can my players use their shapechange spell to turn into paracritters, in third edition, and if they can do they get all the critter powers? For example can they turn into a cockatrice and paralyze their enemies?

i don't think so.
if i remember correctly, you only get the body of said critter, not all of it's abilities . . so with a winged body, you'd be able to fly, but if you changed into the body of a critter that can fly without wings you won't be able to do so . . if you turn into a hell-hound, you will bite like a dog, but not be able to breath fire . . of course i can be wrong on pretty much all of those points, as it's been some time since i tinkered with a gargoyle shaman . .

Shapechange can only be used to turn into normal, non-para critters. Its in the first line of the spell description.

Thanks for pointing that out

NP.

There was also a discussion just recently about whether or not dinosaurs should be considered "normal" critters. I came down on the side of extinct animals, particularly long extinct animals, not being normal. Your mileage may vary.

I guess I come down on the other side as I see the definition as normal vs para-normal, not normal vs paranormal, extinct, exotic.

The only way I might be swayed is if the persons world view was that prehistoric/extinct animals did not really exist, but are only fantasy creatures seen in the geological record - akin to how some stars align themselves in the heavenly skies and acquired names.

My theory has to do with the living memory of the Gaiasphere. Paranormal animals are too new to be mimicked with magic, extinct animals are too old. I also like the thematic element of an extinct animal being "forgotten", like the planet loses something when they're gone.