A subclass of mammals lacking a placenta, including marsupials and monotremes; formerly used as a taxonomic classification.
From 'aplacental' plus the Latin plural suffix '-ia.' This was a formal taxonomic term used in 19th-century zoology, now largely obsolete due to molecular phylogenetics.
Scientists once thought 'aplacentalia' was a natural group, but DNA revealed that placental mammals are actually more closely related to some aplacentals than to others—taxonomy got revolutionized!
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