Saturday, 22 October 2011

Cycads Are Not As Ancient As Previously Thought

The distinctive bark scales of a cycad
There are many living fossils on our planet, relics of more primitive times. Some can give us a glimpse into the past, such as the world famous coelacanth, while others are little more than survivors, such as the colonies of bivalve molluscs which exist in the deep places of the Earth. One of the less well known members, or rather groups, which falls inside the Lazarus taxon are the cycads. These stout trees and shrubs have large fern-like leaves and trunks which are covered in interlocking sections of bark, similar to reptile or fish scales.

These plants first evolved around 250 million years ago during the dawn of the dinosaurs. It was long thought that they had barely changed since they first appeared. However fossil and DNA evidence suggests otherwise. Cycads flourished during the Age of the Dinosaurs, but declined in diversity during the K-T Boundary extinction event 65 million years ago. The 300 species still alive today were all thought to be the species that survived this.

A study conducted by Dr Nathalie Nagalingum from the Royal Botanic Gardens, UK, has shown that modern cycads are far younger than previously thought. By comparing fossils from all 11 extinct cycad groups and the genomes of the living examples, she found that the modern species actually dated to around just 10 million years old, during the late Miocene epoch. The date reveals that the cycad family underwent two waves of evolutionary innovation.

Nagalingum states that the most likely cause of this second wave was climate change. By the late Miocene, the continents were in their modern orientations. The climate cooled due to the large stretches of ocean at the poles and became more seasonal. It is likely that the cycads adapted to take advantage of the rainy season or sunny periods. This study shows that many species of living fossils might not be true members of the Lazarus taxon. Only genetic evidence will prove this one way or another.