That would at least make some sense, but how do you tell what the critter ate and how it raised its young when all you have is <50% of a leg bone?
Reconstruction can be done very easily by a leg bone. How it's shaped can help to determine whether it was a carnivore, or herbivore. Whether it was a biped or quadriped. Mammal, Bird, or Reptile. If you have a leg, you can determine it's hip structure, and so on from there.
Teeth are also some of the most abundant fossiles found, given that their some of the most "surivable" parts of the body. Using teeth make it very easy to figure out what it ate. How it's jaw and head where built can easily be reconstructed from that as well, and then you go from there. Once you get a general idea of the type of creature that it was, you can begin to piece it's social life together by compairing it to other species (living or dead) it shares similiarities to, or other clues that future fossile finds may divulge.
It's anything but an exact science, but it is based on solid guess work. Sometimes new finds come along that will disprove the original theory, and it has to be reworked, but that in my opinion is the greatest part of anthropology. Constant learning.