Placentalia - a subgroup of placental mammals

Sir Richard Owen (1804-1892)

The current literature abounds with the term "placental mammals;" it often appears in a paper's title. Even though marsupials have a yolk sac placenta - and several a chorioallantoic placenta - they are not included in the definition of Placentalia. This is a constant source of irritation to marsupial biologists and a pitfall for those wishing to stay their friends (see between Scylla and Charibdis).

Placentalia was relaunched by McKenna and Bell in their influential volume Classification of Mammals above the Species Level (New York 1997). It is attributed to Sir Richard Owen (pictured above).

Owen 1837 [sic] page 903

This is the passage cited by McKenna and Bell. It may well be the first usage of Placentalia but is far from being a definition.

In its current usage, Placentalia defines a crown group of mammals comprising extant species and their ancestors. It is not the same as Eutheria, which includes lineages that do not have living descendants. Placentalia therefore is popular in molecular phylogenetics and phylogenomics. The reason is evident: when working with DNA sequences from living species, inferences can be drawn only about character evolution in the crown group.

When looking for the Owen reference I found that McKenna and Bell had got the date wrong. Robert B. Todd commenced publication of The Cyclopaedia of Anatomy and Physiology in 1837 but Owen's article is in volume 4 part 2 published 1849-52 (full reference below).

Owen R. Teeth. In Todd R.B. (Ed.) The Cyclopaedia of Anatomy and Physiology Vol. 4 Part 2, pp. 864-935. London: Longman, Brown, Green, Longmans and Roberts, 1849-1852. McKenna and Bell Refer to page 903.