It´s known that an asteroid hit the Yucatan peninsula in Mexico at the end of the Cretaceous Period. It left a 180 km wide crater and from the impact site tsunamis developed and the Caribbean region was buried in ash and other debris. The consequences of the asteroid impact were global. Vajda and her colleagues have previously studied the broad-scale changes in the New Zealand vegetation following the impact, but now they have dramatically improved our view of the timing of events.

At the end of the Cretaceous the vegetation on New Zealand was dominated by conifers and flowering plants. Many of these species disappeared suddenly at the end of the Period and were instead replaced by fungal spores and fungal threads preserved in a four millimeter thick layer of coal. The layer coincides with fallout of iridium, an element rare in Earth’s crust but which abounds in asteroids.

-We have managed to reconstruct the event month by month, with a very high time resolution, says Vivi Vajda. During a very short period – from between a few months to a couple of years – the fungi and other saprophytes which live on dead organisms must have been the dominating life form on Earth. Atmospheric dust blocked the sunlight and led to the death of plants that are dependent on photosynthesis.

The layer of fossil fungi is followed by a 60 cm thick interval containing traces of the recovery flora, which re-established relatively quickly, ground ferns at first, followed after decades to hundreds of years by more diverse, woody vegetation.

A similar layer of fungi and algae is known from a previous catastrophe which happened 251 million years ago at the Permian-Triassic boundary. This was an even greater mass extinction: about 90% of the existing species disappeared. Research will now focus on whether the similar biological signatures at these mass extinctions reflect similar causal mechanisms.

Vivi Vajda’s e-mail address is with phone no 0046 46 222 3959.