Paratheria (mammals)

Paratheria is a deprecated infraclass of mammals that historically grouped the monotremes (egg-laying mammals) with extinct groups that exhibited some similar characteristics, particularly in their dentition and jaw structure. The term was used primarily in the late 19th and early 20th centuries to distinguish these mammals from the Theria (marsupials and placentals).

The central idea behind Paratheria was that it represented an early, distinct evolutionary lineage of mammals separate from the lineage leading to marsupials and placentals. It suggested that monotremes and various fossil groups shared certain primitive traits, indicating a closer relationship among them than to other extant mammals.

However, the concept of Paratheria has largely been abandoned due to advances in phylogenetic analysis and a deeper understanding of mammalian evolution through both fossil evidence and molecular data. Modern cladistic analyses consistently place monotremes as the sister group to the Theria (marsupials and placentals) within the Mammalia. This revised understanding demonstrates a more direct evolutionary link between monotremes and other mammals, invalidating the need for a separate infraclass like Paratheria.

The extinct groups once considered part of Paratheria have since been reassessed and placed in different positions within the mammalian evolutionary tree. Some may be considered early mammaliaforms or basal members of other mammalian lineages, but are no longer united with monotremes under the Paratheria umbrella.

In summary, while historically significant for its role in early attempts to classify and understand mammalian relationships, the term Paratheria is now largely obsolete in modern mammalian taxonomy and phylogeny.