Even if Sharptooth had hit a ledge (whether such an impact would be fatal or not), the distance he fell before he disappeared from sight was more than enough to realistically kill him. (I don’t know how this might have affected the outcome of his fall, but he was also struck by a large boulder on the way down, which caused him to change position mid-fall).
In real life, it’s believed that Tyrannosaurus rex was so large that even simply falling over could have been dangerous for it. It’s been estimated that a full-grown T. rex that tripped while running 40 miles an hour (a speed it may not have even been physically capable of), the resulting impact would have cracked its skull, broken its neck, and crushed its internal organs. By that measure, every sharptooth in LBT has been put through far worse than what a real T. rex could survive. :blink:
As for the scene in the cave, it’s possible that Ducky remained hidden behind that rock longer than was shown (i.e., after she screamed to get Sharptooth’s attention and crouched down to hide, the film cut directly to the shot of her cautiously looking over the rock, which actually occurred several seconds later). It might not seem like a good idea to curl up behind a relatively obvious hiding place for any length of time, with your eyes closed so that you can’t see the huge, dangerous creature you’re trying to entice to come to you. However, it was a natural reaction given to how terrified Ducky clearly was at that time, and if Sharptooth had simply turned around and starting walking towards her, one would expect that the you would expect that the increasing sound and vibrations of his footfalls would have been enough to warn Ducky that he was coming. She wasn’t expecting him to first go farther away from her, and then take another route back towards the cave entrance. (She wasn’t alerted by any sounds of Sharptooth approaching from behind, though; apparently he can be quiet when he wants to.)