The most recent common ancestor is the most recent individual from which both species are descended. The points in which the branches meet shows us points in which there were important ancestor, and the characteristic that is written is the one that appears for the first time there. So, the point in which the branches for Reptiles and Rodents meet can be found where we can read the "Amniotic egg" characteristic, and that's where the most recent common ancestor would be for Reptiles and Rodents.
