USU Botanist Says Dino-Age Plant May Not Be 'Living Fossil' After All

So-called "living fossils," modern survivors of distant, more diverse lineages, fascinate scientists because they closely resemble their ancient ancestors. Unlike a Neanderthal that would stick out like a sore thumb in a crowd of modern humans, a living fossil such as a horseshoe crab or a gingko tree looks remarkably similar to its prehistoric progenitor.

Utah State University botanist Hardeep Rai studies cycads, palm-like plants often described as living fossils found primarily in the world’s tropical regions and Australia. Mostly endangered in the wild and popular as ornamental plants, cycads made a memorable backdrop for Hollywood’s Jurassic Park.

“Unlike most of today’s species that are rapidly adapting to changes in their environment, cycads’ mutation rates and diversification are dramatically slower,” says Rai, a research scientist in USU’s Department of Wildland Resources.

Scientists have long guessed that cycads had their last major episode of genetic diversification — an evolutionary growth spurt, if you will — during the age of dinosaurs some 150 million years ago. Trampled on, feasted upon and fertilized by the ancient beasts, the plants underwent accelerated diversification during the animals’ tenure and subsequent demise and — that was that.

Yet, based on recent study, Rai and colleagues believe cycads may have experienced a more recent burst of diversification. Findings by Rai, Nathalie Nagalingum and Sarah Mathews of Harvard University, Charles Marshall and Tiago Quental of the University of California, Berkeley and Damon Little of the New York Botanical Garden, appear in the Oct. 20, 2011 issue of Science Express.

Based on computer-automated analyses of the largest taxonomic dataset of cycads to date and their nuclear DNA sequences, the thick-stemmed plants appear to have undergone significant speciation within the last 12 million years.

“The hypothesis that dinosaurs are responsible for the approximately 300 species of cycads we know today no longer fits,” Rai says. “Some worldwide event caused an explosion of evolutionary change. The question is: What happened?”

The answer, he says, may lie in findings about a complementary, six-legged species that aids the plants’ reproduction: the weevil. Both friend and foe to the cycad, the insect is the plant’s main pollinator. A 2008 study of the weevil's lineage suggests that the plant-eating beetle underwent rapid and significant diversification around the same time as the cycad. Rai and his colleagues surmise that events leading to continental shifts and associated changes in seasonality may explain the simultaneous genetic "explosion" that occurred with the cycad and the weevil.

"We have two independent lines of evidence that point to a catastrophic global event," Rai says. "Although the cycad lineage is ancient and it's often cited as a classic example of a living fossil, that may not be the case. Cycads may not be living fossils, after all."

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