Extant Cycads may not be “Living Fossils”
Cycads are palm-like seed plants with massive stems which produce a crown of fern-like leaves. These strange looking plants may be relatively short, but fossil data indicates that some extinct species may have grown to more than fifteen metres tall. A new study indicates that those species alive today are not closely related to the cycads that were grazed by the dinosaurs and other Mesozoic herbivores. It seems that extant cycads may have evolved from species that diversified to exploit changing world climates around 5-12 million years ago.
That is the conclusion from a study led by Utah State University scientists, the term “living fossil” so often used when referring to organisms that resemble those preserved in the fossil record may not apply to the cycad family.
The evolutionary origins of cycads remains unclear, they are probably descended from pteridosperms (seed ferns) and their are fossils to indicate that these plants were around during the Permian Period, although they may actually have evolved earlier, in the Carboniferous.
A Model of a Cycad
The picture (above) shows a replica of a cycad tree in the CollectA model range, to see this series that is available from Everything Dinosaur: CollectA Prehistoric Plants and Dinosaur Models.
Commenting on his research findings, Hardeep Rai, a post-doctoral research fellow with Utah State University’s department of Wildland Resources stated:
“The current form of cycads looks very much like cycads did when they were at their peak 150 million years ago in terms of the number of species and their coverage of the Earth.”
However, the research team concluded that of the three hundred or so known species alive today, these plants resulted from a fairly recent branching of the cycad family tree.
Discussing the research findings, Nathalie Nagalingum of the Royal Botanic Garden in Sydney, New South Wales said:
“Today’s species did not exist during the reign of the dinosaurs. They evolved independently of dinosaurs only ten million years ago. The recent radiation of cycads radically changes our view of these emblematic living fossils.”
It is very likely that the feeding activities of dinosaurs and other herbivores during the Late Palaeozoic and into the Mesozoic affected the evolution of plants, just as the grazing of mammals influences flora today. However, this study suggests that those cycad species alive today are not directly related to more primitive cycad genera.
Gymnosperm Tree of Life Project
The study arose from the National Science Foundation-funded Gymnosperm Tree of Life project. Co-authors of the research include Berkeley’s Charles Marshall and Tiago Quental and Damon Little of the New York Botanical Garden.
We at Everything Dinosaur, discussed the cycad family back in July when we produced an article commenting on the presence of cycads today. We may have to re-write this piece in the light of this research.
To read more about cycads: Yes, we have Cycads Alive Today.
Cycads were most abundant during the Jurassic and Cretaceous, before the evolution of the flowering plants (Angiosperms) led to the decline of this type of flora. Extant cycads are classed into eleven genera found in three families and certain types of cycad are popular plants with gardeners, although they are high maintenance in a chilly UK climate.
Prior to this new study, the fragmented distribution of modern cycads was thought to indicate that the modern species are the descendants of previously more widespread genera. The fossil record shows that cycads were particularly widespread during the Mesozoic, even making up a significant proportion of the flora to be found in polar regions. However, unlike their modern equivalents, these prehistoric polar cycads lost their leaves in winter and they had vine-like stems.
Although, cycads superficially resemble palms and have branching, pinnate, fern-like leaves they are not closely related to palm trees or indeed to ferns.
The research team used a method called molecular dating to analyse the DNA of about two hundred living cycad species. Scientists calibrated these species’ genetic profiles against the fossil record and found they branched off from a common ancestor between five and twelve million years ago.
The concept of a molecular clock is that evolutionary change occurs at regular time intervals. If it is assumed that the rate of genetic change (mutation) in the DNA of an organism does not change, i.e. the rate is relatively constant, the molecular genetic difference between two species can be measured and the time when they diversified from a common ancestor calculated. Using this data, Hardeep Rai and his fellow researchers were able to calculate when these species of cycads in the study diverged from their common ancestor.
“The hypothesis that dinosaurs are responsible for the approximately 300 species of cycads we know today no longer fits. Some worldwide event caused an explosion of evolutionary change. The question is: What happened?”
The scientists have postulated that climate change triggered this burst of speciation with the help of specialised insect pollinator, weevils and beetles that underwent evolutionary change at the same time.
Around 12 million years ago, the world’s continents reached their current positions. Instead of warm climates around the globe, temperate zones emerged at the higher latitudes and climates became seasonal. Diversification seen in today’s cycads many have occurred in response to these environmental changes.