Cycads not ‘living fossils’

Modern members of the plant group date back a mere 12 million years

Once thought to be the last remaining members of a plant lineage that went extinct with the dinosaurs, modern-day cycads are now believed to have diverged from a more recent common ancestor.

Genetic analysis suggests that modern-day cycads like Cycas thouarsii (shown) are not “living fossils” of the dinosaur era, but arose from a common ancestor that lived about 12 million years ago. © Science/AAAS

Although cycad populations suffered major losses about 65 million years ago when dinosaurs — their once primary dispersal agents — went extinct, the plants later experienced a renaissance due to a global climate shift, a new study suggests. Living cycads diverged from an ancestral species that flourished around 12 million years ago, not from older dinosaur-era relatives, an international team of researchers reports online October 20 in Science.

To estimate the divergence time of living cycads, researchers used a technique called molecular clock analysis. First they measured the genetic differences separating 200 living cycad species. Since certain genetic changes typically accrue at a fixed rate once species radiate from a common ancestor, scientists were able to use this cycad DNA data, in conjunction with the fossil record, to predict a much more recent divergence. – Nick Bascom

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