Ancient Sea Scorpion was Bigger than a Human

http://news.yahoo.com/s/nm/20071121/sc_nm/scorpion_fossil_dc_1

This is as critter akin to a lobster. They found a fossilized claw that put the whole creature at about 8 feet long. Probably about the size of a big bull sea lion. How scary would that be? For the answer, see the inevitable thriller about the people who accidentally run across a group of these things that are still around.

One of the problems with monster SF is that things with exoskeletons don't usually get very big (gravity, inefficiency with exterior supports, the scaling of surface area, and the amount of energy to create a big exoskeleton). The article mentions the possibility of higher O2 concentrations in the air. Also, these were sea creatures, so they have buoyancy to help counteract gravity.

Now imagine what sorts of creatures could evolve on a low gravity planet with high O2 levels. Throw in some stranded astronauts and the story writes itself.