heads and did the few I could. Dinosaurs for the most part could not sprawl, like I said before. The femur would not allow it. Dozens of Psittacosaurs have been found in a sprawling position. If it is a life position like I purposes then I have to explain why they could. I’ve heard the argument that there’re muscles, tendons, whatever but we do not know for the most part whether or not the muscles would allow it without having a living animal. Maybe they could (which I believe) and maybe they couldn’t. But for argument sake, I say they could. Now an offset femur would allow a broader range of movement. Since the top of the femur has a cartilgouse cap, I contend the femur could be moved even more into the acetabulum, as can be seen in figure 23.3 (F). Crocodiles walk in a sprawling and completely parasagittal posture. Ever see a crocodile run? So yes, animals do.

A little YouTube research shows that when crocodiles are walking quickly, they still rotate the shoulders and femurs out and around. At no point are the legs directly underneath the body. When they "gallop," it's a different motion: the hind limbs act in unison, pushing the body up and forward, and the fall is caught by the forelimbs, which also act in unison. Still not parasagittal, though. In order for Psittacosaurus to fully sprawl its hindlimbs, we're talking about essentially a cartilage cap that would separate the bone of the femur from the acetabulum...right?

Z) Let's move on the foot. The authors suggest that the foot was broad, and that large attachment scars existed on the shafts of metatarsals 1-4, suggesting that the foot was used for "more than just walking."

TLF) This is Rozhdestvensky (1955), he said this and I followed it. The phalanges and unguals are dorosal/ventrally flattened, making the foot ‘wider/flatter’ than any other dinosaur.

I know, I just didn't have the energy to flip through the citations index. I seem to recall hadrosaurs and ankylosaurs having pretty flattened unguals. Do Psittacosaurus' unguals have lateral "shelves" that would suggest webbing as in modern otters and beavers?

Z) Perhaps running, or jumping, or simply being active.

TLF) Ok, but I can’t see it jumping.

I can't see it swimming. :-P

Z) Where, exactly, can I find a rubrik telling me how much muscle is required for walking, and how much is excessive? Strong feet do not necessarily equal a "swimming kick."

TLF) Actually it was Sereno (1987) who said the foot had more muscle attachment, meaning a stronger ‘foot’ stroke. And I can see how this would work for a strong swimming stroke.

Yeah, but even that's an ambiguous statement. "Foot stroke," I mean. I'm betting that Sereno took it to mean a powerful push-off step, but...eh. It's just vague language. That's not your fault, and you're certainly free to interpret that however you want, I just wish...Sereno used more precise terms there.

Z) Also, again I say, very few habitually semi-aquatic animals use a scissor kick. Frogs use that same motion for jumping. Maybe Psittacosaurus was also an excellent leaper, and all that padding and muscle was used for shock absorbtion.

TLF) Are you serious? What morphology of the animal leads to this?

I was being facetious. I admit that it's hard to tell sometimes.

Z) Also, you don't need a lot of muscle in the foot itself for swimming. You need a lot of muscle in the part of the leg that provides propulsion--the thigh. Look at moose. Moose are perfectly capable swimmers (go figure). They have hooves. But they do have enormous thigh muscles.

TLF) So are Elephants.

You mean elephants are good swimmers? Sure, but my point is that, ultimately, if a moose can swim across Turnagain Arm, foot morphology is not the biggest factor what it comes to being a good swimmer--a powerful thigh muscle is. How powerful was Psittacosaurus' thigh?

Z) How about the forelimb? According to the authors, Psittacosaurus couldn't pronate or supinate, so the palms faced medially, sort of like theropods.

TLF) This is Senter (2007).

I know it is. I've read most of his "arm bone ranges of motion" papers. They're quite good, though I do have some problems with them--just because specific joint has a certain range of motion does not mean that the living animal could access such a range. Look at your elbow or knee, for example. The osteological range of motion is greater than the living range because soft tissue gets in the way. If anything, his maximum ranges of motion are too forgiving! But the main point here--that Psittacosaurus' arms had a similar range of motion to theropods--is quite valid.

Z) It had a tiny little hand with three main digits and a vestigal Digit IV. The proportions of the fingers bring to mind basal theropods Eoraptor and Herrerasaurus, but shorter and stockier and probably stiffer. Ford & Martin suggest that the fingers were webbed, and that flexion of the first digit (which is very small) may have folded the web during the return stroke. So now we're talking about a doggy-paddle. A very bad doggy-paddle, because the palms face medially. Not even frogs doggy-paddle. So you've got a frog kick combined with a horrible forelimb doggy-paddle.

TLF) I never said doggy-paddle! Read Senter (2007) and the movement of the forelimb in basal neoceratopsians and tell me what you think the forelimbs were for. Chinnery, Sereno said they were able to walk on the forelimbs and bring food to their mouths. Senter (2007) showed that they could do neither. But his research does make for a beautiful swimming stroke.

Well, a doggy-paddle is what you get when the forelimbs are moved through the water in an arc parallel to the body. I doubt the thumb was big enough or strong enough to facilitate any kind of propulsive catch on the return stroke. If Psittacosaurus wanted to use its forelimbs to swim, it would've had to wave them to the side of the body like plesiosaurs or use an ineffective doggy paddle. Now, if it really did have sea turtle flippers, it might be able to pull of that first one...

Z) And then, the best part: "...the manus of psittacosaurs may have been held together by thickened skin (e.g., sea turtles)."

TLF) Again, Rozhdestvensky (1955), said not only the pes but the manus unguals and phalanges were dorsally flattened. Why not have a webbed hand. Oh, and by the way it was also Rozhdestvensky (1955) who said the hind feet may have been webbed.

I'm really just making fun of the choice of example. I immediately got a Mock Turtle vibe. A "mit" of skin, like a duckbill, is very different from a sea turtle's flipper. Gotta admit, it's a funny image!

Z) They even handily show a picture of a sea turtle's FLIPPER on page 334, compared to a psittacosaur paw. They look NOTHING ALIKE. Their other examples look even less like Psittacosaurus: a whale, a sea lion, and a penguin. Did you guys even look at your other figures?

TLF) Yes, the manus in Psittacosaurus is more of an inverted sea turtle’s flipper.

I mean...com'on, that's being kind. I'd also like to note here that all the animals in the example are fully marine. Wouldn't a better analogue be an otter or a freshwater turtle?

Z) Furthermore, the authors muse that the limited range of motion of the forelimb could not be used for digging or food gathering, so they must have been used for swimming. Interestingly, they cite a study by Phil Senter who played around with psittacosaur limbs to figure this out. He's come to similar conclusions about dromaeosaur arms, so maybe dromaeosaurs used their feathered arms for swimming, too and that, below all those feathers was a meat-encased flipper. So now we've got an animal who uses a frog kick and a doggy-paddle with sea turtle FLIPPERS

TLF) What, you don’t agree with Senter’s psittacosaur arm movements? But you’re okay with his dromaeosaur? What other conclusion can you come up with for the psittacosaur arm movement? They couldn’t walk on their forelegs or gather foot to their beak. You have a problem with this, talk to Senter.

No, I'm saying that if you label psittacosaurs as being semi-aquatic because of its range of motion in the arms, you COULD say the same about dromaeosaurs, but that would be silly in raptors, because they have big feathers on their arms. I have no idea why psittacosaurs would have arm movements similar to dromaeosaurs (apart from the shoulder), but I don't know if you can pin psittacosaurs as being semi-aquatic when dromaeosaurs clearly weren't, but they have the same range of motion in the (non-shoulder) arms.

Z) Now, they do raise interesting point: many psittacosaur specimens are found with gastroliths. Why use gastroliths for breaking down food when you're already doing that with your teeth self-sharpening, leaf-slicing teeth? Perhaps the gastroliths were used for ballast, as they are in crocodiles. It is unusual that Psittacosaurus used gastroliths--the only other dinosaurs with gastroliths (to my knowledge) are sauropods and ornithomimids, neither of whom chewed their food, so gastroliths would help with digestion in this case. It's also possible that Psittacosaurus ate a wide variety of foodstuffs, some of which were not sufficiently broken down by chewing alone, and so needed further gastrolith processing. They could have also been used to achieve negative bouyancy while submerged.

TLF) Did you actually read the article? Did you miss the part where I said it was Phil Currie who came up with this? No, I guess you forgot that. In fact I contacted Phil just to make sure. He said the teeth are sharp enough that they didn’t need gastroliths and HE is the one who said they used them for ballast. Have a problem with that, take it up with Phil.

I...have no response. I'm just saying that there are other explainations for gastroliths in Psittacosaurus. Sinraptor and Caudipteryx have gastroliths, too. I can see their use in the oviraptoroid, but Sinraptor? Go figure.

Z) Next, we move to the tail. The authors consider the tail to be ""long"" (their word is actually in quotes, as if admitting that, no, it's not really all that long). They also argue that the tail is quite deep. It is not. Hadrosaur tails are deep. Stegosaur tails are deep. Psittacosaur tails are not. The authors similarly note: "The neural spines are proportionately tall in all species and are particularly tall in P. sinensis. In P. mongoliensis and P. sinensis distal neural spines are flattened side-to-side, and fan-shaped.... Thus the tail may have been laterally compressed, which would help in swimming as in some modern lizards.... (or crocodiles)" Help me out here, folks: what's the ossified tendon situation in psittacosaurs? I'm not sure myself. But here's what I do know: it's just as likely that Psittacosaurus used its tail for swimming as any other dinosaur with a tail unhindered by ossified tendons.

TLF) Ossified tendons are over rated and I did an article (or at least I think I did) for Prehistoric Times. Tendons don’t stiffen they strengthen. Ok, you’ve made my argument then. Yes, Hadrosaur tails are deep which is why they were thought they swam, and maybe they did.

But if they were ossified, isn't there a risk of breakage? As for duckbills, they're not considered semi-aquatic. If anything, they just happened to be good swimmers. My point is that if hadrosaurs and stegosaurs are not considered semi-aquatic with tails like those, how can you say it about Psittacosaurus' pretty average caudal series?

Z) This is after spending the previous paragraph comparing the tail to that of a crocodile. So, just so we're all keeping track, we've got a frog-kicking, flipper-handed doggy-paddler with the deep tail of a crocodile but the compressed tail of a lizard. Clearly, Psittacosaurus was the ultimate semi-aquatic vertebrate.

TLF) Yep.

But some of those swimming modes are contradictory and wouldn't have evolved in concert with each-other!

Z) What about the nose and orbit? They are "dorsally high" and favorably compared to those of crocodilians, hippos, and capybaras.

TLF) And the problem? I did this kick-ass illustrations of different Psittacosaur skulls and compared it to a Capybara but the editor took it out due to space. Also some Psittacosaurs have a more foreword facing orbits than Tyrannosaurus rex! Reminds me of a dicynodont.

Brother, you have post those skulls on Facebook. I must see! I've noticed the dicynodont resemblence before, especially in the bulky-headed guys like P. sinensis.

Z) What about the skin? It's thick...and strong! So it probably strengthened the limbs and tail for swimming. You need thick skin to swim! Just ask any amphibian! Or lizard! Or semi-aquatic mammal like the capybara! You know what kinds of animals DO have thick skin? Fully aquatic animals. Ichthyosaurs and whales.

TLF) And you know Psittacosaurs didn’t have thick skin?

I'm saying that thick skin doesn't really scream "semi-aquatic." It may scream "fully aquatic" or "semi-aquatic in very cold waters," but otters and martins and muskrats and capybaras and beavers don't have particularly thick skin. They have insulation. Some more than others.

Z) My big long bristle rant.

TLF) This is something that I will admit is a stretch (maybe). The reason why the whole article came to place is explained in the PT article.

I look forward to reading that. I agree that you agree that the bristle-fin thing is a bit odd.

Z) Finally, the authors suggest that psittacosaurs "may have fed in lakes or rivers, perhaps crawling in the mud in search of aquatic plants...

TLF) This is Suslov. Did you really read the article?

My gripe here is with the choice of verbage. "Crawling" makes it sound like an herbivorous protorosaur or something.

Z) However, a variety of forelimb to hindlimb relative lengths suggest that some psittacosaurs were likely more terrestrial than others...." Nice save, guys.

TLF) What, I’m wrong? Did you check that cool figure of the different Psittacosaurs? Oh, right, I was wrong and they have the same limb lengths.

Not at all, your illustration is top-notch! I just felt like it was a cop-out statement.

Z) Their final comparison is with a beaver, who I guess lives in the same kind of environment that psittacosaurs are found.

TLF) Hey, I thought you said we didn’t compare them to a living animal?

The comparison was more in terms of choice of environment.

Z) So here we have a frog-kicking, flipper-handed doggy-paddler with the tail of both a crocodile and a lizard, the skin of an ichthyosaur, the tail of a salamander, and the environmental preference of a beaver. No other animal has evolved so many different, sometimes contradictory, strategies for semi-aquatic life. Psittacosaurus really wanted to get it right. Clearly, had its reign not been cut short at the end of the Mesozoic, we might well see this creature swimming the post-Cretaceous seas.

TLF) Nice, I like it. The Editor took out my illustration of this, that sucks!

Was it similar? Can we agree that the fin looks a little wonky?

Look, all kidding aside, the problem(s) with Ford & Martin's idea is that almost every argument they make is an example of false conclusion. This is the same kind of argument you see Horner making in regards to Tyrannosaurus rex being a scavenger. "It's got tiny little arm" does NOT mean it had to, therefore, be a scavenger. Plenty of hunting animals don't use their arms to hunt. "It couldn't run fast" does NOT mean it had to scavenge because it's prey was running slower than it was. By the same token, having dorsally high eyes does NOT mean you spend a lot of time underwater. Crocodiles and hippos actually have somewhat telescopic eyes (that is, the eyes are above the skull table). This is not the case in Psittacosaurus, and in fact the eyes of many dinosaurs are proportionately as high or higher on the skull than Psittacosaurus.

TLF) Then why the high eyes and nose? You don’t need telescoping eyes for a swimming animal.

Well, no, but my point is that Psittacosaurus' orbits aren't any higher than in many clearly-terrestrial theropods, or some sauropods. There are a few dinosaurs with proportionately higher orbits that haven't been accused of a semi-aquatic lifestyle.

Z) Broad feet does not necessarily imply a semi-aquatic lifestyle, either. Plenty of animals without broad feet swim (dogs, hippos, moose) and plenty of dinosaurs had strong, broad feet and are not thought of as semi-aquatic (tyrannosaurs, duckbills, ankylosaurs).

TLF) Again you have a problem with the feet see Rozhdevensky. No other, NO OTHER dinosaur has the dorsally flattened unguals and phalanges, NONE! Hadrosaurs and Ankylosaurs have broad phalanges but not fattened ones.

Awright, I'll roll with it.

Z) *rants about Larry Martin*

TLF) I did not think that was funny. You have no idea on how much time I’ve spent, read, researched, etc on this. You neglected to state that there were several other paleontologist who came up with this idea and made it look like it was us and that we are idiots.

I totally understand and apologize. Just out of curiosity, why put Larry's name on the paper at all? This could've been all you, man! Whether I think it's wrong or not, just getting published is pretty pimp.

TLF) You also didn’t mention the article that said large ceratopsians may have been semi aquatic. Description of a Complete and Fully Articulated Chasmosaurine Postcranium Previously Assigned to Anchiceratops (Dinosauria: Ceratopsia) JORDAN C. MALLON AND ROBERT HOLMES, P 189-202.

No, I did see that paper. I'm not really against the idea, but the thrust of the argument comes from paleoenvironmental association. I wouldn't say that ceratopsids exhibit any really convincing habitually semi-aquatic features. Waders, maybe, but hippo-like waders? Color me skeptical.

Z) Anyway, my opinion is that it's a poorly-researched bit of speculation on the part of the authors, and is nowhere NEAR a slam-dunk. Just my opinion, of course. Maybe you readers out there in Readerland have something different to say about it.

TLF) What, you forgot to mention the part about them being found more in a lacustrine environment then Aeolian. Must believe that part!

See above. :-)

Thanks for Tracy for providing this well-thought and well-written response to my rambling critique! You are a scholar and a gentleman, sir, though we may still get in a fist-fight someday about that whole BCF thing. I shall endeavor to make your acquaintance at SVP, perhaps in Vegas? Now, I demand you post your rejected Psittacosaurus pictures on Facebook or some similar website.

8 comments:

Dino Hunter said...

Nice, I like it. I'll get back to your comments when I have more time. My response this time will be shorter.

And as per your request, consider it done.

Dino Hunter said...

One of the things I didn't like was the editor wanted things taken out of the text that I thought was relavent; i.e. this was taken out about gastroliths There is another possibility in that Psittacosaurus was omnivorous and not just herbivorous. If they were omnivorous than a swimming Psittacosaurus with self-sharpening teeth could not only eat small vertebrates but also fresh water/terrestrial plants. If so then there is the possibility that the gastroliths were used in digestion and ballast.

And a table of Psittacosaurs with gastroliths.

Psittacosaurus mongoliensis = Protiguanodon mongoliensis OSBORN, 1923 Holotype: AMNH 6253: Fragmentary skull, dentary, and skeleton.
112 gastroliths. Mount Ussuk, Tsagan Nor Basin, Hovd Province, Mongolia.
Psittacosaurus mongoliensis SERENO, 1987 AMNH 6544: Fragmentary
skeleton including gastroliths Red Mesa, Artsa Bogdo basin, Ööshinn Nuur (Oshih Basin), Northern Gobi, Mongolia.
Psittacosaurus sp MAYR, PETERS, PLODOWSKI, & VOGEL, 2002
SMF R 4970: Nearly complete specimen with gastroliths, skin impressions and fiberial spines.
Unknown, possibly from Sihetung near Beipiao City, Liaoning Province, China.
Psittacosaurus sp BEHRENDT, 2006 Number: Not given: Skull and skeleton. Note: Gastroliths. Western Liaoning Province, China.
Psittacosaurus mazongshanensis XU, 1997 Holotype: IVPP V 12165, fragmentary skull and skeleton with gastroliths. Western Liaoning Province, China.

Jaime A. Headden said...

Tracy, I don't have the paper, so I am interested. You cite the dorsally flattened unguals as aquatic adaptations. What is the citation for this hypothesis?

Matt Martyniuk said...

I'm also curious about the flattened unguals. What evidence is there that this is an aquatic adaptation rather than one for say, a fossorial lifestyle as has been (IMO more convincingly) argued for Psittacosaurus?

Dino Hunter said...

Flattend unguals

Rozhdestvensky, A. K., 1955. New data on Psittacosaurus-Cretaceous ornithopods. In Questions on the geology of Asia, 2: 783-788.

Its in Russian.

Jaime A. Headden said...

Rozhdestvensky is wrong. Sorry. If this is the source, and no further backup on data regarding unguals in aquatic amniotes has supported this (none that I've read; I asked the above question for the purpose of seeing if this wasn't just taken from Rozhdestvensky), then I can confidently say that the shape of the dorsal surface of an ungual has NO influence on an animal's ability to swim, nor does it present in a reasonable majority of aquatic amniotes. It is lacking, for example, in terrapins, any lacertilian really, and virtually all mammals. Otters, I think, have been shown to have slightly dorsally-flattened unguals, but this is a horribly narrow exception.

Jaime A. Headden said...

Oops, spoke too soon. Some pedal unguals in large theropods, such as carcharodontosaurine allosauroids, are dorsally flattend _somewhat_. The unguals are otherwise generally identical to other allosauroid pedal unguals in morphology. Slight dorsal flattening also seems to occur in some other large theropods, as may be incipient in form in tyrannosaurids. But there's a distinction to be noted here: what actually causes "dorsal flattening" in these unguals? In some taxa, such as psittacosaurs, the lateral grooves are high and the ventrolateral and -medial corners are broad and pointed, and resemble nothing so much as ornithomimid manual unguals; the position of the grooves means that there is limited space between then on the dorsal margin of the ungual, which MIGHT simply be reduction of an arc into a "flat" aspect, whereas it may otherwise be rounded. In other words, flattening is an artefact of an abbreviated arc.

David Marjanović said...

TLF seems to think you're arguing with him, when in fact you're arguing with his paper.