Lichens thrived while those around them fell
Mass extinction allowed new combinations to rise up, DNA analysis shows. Nick Carne reports.
It seems not everything got fried when an asteroid smacked into the Earth 66 million years ago.
New research suggests some types of lichens – organisms made of fungi and algae living together – seized the moment and evolved into new forms to take up plants' role in the ecosystem.
The “moment”, of course, saw ash blocking out the sun and cooling the planet, which is why many plants, early birds and insects and, of course, dinosaurs were wiped out. However, fungi don't rely on sunlight for food and the fossil record shows an increase in fungal spores at this time.
Since lichens contain a plant and a fungus, scientists from the Field Museum in the US wondered whether they were affected negatively like a plant or positively like a fungus.
"We thought that lichens would be affected negatively, but in the three groups we looked at they seized the chance and diversified rapidly," says lead author Jen-Pang Huang, now at Academia Sinica in Taiwan.
"Some lichens grow sophisticated 3D structures like plant leaves, and these ones filled the niches of plants that died out."
While Huang and colleagues didn’t have many fossil lichens from that time to look at, they did have lots of modern lichen DNA.
From observing fungi growing in laboratory settings, scientists know generally how often genetic mutations show up in fungal DNA. How frequently a letter in the DNA sequence accidentally gets switched during the DNA copying process is called the mutation rate.
If you know the mutation rate, the researchers say, when you compare the DNA sequences of two different species you can generally extrapolate how long ago they must have had a common ancestor with the same DNA.
They fed DNA sequences of three families of lichens into a software program that then worked out what their family tree must look like, including estimates of how long ago it branched into the groups we see today.
They supplemented this information with the few lichen fossils they did have, from 100 and 400 million years ago, and the results pointed to a lichen boom after 66 million years ago – at least for some of the leafier lichen families.
"Some groups don't show a change, so they didn't suffer or benefit from the changes to the environment," says Thorsten Lumbsch, the Field Museum's curator of lichenised fungi.
"Some lichens went extinct, and the leafy macrolichens filled those niches. I was really happy when I saw that not all the lichens suffered."
The results, Lumbsch adds, underline how profoundly the natural world was shaped by this mass extinction.
"If you could go back 40 million years, the most prominent groups in vegetation, birds, fungi – they'd be more similar to what you see now than what you'd see 70 million years ago. Most of what we see around us nowadays in nature originated after the dinosaurs."
Lichens are great environmental indicators, says Huang: “by simply doing a biodiversity study, we can infer air quality and pollution levels".
They’re also unpredictable.
"We expect a lot of patterns from studying other organisms, but fungi don't follow the pattern. Fungi are weird," he says.
The findings are published in the journal Scientific Reports.