Let's say you have two mammals who are the only two in their Order to share a primitive feature on their molar. Does that in and of itself suggest an affinity between them?

It is a strong hint, but not definitive proof. There are lots of ways that a primitive trait can be lost and later(perhaps much later) re-acquired.

Let's say you have two mammals who are the only two in their Order to share a primitive feature on their molar. Does that in and of itself suggest an affinity between them?

 

It does suggest an affinity, but it's not enough. Phylogenetics trees are built with many characters, not a single one.

It does suggest an affinity, but it's not enough. Phylogenetics trees are built with many characters, not a single one.

 

Well, obviously. I was just wondering if sharing primitive features was one of the characters that could be used to build a phylogenetic tree.

Well, obviously. I was just wondering if sharing primitive features was one of the characters that could be used to build a phylogenetic tree.

 

Yes, but the thing is; phylogenetic trees are hard to built, you have to be careful about the number of species and the number of traits. For only 20 species, there's ~8 000 000 000 000 000 000 000 possible rooted trees.