Ameridelphia

Ameridelphia is a historical and informal term once used in mammalian taxonomy to refer to a group of New World marsupials. Traditionally, it encompassed the American orders Didelphimorphia (opossums) and Paucituberculata (shrew opossums), and in some older sources even included Microbiotheria. The idea was that these American marsupials formed a distinct lineage separate from Australasian marsupials.

In contemporary systematics, Ameridelphia is not recognized as a valid clade. Molecular and morphological evidence indicates that Didelphimorphia and Paucituberculata do not form a single evolutionary lineage, and in many schemes Paucituberculata is placed as related to or sister to Australidelphia rather than to Didelphimorphia. Microbiotheria, which includes the monito del monte (Dromiciops gliroides) from South America, is now considered part of Australidelphia, further undermining the idea of a cohesive Ameridelphia. As a result, the term is largely abandoned in modern classifications.

The terminology reflects geography rather than solid evolutionary relationships, and today it is largely of historical interest. Most current classifications divide marsupials into Australidelphia (Australasian marsupials and Microbiotheria) and non-Australidelphia or didelphimorphian lineages (the latter sometimes represented by Didelphimorphia and Paucituberculata in older literature). Because of ongoing revisions and differing data sets, some sources may still discuss Ameridelphia, but it is not treated as a valid natural grouping in most contemporary taxonomies.