Placentoid

Placentoid is a term used in biology and paleontology to describe organisms that possess a placenta or placenta-like structure. The word combines Latin placenta, meaning “flat cake” or “placenta,” with the suffix -oid, meaning “resembling.” In common usage, placentoid can refer to placental mammals or to lineages characterized by placentation, but it is not a formal, current taxonomic group in modern classifications.

In mammals, the placenta is an organ that connects fetal tissues to maternal blood supplies, enabling nutrient transfer, gas exchange, and waste removal during intrauterine development. Most living mammals are placentals, a group traditionally aligned with the clade Placentalia or Eutheria in scientific classification. There is variation among placentation types, including differences in the degree of fetal-maternal tissue contact and invasion, which can influence reproductive strategies and developmental timing.

Historically, placentoid has appeared in scholarly writings to describe mammalian taxa with placental development or to distinguish placental mammals from marsupials and monotremes. In contemporary taxonomy, the term is largely superseded by formal clade names, though it may still appear in paleontological or comparative anatomy discussions to emphasize placentation traits, convergent features, or ancestral states of reproduction.

See also: Placentalia, placenta, placenta types, mammal, viviparity.