How do evolutionists construct descendency trees from fossils?

[Al-Layth]

Looking at the result of a great number of fossil expeditions, I understand how you can construct the full or partial skeletons of a number of different animals, you would be able to certainly identify certain bones as belonging to a distinct animal. That's great. you now know there were these very similair looking animals with slight variations and you have a collection of their skeletal models.

My problem is, how do you know which animal descended from which?
Determining the age of the bones cannot be enough because its possible multiple species existed at the same time.

And from what i've read, the anthropologists of the past have made many mistakes as regards to determining descendancy as well.

So what actually are the current best methods for determining the correct descendancy tree from a collection of animal skeleton you constructed from the discoveries from a number of fossil expeditions?

 

[BillTre]

There several methods for doing this.
Cladistics is one of the better known methods.
It is a fairly large complex subject.
Here is a wikipedia article on cladistics you might want to read.
These methods can be used for physical traits as well as molecular traits and can be applied to current as well as extinct species.