When was the First (Placental) Mammal?

There are three sorts of mammals: monotremes (platypuses), marsupials (kangaroos) and placentals (us and lots more).

How do you find the first placental mammal? I suppose you could find its fossil, be able to determine it is placental, date it, and (the difficult bit) prove that there are none which came before. This is unlikely and your conclusions are likely to be challenged. 

However there are a few fossils which are accepted as being placental mammals dated as living after the extinction of the dinosaurs. But were they the first? Were there placentals living alongside the dinosaurs? There are no accepted placental fossils in rocks below the K-Pg boundary, but that does not prove much.

There is another way, using molecular clocks. Much has been written about the use of molecular clocks - the more you get into it the more difficulties appear. See the diagram below to get a flavour of the controversies.

Thick purple lines are crown orders, green lines are stem orders, and black lines are stem placental families.

(A) Explosive model: all placental mammal diversification and origination occurred just after the K-Pg boundary.

(B) Soft explosive model: placental mammals originated just before the K-Pg boundary, but intraordinal diversification only occurred after the boundary.

(C) Trans-KPg model: both interordinal and intraordinal diversification occurred around the K-Pg boundary.

(D) Long fuse model: placental mammals originated in the middle of Late Cretaceous, but intraordinal diversification did not begin until after the K-Pg boundary.

(E) Short fuse model: placental origination and crown order diversification occurred during the Cretaceous.

The author of THIS ARTICLE, working at Bristol University, has avoided much of these controversies by comparing families and using statistical methods, of which I know little, to conclude that placental mammals originated in the Cretaceous

The article is based on THIS ACADEMIC PAPER. An indication of the depth of study used in the article can be gauged from the following figure.

Clade age and extinction time estimates for placental mammal families

Each line represents a family (arranged by order and clade but without further phylogenetic information), with 95% credible intervals in colors at the root estimates and extinction estimates (where applicable). Gray lines fill in the lineage. 93 families have credible intervals extending into the Cretaceous, but many originated after the K-Pg boundary. For stem and crown order classifications for each family, see Data S1.