(referring to the ischial boot)
Ornithomimids have it, tyrannosaurids don't. Not the distribution one
expects of a size-related character.
There is supposed to be an ornithomimid that lacks an ischial boot. And the
character is subject to individual variation in _Tyrannosaurus rex_, the
boot is absent in CM 9380 and AMNH 5027 as well as most other tyrannosaurid
specimens, however a boot is present in TMP 81.61, as well as a knob
posterodistal to the ilioischial peduncle. Is it possible that this is a
proximodorsal process, the structure found in _Bambiraptor_,
_Archaeopteryx_, _Unenlagia_, etc.?