Simplify the following text for a Grade 9 native English-speaking person.
Original: Marsupials present in South America included didelphimorphs ( opossums ) and several other small groups ; larger predatory relatives of these also existed , like the borhyaenids and the sabertooth Thylacosmilus ( sparassodont metatherians which are no longer considered to be marsupials ) .
Simplified: The borhyaenids and the sabertooth Thylacosmilus are no longer considered to be marsupials . They are sparassodont metatherians , the sister group of the marsupials .