What is Oligo-Miocene radiation?

Question

Some studies about Earth's fosilized plants/animals mention a "Oligo-Miocene radiation".

Examples:

Our study therefore strengthens previous suggestions that the absence of very large penguins today is likely due to the Oligo-Miocene radiation of marine mammals.

from Mayr et al (2017), doi: 10.1038/s41467-017-01959-6

Molecular phylogeny and dating reveals an Oligo-Miocene radiation of dry-adapted shrubs (former Tremandraceae) from rainforest tree progenitors (Elaeocarpaceae) in Australia

from Crayn et al. (2006), doi: 10.3732/ajb.93.9.1328, see also www.ncbi.nlm.nih.gov/pubmed/21642198

So, what is this "Oligo-Miocene radiation" ?

Answer 1

The ‘Oligo-Miocene’ part

The Oligocene and the Miocene are epochs of geological time. The Oligocene lasted from ~33.9 million years ago to ~23 million years ago; the Miocene followed immediately after the Oligocene, and lasted until ~5 million years ago.

Oligo-Miocene refers to events that happened around the boundary between the two epochs at ~23 million years, or that happened throughout the Oligocene and Miocene epochs.

The ‘radiation’ part

‘Radiation’ here is evolutionary radiation, a biological term usually referring to an increase in the number of species or in the differences between them. The Palaeontology Online glossary defines ‘evolutionary radiation’ as follows.

An evolutionary radiation is an increase in either taxonomic diversity (i.e. the number of species) or morphological disparity (how they differ in anatomy). This can result from an adaptive change or the opening of ecospace.

The combination

Putting the two parts back into the contexts where you found them: the first article mentions an increase in the diversity or marine mammals, and the second in the diversity of dry-adapted shrubs. Both these increases occurred between the start of the Oligocene and the end of the Miocene.

Answer 2

Radiation here has nothing to do with radioactivity. In this context it is refers to a diversification of marine mammals. It is a common term used in evolutionary biology.

Radiation: 'divergence out from a central point, in particular evolution from an ancestral animal or plant group into a variety of new forms.'