Fern spikes: the first green after catastrophe

In sediment cores that cross the Cretaceous–Palaeogene boundary, the pattern is consistent worldwide. Below the boundary: diverse pollen. Above it: almost none, replaced by a sudden flood of fern spores.

The fern spike

The spike is not subtle — in some cores fern spores go from a modest share of the sample to over 70 per cent within a few centimetres of rock. Ferns are the classic disaster taxon:

• spores travel enormous distances on wind
• they colonise bare ground quickly
• they need no pollinator and no seed dispersal partner

The same signature appears after modern volcanic eruptions. Ground stripped bare at Mount St Helens turned green with ferns long before shrubs returned.

What it tells us about the sky

A collapse in flowering-plant pollen followed by a fern takeover implies something knocked out photosynthesis broadly and briefly — consistent with dust and sulphate aerosols blocking sunlight for months to a few years.

Herbivores lose their food supply first. Then the animals that ate the herbivores. This is why body size correlates so strongly with extinction risk at the boundary.

Recovery

Forest structure returned within a few thousand years, but composition changed permanently. The Palaeogene forests that emerged were dominated by flowering plants, and the great gymnosperm forests never regained their former range.

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Cover image: USFWS/Southeast — Public domain (source).