Cycads are not “living fossils” from Dinosaur Age
Today’s cycads ‑ plants famed as “living fossils” because they’ve survived since the last dinosaurs munched on them 65.5 million years ago ‑ are really only a few million years old, according to a new study by University of California, Berkeley, scientists and their international team.
Often mistaken for palms, cycads are actually cone-bearing plants that flourished during the dinosaur era, and have survived in tropical and subtropical pockets to the present. The UC Botanical Garden hosts a nationally recognized collection of cycads, many of them rescued from plant smugglers.
The new study uses molecular evidence to show that the surviving cycad species are actually not relics of the dinosaur era, but the result of an evolutionary explosion among cycads that began about 12 million years ago.
“All the cycad species we examined diverged from their nearest relatives in a really narrow window of geologic time, well after the dinosaurs became extinct,” said co-author Charles Marshall, director of the University of California Museum of Paleontology and a UC Berkeley professor of integrative biology. “This was a global event, and then the diversification essentially stopped in the last couple of million years. There is no other group of plants that has this remarkable pattern of diversification.”
“We can now say that living cycad species are not ancient or leftovers from dinosaur times,” said Nathalie Nagalingum, a research scientist at the Royal Botanic Gardens in Sydney, Australia, who led the study while a post-doctoral fellow in Marshall’s laboratory at Harvard University and subsequently UC Berkeley. “They evolved independently of dinosaurs only 12 million years ago. The recent radiation of cycads radically changes our view of these emblematic living fossils.”
The study was published online Oct. 20 in Science Express.
Nagalingum, Marshall and their colleagues studied all 11 groups of cycads and two-thirds of the world’s 300 species, developing a molecular clock that told them how recently living cycads diverged from one another. If cycads had truly dated from the dinosaur era, the times of divergence between the living species would have dated back to their heyday in the Jurassic Period, which began 200 million years ago, through the Cretaceous Period, which ended with the dinosaurs 65.5 million years ago.
Instead, they found the living species originated within the last 12 million years or so.
“Cycads are poster-child living fossils, yet the living species are really young,” Marshall said. “So, while the group as a whole are living fossils, the species themselves are not.”
“It was amazing that all the cycad groups across the globe ‑ in Australia, Africa, Southeast Asia and Central America ‑ began to diversify at the same time,” Nagalingum said. “This indicated that a global trigger may have been responsible. It seems that the trigger was a change in the climate, that is, when global cooling began and when the world started having more distinct seasons.”
Nagalingum noted that cycads today are among the most endangered plants.
“Cycads are very slow-growing plants, so it’s hard to predict whether cycads can survive, now that climate change is occurring at a much faster rate,” she said.
Coauthors of the Science paper with Nagalingum and Marshall include Tiago Quental, a former UC Berkeley post-doc now at the Universidade Estadual de São Paulo, Brazil; Hardeep Rai of Utah State University; Damon Little of the New York Botanical Garden; and Sarah Mathews of Harvard University’s Arnold Arboretum.