The astragalus is one of two major bones in the archosaurian ankle (and indeed the ankle is rather important when it comes to archosaur phylogenetics). The astragalus and calcaneum are generally two fairly small and blocky elements that lie between the tibia and fibula and the metatarsals. In the theropods, the astragalus has an ascending process – a flat, triangular plate of bone that sits in front of the tibia and actually sets into a recess of the anterior face of the tibia. (This is also seen in basal saurischians, but it’s not as exaggerated as in the theropods). In some cases it can be really big, reaching a considerable distance up the face of the tibia (as I recall, up to about a third of the way up).
This is a nice example, a Tyrannosaurs astragalus with the subtriangular ascending process clearly visible and set into (and indeed fused to) the tibia. This is one of those classic little quirks of anatomy that, if noting else, provide and excellent little systematic feature – if you find even a small fraction of a skeleton out in the field and it has this, then you know you have a theropod on your hands.
Dave Hone's Archosaur Musings 
and it is one of those excellent little biomechanical features: if you see such a process you know you have a hinge-like ankle joint on your hands, instead of one that allows rotation around the vertical axis. And that automatically means an upright (erect) limb posture, because a sprawling gait forces rotation in the ankle, not just flexion and extension.
Good point, thanks for sharing Heinrich!
Of all the obscure bones in archosaur anatomy this one is well known to a amateur like me.
In Australia our only large predatory dinosaur was known from a single Allosauriod astragalus find over 30 years ago.
As it turns out, it was nicely diagnostic and a more complete specimen was described this year: Australovenator.
So for my entire life we hung our reconstructions off this single diagnostic bone, often described as belonging to a ‘Small Polar Allosaur’.
Interesting, I had heard of the ‘small polar allosaur’ but didn’t realize it was the same thing as Australovenator.
I’m pretty sure they have Australovenator’s for comparison and thats has been the assertion.
Yeah, for a couple of decades there Astragalus and Allosaurus were pretty much synonymous.