First proposed by Thomas Henry Huxley in 1880 , it is nearly synonymous with the earlier taxon Marsupialia , but it also contains the nearest fossil relatives of marsupial mammals . First proposed by Thomas Henry Huxley in 1880 , it is nearly synonymous with the earlier taxon Marsupialia ( Illiger , 1811 ) though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals .