I know that the answer to the question "Which came later, the last common ancestor, or the marsupials?" is in "The oldest known fossil among the Eutheria ("true beasts") is the small shrewlike Juramaia sinensis, or "Jurassic mother from China", dated to 160 million years ago in the Late Jurassic. A later eutherian, Eomaia, dated to 125 million years ago in the Early Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage. In particular:". Can you tell me what it is?
The answer to this question is:
the marsupials