Molecular dating of Placental mammals

Molecular dating allows to date the nodes of phylogenetic trees using variation in molecular sequences (most frequently DNA sequences) as well as, typically, information from the fossil record.

This means that, as our knowledge of the fossil record and our methods to use molecular data improve, we can obtain an improved understanding of the evolutionary history of a given group of organisms.

There has been some controversy over the evolutionary history of Placental mammals with respect to the Cretaceous-Paleogene (KPg) boundary.

It is widely accepted that at this boundary a catastrophic event occurred causing a mass extinction, including the extinction of non-avian Dinosaurs.

The fossil record of Placental mammals suggest that they were largely affected by this event and they diversified explosively after the KPg boundary.

Contrasting with this reading of the fossil record, molecular dating studies based on DNA sequences had repeatedly suggested that diversification among orders of Placental mammals had occurred largely before the KPg boundary.

I have worked on this topic during my postdoc in Matt Phillips’ lab. Indeed, Matt Phillips – a renowned expert in molecular dating – before I started working on this topic had already proposed a model – the “soft explosive model" – of Placental diversification based on careful analysis of molecular data (Phillips 2016 – Systematic Biology). This model suggests that most of the divergence between Placental orders occurred shortly after the KPg boundary. This model, then, represents a much-needed reconciliation of molecular dating with fossil evidence.

The work me and Matt co-authored provides further support to the soft explosive model. In particular, it shows that previous molecular dating studies had been mislead by the use of fossils of uncertain affinities, as well as by variation in substitution rates associated to variation in body size. In addition to this, previous estimates of ancestral body size for crown mammals were affected by the molecular dating.

• Phillips, M.J., Fruciano, C. 2018. The soft explosive model of placental mammal evolution BMC Evolutionary Biology 18: 104. 

The fossil record of Placental mammals (bars) shows a clear increase after the KPg boundary. Arrows show the approximate time of inter-ordinal diversification in various molecular dating studies.

Using an improved set of fossil calibrations for the molecular dating analysis provides support for the soft explosive model (a). This contrast markedly with the results obtained on the same data but with a previously used, less appropriate, set of fossil calibrations (b).