I have a question, looking at this and similarly-shaped dinosaurs. They look quite heavy, and yet their feet/legs are always so small and delicate. How did they support their weight like this for extended periods of time?
Now, this one I actually do have a hard time seeing balanced, like that. Wouldn't it have to be even more vertically posed, so not to fall on its face? The tail seems to offer next to no counter-weight, even the belly is positioned in front of the pelvis, and would pull it forward, at least from what it looks.
It helps to see it in 3D (like in a skeletal mount). The belly is widest just in front of the pelvis, and the guts continue into the pelvis on these guys. As a result the CoG should be almost exactly where the front foot is placed (remember it's walking, so it's not in perfect static equilibrium).
that analogy makes a lot of sense to me- I believe that ground sloths, therizinosaurids, and perhaps prosauropods follow a very similar body plan due to fulfilling the same ecological niche. I wonder if someone has taken the time to name such specific niches?
In short...no. Like all digitigrade animals they could probably crouch on their ankles if they wanted to, but there's no way they could take a normal stride like that; the knee would have to be dramatically modified, and having seen several I can say they aren't.
It's still a matter of some debate, but having seen the specimen up close the pelvis is modified in countless ways to accommodate such a pose...but the simplest way to convince yourself is to just picture the animal with its back horizontal...it would fall flat on its face without the legs protracted so far forward that it couldn't really walk (although it may have needed to adopt such a position to drink water).
Very cool. I'd note that it looks as though you've restored the femur as though it were strictly vertical, rather that laterally splayed as the femoral caput and the clearance required for femoral protraction indicate. Is this just standard form, or am I under-evaluating your restoration?
In this case you are actually underselling it; I mounted a cast last spring and I measured the perspective in (the femur in these guys splays at something like 30-35 degrees). I should really do an anterior and dorsal view if I get the chance (by which I mean...early winter at the earliest).
The same is actually true of more theropods than most people realized. I figured this was so, and have illustrated femoral posture with a degree of foreshortening due to positioning the femur with the caput level within the acetabulum. The tends to position the femur outwards. It's exagerrated, for example, in microraptorians as they not only have a slightly elevated caput but also a laterally curved femoral shaft (i.e., the femur in total is slightly S-shaped in cranial view).
I have to completely disagree with the microraptoran's comment. I still feel that the Hwang paper shows the only undistorted femur, and the femoral head is clearly typical of other dromaeosaurs and in no way offset: [link] (with interpretive lines here: [link] )
The other specimens that supposedly show an upturned femoral head are all the result of compaction as the femoral head is compressed.
In Nothronychus, on the other hand, the life of the femoral head is angled nearly 45 degrees from the long axis; it's comepletely unlike any non-therizinosaurid dinosaur specimen that I've seen (both femora show the same angle).
"Ingenia" yanshini actually comes close with a very heavily offset caput, and includes uneven distal extensions of the femoral condyles (the fibular condyle is FAR distal than the tibial one).
As for microraptorans, we have virtually no other femoral capiti to compare to, much less remark on a trend that excludes taxonomy. I understand that some distortion indicates the caput is adversely elevated, but not to what extent, and the taxa in which we have are different from one another, some being seemingly more terrestrial than others. This change in habitat should alter the expectation of behavior of the femur, both attitude and range of movement. I agree that the Hwang et al. Microraptor zhaoianus femur is relatively secure, close, but it is also slightly crushed and this leads us to an issue that it also has a slight elevation (maybe 5 or so degrees).
It's actually more weird than you realize. The pubes and ischia are not united like in normal theropods (a condition that is partially evovled in primitive therizinosaurs like Falcarius) so the guts can actually sit in between them. Also, the front side splays outward to a truly shocking degree. They almost look like bipedal ankylosaurs. I'll try to remember to post a photo when I get a chance.