This is the part where I beg my readers for help, kind of like how I did back with Nyctosaurus and Dimorphodon. This time it's for therizinosaurs. I really appreciate all the comments in the previous post. But I now have plenty of other questions. Let's see what kind of answers I can get!
1) Plantigrade vs. Digigrade: Make your case(s), folks. Andrea Cau's post on the subject, and the supposed therizinosaurs footprints are very convincing. However, Qilong makes some good points, too. Honestly, I'm having a hard time drawing a plantigrade, standing therizinosaur. There's no "heel," for one thing. Plantigrade mammals have nice big heels, but dinosaurs didn't. And when you think about it, a giant, 40-foot, 3-ton, big-bellied herbivore walking around on two plantigrade feet makes about as much sense as that same animal walking around digigrade, forever risking toppling forward because the majority of its bulk is in front of its feet. I know that knuckle-walking isn't actually an option, but it almost seems like therizinosaurs should be quadrupeds somehow!
2) In all of your humble opinions, do you think the massive, grown-together pubis/ischium complex is indicative of a sitting habit, as is the case in chalicotheres?
3) Exactly how ridiculous should I make the back on a standing therizinosaurs? Many of the illustrations I've seen, including skeletals, show the dorsal vertebrae trending almost straight upwards in front of the pelvis. This is especially apparant in the most recent description of Nothronychus. It just doesn't look natural!
4) Given that therizinosaurs are basal maniraptors, do you think their arms and hands had the same amount of "folding" that, say, oviraptors and paravians demonstrate? Or better yet, did early representatives have that, like Falcarius, but later therizinosaurs lost it?
5) In more derived therizinosaurs, does the hallux actually contribute to the footprint? That is, are derived therizinosaurs actually four-toed?
6) The "feathers" of Beipaosaurus look a little bit like quills. Do you think that larger therizinosaurs would have retained some of these quill-like feathers, perhaps for intimidation? I can certainly see the advantages to ruffling up a bunch of quills and making yourself look larger and spiney!
That is all. Please post your thoughts in the comments! These animals have very strange skeletons!
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7 comments:
Just because there wasn't a heel comprised of oversized, mammal-style tarsals, and just because the animals had the common sense not to walk on their metatarsals, doesn't mean that there wasn't anything back there supporting the caudal side of the foot behind the digits! Look at elephants, for example - huge, thick, callouses and masses of fibrous tissues for pads actually make up MOST of the foot as it contacts the ground. Even bigger ground birds have pretty substantial metatarsophalangeal pads, and I strongly suspect that that's the explanation for the morphology of the therizinosaur tracks...not plantigrady.
I vote "no" on sitting habit - sitting would have been similar to birds, not chalicotheres.
The elongate presacral region and shortened tail will mean, I think, that the posture cannot be completely horizontal as in most theropods - maybe if therizinosaurs were Groucho-walkers, but I don't think they were (not that I've looked into it at all...) But I don't know much about what their posture actually WAS...
Yes, derived therizinosaurs were functionally tetradactyl (in the pes).
Me likey the fluffy therizinosaurs! Giant Roadrunners of Doom!
From a purely aesthetic standpoint, I like quills on my "Theries". Though I also imagine them with a loose coat of long, fluffy feathers like an Emu.
/ Almost types "Emo" three times. <:/
Therizinosaurs had both protofeathers and quills. How much protofeathers were retained in a large therizinosaur is anyone's guess, but I would suspect they kept their quills, which seem to be, if anything, better for display than insulatory.
And, yes, derived therizinosaurs were effectively four toed.
I'm afraid I can't add anything else to the discussion...
Its quite possible that therizinosaurs walked digigrade, then reverted to a plantigrade stance when they ate, sort of a nice medium between fully sitting and standing while eating. However, I don't think it seems likely that therizinosaurs walked around plantigrade. Not only does it lack a heel, but most plantigrade animals tend to have very short metatarsal bones. Therizinosaurs don't, except in the largest species, but that's more of a size thing anyway. In fact, the only plantigrade animal with a long foot I can think of is humans, and they have a rather strange foot anatomy with a gigantic heelbone anyway.
Some scientists think that the big pot-belly of the therizinosaur actually helped it maintain a bipedal stance, by shifting its center of gravity further back on the animal. this would combat the problems of its falling over.
I don't know about the pubis/ischium complex though, and any implications it would have for sitting behavior. Nor do I have any idea on how therizinosaur hands could fold.
As it turns out, therizinosaurs' spines did curve upward like that. While not fully erect, the spine did curve at about a 45 or 60 degree angle from the hips (not sure which, just know it wasn't horizontal, and not erect). Paul actually did some analyses of spinal posture in therizinosaurs. This is the paper...
Body and tail posture in theropod dinosaurs. The Carnivorous Dinosaurs, 2005
I wouldn't be surprised if Therizinosaurus proper retained some feathers despite its size. I mean giraffes have short hair despite their size, same with moose and bison. Even elephants redeveloped long hair when they went into colder regions. Perhaps it would be something more like an ostrich or something, with a "cape" of feathers on the head, upper back, and arms.
thank you
1. Digital morphology is characterstic of other digitigrade theropods, including tyrannosaurs but also smaller animals like birds. The interesting thing about therizinosaur toes is that they are very slender, and this has a lot to due with the skewed weight distribution across four toes, rather than three, meaning that the second and third toes are bearing weight almost evenly, rather than being skewed towards the third digit.
Metatarsals are, while broad, still cylindrical; and again, while some of the metatarsals are not appressed directly to one another, they likely did contact one another along their length, unlike mammals. Similarly, they resemble "prosauropod" metatatrsals just enough to support the claim of digitigrady by comparison, as tracks associated with prosauropod remains also imply they were digitigrade, and thus that their similar morphology should relate to similar function.
2. There is no known study incorporating the Erlikosaurus-type pelvis into a morphometric study. Note that the pelvis of chalicotheres, and that of megatheriine megatheriid sloths, are extremely opisthopubic and support a verticalized posture as an extension (as in humans) of their static posture when feeding (or doing a lot of other things). Note also that in sloths, and to an extent some chalicotheres, the ilia are very broadly splayed. These features have not been studied.
3. The anterior angle of the dorso-sacral contact is based on nanshiungosaurus bohlini, which preserved an angled junction between the two. This has been applied universally to other sacra in therizinosaurs by Paul. I've not seen any evidence showing the angle is so extreme, especially in some of the more recently-described taxa. Segnosaurus also preserves a sacrum, but it is not mentioned whether the angle is so distinct.
4. This is a gradational issue: basal therizinosaurs like Falcarius and Beipiaosaurus show more folding capability than does Therizinosaurus. One should not infer the capability of one wrist to any others without manipulating or approximately do so the wrist itself (see Senter's work on direct manipulation as extreme and median levels of movement). The wrist in the basalmost taxa appear to be able to come to about 90 or so degrees, but that's based on Beipiaosaurus, and the angle may be less.
5. Probably, if the first metatarsal is correctly placed. The first metatarsophalageal joint, or first "knuckle" of the first toe comes low enough to the end of the second that if the foot was placed on the ground, the first toe would extend outward (and it was not reversed) such that it would need to be extremely flexed to escape the ground, or contribute in the step. Its robusticity seems to support this, but the foot bones have not been subjected to microscopic study to determine if the tissues relate to weight-bearing bone.
6. "Quill" is pushing it. The feathers appear to be flat and very stiff, and sparsely placed throughout the body. If Beipiaosaurus is an herbivore, these structures may be defensive.
On the knuckle walking thing, I believe there was one that suggested that the largest therizinosaurs could in theory use their finger to do such, but of course that isn't widely accepted
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