The first placental mammal

From O'Leary et al. Science 2013; 339: 662-7

Reprinted with permission from AAAS

When did placental mammals diversify to found the known orders? The old story used to be that an asteroid impact killed all the non-avian dinosaurs, but mammals survived and diversified in the post-impact Palaeogene. More recent molecular studies have challenged this. They place the origin of some orders or most in the Cretaceous. However, a paper in today’s issue of Science asserts the previous version was right all along.

First a bit of terminology: Notwithstanding that every marsupial has a placenta, “placental mammals” refers to the clade comprising all living non-marsupial (and non-monotreme) mammals and their common ancestor, i.e. crown placentals or Placentalia. Some of us use Eutheria to avoid upsetting our Australian colleagues. In the supplementary material from today’s report I am taken to task for such laxity!

What did the revisionists, Maureen O’Leary and colleagues, actually do? Quite a bit. For 86 living and (as far as possible) 40 fossil species they scored 4541 phenomic characters including, importantly, some that relate to reproduction, fetal membranes and placentation. This gave them sufficient information to combine with molecular data without the former being swamped by the latter.

The most important result was that none of the living orders arose before the Cretaceous-Palaeogene boundary – this marks a series of catastrophic events including an asteroid impact. What did survive was a hypothetical placental ancestor. The creature shown above is their best estimate of how that ancestor might have looked.

They also tell us how it reproduced. Its single young was born hairless with eyes closed. The uterus was bicornuate and the chorioallantoic placenta was haemochorial. In the appendices they add that implantation was superficial, amnion formation was by cavitation (a surprise that one), there was a temporary yolk sac placenta and a small allantoic vesicle (written vessel but we get the point).

The new chronology is attractive not least because it is better in agreement with the fossil record. It is no longer necessary to postulate long ghost lineages where an order existed for millions of years without leaving any fossils. However, as the accompanying comment implies, this is unlikely to mark the end of the debate.