The borhyaenids and the sabertooth Thylacosmilus are no longer considered to be marsupials . They are sparassodont metatherians , the sister group of the marsupials . 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 ) .