Therizinosaurs are, in my opinion, the strangest dinosaurs. I covered the group in some detail back on the original blog, and Brian did a wonderful job of summarizing the group and discussing its newest member. I've been thinking about these hulking beasts for quite some time, specifically, how they got around. You see a lot of illustrations of derived therizinosaurs standing almost-but-not-quite upright, the majority of their substantial weight far in front of their legs. It looks almost comical--it's as if the animal is trying very hard to avoid falling flat on its face. I have also seen the occassional restoration (Luis Rey comes to mind) of a derived therizinosaur standing in usual theropod posture, horizontal back, arms folded to the side. This posture is ridiculous for two reasons. First, advanced therizinosaurs like Erlikosaurus and Therizinosaurus had arms which functioned more like ornithomimosaur arms than dromaeosaur arms. A relatively new study by Lindsay Zanno suggests that therizinosaurs had an increased dorsal reach and wrist flexibility, as well as a reduction in individual finger length. The arms also became robust the muscular. The claws, which weren't so much sickle-shaped as weakly curved, are also analogous to ornithomimosaur claws, although obviously much longer.
The second reason is that, due to their expansive guts, enlarged arms, and short tails (which, in Beipiaosaurus at least, ended in a pygostyle), a therizinosaur's center of gravity would have been closer to the pectoral girdle, rather than the pelvis. In most theropods, the bipedal posture works thanks to the long, heavy tail, and short body which lacks an herbivore's stomach. The center of gravity of, say, Allosaurus, is just in front of the hips. The reason a running allosaur does not fall onto its face is because the large tail counterbalances the body.
Sadly, this is not the case in therizinosaurs. We know that these Cretaceous giants had enormous cuts for several reasons. First, their pubes point strongly backwards, and the pelvis itself is ridiculously wide. While the pubes and ischia meet at their distal ends, there is a significant space between the left and right halves. This condition is seen in other herbivorous dinosaurs. Also, in a feature unique among all dinosaurs, therizinosaurs re-evolved pedal digit I to not only touch the ground, but bear weight. In other theropods, pedal digit I is a vestigal organ, which a splint-like metatarsal that does not even reach the ankle. The vestigal toe itself is something of a dewclaw. In birds, this toe became reversed, moved down on the foot, and became the seminal "reversed hallux." Therizinosaurs enlarged the 1st toe, regrew their metatarsal, and, soon after Nothronychus, became functionally four-toed. This is obviously an adaptation for carrying weight.
While I'd like to see his reference material, Jaime Headden has put together a wonderful visual summary of known therizinosaurs (minus Falcarius and Suzhousaurus). Of the known genera, Alxasaurus, Segnosaurus,a nd Neimongosaurus are the best-represented. The best skull material is from Erlikosaurus (and Falcarius), and Nanshiungosaurus. But what really fascinates me about the therizinosaurs, and this relates directly to their posture, is their extremely strange pelvis. Falcarius, Beipiaosaurus, and Alxasaurus are the most primitive members of the group, so their pelves are a bit closer to the ancestral theropod condition. Segnosaurus and Nanshiungosaurus preserve the best pelves, and here's a better look:
A and B detail the pelvis of Segnosaurus, a reasonably derived therizinosaur. What blows my mind is not the width of the pelvis or the retroversion of the pubis, but the flared iliac blades! Those things jut outward at, according to The Dinosauria, 2nd Ed., a 90-degree angle to the line of the sacrum. How, exactly, did the thigh muscles attach to that ilium? It boggles the mind, and I never know how to restore therizinosaur legs when I attempt to draw the beasts. Even stranger, though, is Nanshiungosaurus, whose pelvis looks reversed and broken, somehow.
Anyway, I hope I've shown that a strictly bipedal posture seems...impractical for advanced therizinosaurs. A quadrapedal posture seems more likely, but again, we have a problem. First, complete arms from derived therizinosaurs are virtually unknown. Segnosaurus and Erliansaurus are perhaps the best examples outside of Therizinosaurus. See, what we're trying to do is see the arm vs. leg proportion, which is why Therizinosaurus' complete arms are unhelpful--its legs are pretty much unknown. Beipaiosaurus actually has incredibly long arms, but it's also fairly primitive, so long arms could have simply been inhereted from its maniraptoran ancestry (although Falcarius' arms aren't ridiculously long).
What I'm getting at is that perhaps, like chalicotheres, therizinosaurs walked on their knuckles. I know that sounds crazy at first, but there are some surprising similarities. First, both advanced therizinosaurs (if Therizinosaurus is any guide) and chalicotheres had fairly long metacarpals followed by short, stubby, and robust phalanges. In chalicotheres, only two fingers were weight-bearing, and were the same length. Although it can't be known for sure yet (incomplete material), it looks like the 2nd and 3rd fingers of derived therizinosaurs were approximately the same length. Both animals had adaptations for reaching and wrist-twisting to reach vegetation and manipulate branches. Therizinosaurs also converged on chalicotheres (or, perhaps the other way around) in having large, robust ischia to support the weight of the body in a sitting position.
However, chalicotheres a significant difference over therizinosaurs: the legs are incredible short, while the arms are incredibly long. In therizinosaurs, as far as I can tell, the legs are longer than the arms. In some cases, the arms and legs may have been subequal in length. However, therizinosaurs certainly did not even begin to approach the proportions set by chalicotheres. But there is another knuckle-walking mammal that had limb proportions similar to those of therizinosaurs: ground sloths. In fact, ground sloths may be a more fitting analogue. Ground sloths had flaring iliac blades and enormous ischia for sitting, after all. They also had long, uncurved claws for scratching and pulling on vegetation.
But ground sloths, too, have their differences. For one thing, they are incredibly robust in all aspects of the skeleton. The limb proportions are a bit more toward the therizinosaur side of the equasion, but the forelimbs are still a bit longer than the hindlimbs. Like modern sloths, though (and chalicotheres!), ground sloths would have been knuckle-walkers, with the ouside of the hand facing outward. This is unlike the knuckle-walking of great apes, in which the front of the hand faces forward. Given the maniraptoran nature of the therizinosaur paw, the same motion would have been true of those Cretaceous beasties. The hands were probably unable to swivel into a gorilla-like posture, and placing the palm flat on the ground is completely out of the question, given the structure of the wrist as well as the sheer weight of the animal (I can just hear those bones snapping!).
I've been trying for days to draw a knuckle-walking therizinosaur, but the whole pelvis area is killing me. If somebody can tell me how the muscles of the thigh attach to a flared ilium, I would appreciate it. At any rate, this is an idea that I've been thinking about continuously for a few weeks now. I should mention that Russel & Russell thought up the chalicothere angle in 1993, a fact I learned only after beginning to seriously research my knuckle-walking hypothesis. There's nothing more disheartening than learning that the idea you thought was so new was already proposed almost fifteen years ago. If anyone has that paper, by the way, feel free to send it my way! :-) The full citation is:
Russell & Russell (1993). Mammal-dinosaur convergence. Evolutionary convergence between a mammalian and dinosaurian clawed herbivore. National Geographic Research (9): 70-79.
In the meantime, I'll keep working on my knuckle-walking therizinosaur.
Pictures of Chalicotherium and Megatherium taken from Wikipedia. Jaime Headden's therizinosaur panoply taken from...the internet. Therizinosaur pelvis diagram taken from The Dinosauria, 2nd Ed.