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Cycads not so ancient after all

Same, but different Cycads may look like they belong in a Jurassic swamp, but a new study has revealed they evolved long after dinosaurs disappeared.

It was thought that cycads — a group of non-flowering plants that resemble palms — had survived unchanged since they first appeared on Earth around 270 million years ago.

But the study, led by Dr Nathalie Nagalingum from the Royal Botanic Gardens, debunks the view that modern cycads are 'living fossils'.

Cycads flourished during the time of the dinosaurs, but declined when dinosaurs died out about 65 million years ago. It was generally thought that the remaining 300 species that we see on Earth today were leftovers from that time.

"The reason we call cycads 'living fossils' is that in the Cretaceous and Jurassic, when the dinosaurs were around, we see lots of cycads in the [fossil] record. They appear all over the world and they were really common," says Nagalingum.

"So it was just thought that … the species that lived in dinosaur times just dwindled to what we see today."

But the analysis of plant DNA and fossils reported in today's issue of the journal Science shows that modern species evolved around 10 million years ago.

"When you go back to the Jurassic a lot of the leaves look very similar to the leaves we see today, and so it was just thought that cycads were unchanged through time," says Nagalingum.

"But that's not the case when we looked at the DNA."

"[Modern] cycads aren't leftovers from dinosaur times, they're actually really recently evolved and very young plants," she says.

Nagalingum and colleagues compared DNA sequences and fossils from all 11 groups of cycads and two-thirds of the 300 species. The results showed that species around the world evolved rapidly during the late Miocene then slowed down and halted about two million years ago.

"What this tells us is that everything that was around in dinosaur times became extinct, and then everything that we see today flourished 10 million years ago. So it's like a second wave of cycads."

Climate factor

Nagalingum says climate was the most likely factor driving this new wave of cycad evolution.

"The reason we think it's climate is because you have different types of cycads in different parts of the world diversifying at the same time. And for something to affect everywhere in the world at the same time, that's likely climate."

By the late Miocene period the continents had drifted to their current positions, and the climate became cooler and more seasonal. This may have helped cycads to thrive in the tropics and sub-tropics.

"When you get seasonality you get summer rainfall in the tropics," explains Nagalingum adding that summer rains may have spurred a flush of new growth.

"It might have been that regular reproductive cycles spurred their evolution," she says.

Despite their recent evolutionary success, two-thirds of cycads around the world are classified as threatened. A quarter of these threatened species are found only in Australia.

Nagalingum says we need to know more about the genetics of these plants to predict how they will adapt to future changes in climate.

"Cycads are slow-growing plants so it's hard to predict whether cycads can survive, now that climate change is occuring at a much faster rate," she says.