What exactly is it meant by "gracile"? Why are so many people using this word when it comes to describing dinosaur legs? And how come people keep saying T.Rex had extremly "gracile" legs, even more so than the raptors! How is this possible? What implications does this have for T.Rex or the Tyrannosaurids in general?
from C.E.R.A.T.O, age ?, ?, ?, ?; July 24, 2001

TOM: "Gracile" means "lightly built" or "slender"; its opposite is "robust" (heavy, sturdy). In the modern world, for example, the legs of wolves and cheetahs would be considered "gracile" in comparison to the more robust legs of lions or leopards.

I don't think anyone would refer to ankylosaur legs as gracile (;-), but in general dinosaurs had relatively long slender legs compared to other big reptiles. Tyrannosaurids in particular had very long and slender legs for their size: young tyrannosaurids (or adults of smaller species) had legs with precisely the same proportions as those of ostrich dinosaurs (ornithomimosaurs). In fact, when the first tyrannosaur legs were found they were thought to have come from a new giant species of ostrich dinosaur!

For a given body size, tyrannosaurs and ornithomimosaurs had longer shins and metatarsi (long bones of the feet) than other meat-eaters of the same size. In contrast, dromaeosaurs (raptors) actually had relatively short feet compared to other theropods of the same body size. This suggests that tyrannosaurus and ornithomimosaurs were faster than average meat-eaters of the same body size, and that (Jurassic Park notwithstanding) dromaeosaurs were slower (although probably very agile).

This suggests that tyrannosaurids may have been built to run down their prey after a bit of a chase, while raptors may have been more ambushers. Modern comparisons would be a wolf for tyrannosaurs and a lion or tiger for raptors.