Simplify the following text for a Grade 11 native English-speaking person.
Original: Multituberculates are usually placed outside either of the two main groups of living mammals -- Theria , including placentals and marsupials , and Monotremata -- but some cladistic analyses put them closer to Theria than to monotremes .
Simplified: Multituberculates are usually placed outside both the two main groups of living mammals , the Theria ( placentals and marsupials ) , and monotremes . Some cladistic analyses put them closer to Theria than to monotremes .