DANIEL PEPPE - About 66 million years ago, an asteroid crashed into Earth just off the Yucatan Peninsula in Mexico. The impact threw huge amounts of rock fragments, dust, and chemicals into the atmosphere, causing sulphuric acid rain and a massive dust cloud that blocked the sun for more than a decade.
Famously, this catastrophic event led to the extinction of the dinosaurs. Less well-known is the fact that more than 50% of plant species in North America also disappeared. Not only did the dust cloud cool the planet, it also prevented plants from photosynthesising, causing them to die.
These plants were the food source for dinosaurs and other living organisms, and their extinction changed the landscape forever. After the dust settled and plants began to return, the Earth looked like a very different place.
Common illustrations of dinosaurs rarely show the rich, diverse plant life that surrounded them and provided food to many dinosaur species. This might lead you to assume that dinosaurs lived in a world where plants were not very common. However, through the study of fossil plant remains, palaeobotanists (scientists who study plant fossils) have been able to determine that the age of dinosaurs was actually teeming with an astounding variety of plant life.
The best fossil plant sites from the time period before and after the K-T (Cretaceous-Tertiary) boundary are found in North America. Just before the K-T boundary mass extinction, fossils show us that there was an incredible diversity of plant life.
In many places, such as Montana and Colorado, there were many more species of plant living on the landscape than there are today.
Flowering plants (angiosperms) evolved in the earliest Cretaceous and rapidly diversified so that by the end of the Cretaceous they dominated most plant communities.
During the latest Cretaceous, many modern families of angiosperms were common across North America, such as relatives of sycamores (Platanaceae), dogwoods (Cornaceae), palms (Arecaceae) and magnolias (Magnoliaceae) – the last of which is pictured above behind a T. rex.
The taxonomic affiliation of other common species is unknown, and these species may have been members of now extinct angiosperm groups. Despite the dominance of angiosperms, other types of plants, such as cycads, ferns, and conifers, were also common.
The asteroid impact and its after effects completely devastated this flora. After the K-T boundary, plant communities recovered slowly and looked very different from those of the Cretaceous.
As the dust settled and temperatures began to return to normal, landscapes that were once covered in trees were instead dominated by significantly fewer species. These included fast-growing ferns and herbaceous angiosperms (herbs). After hundreds to thousands of years, the surviving species of plants began to repopulate the landscape.
During the Cretaceous, when the dinosaurs walked the earth, most of these plants, which included members of the sycamore, dogwood, and katsura families, had been rare and restricted to swampy, wet regions. With the extinction of so many competing plants, they were now able to dominate all environments.
The post-extinction world also saw the evolution of many modern groups of angiosperms, including the first occurrence of species closely related to modern maple, willow, birch, oak, dogwood, and beech. For other types of plants, such as the cycads, the most common species in the plant group went extinct, and these groups became permanently less diverse.
Amazingly it took millions of years for North American plant communities to fully recover from the catastrophic extinction. Even on the opposite side of the Earth from the site of the asteroid impact, plant communities appear to have been hit hard.
Records of fossil plants from New Zealand show that although there was a much smaller extinction here than in North America, there was also a catastrophic die-off of plants at the K-T boundary and forests were replaced by ferns and herbs for several thousand years. These records from opposite sides of the globe demonstrate that although the K-T boundary is best known for the extinction of dinosaurs, it was also catastrophic for plant communities around the world.
As many as 50% of all plant species went extinct and ecosystems took thousands to millions of years to fully recover. This combined mass extinction of dinosaurs and plants at the K-T boundary fundamentally changed the world. Life on land was never the same again.
- Dr. Daniel Peppe is an Assistant Professor in the Department of Geology at Baylor University. His research is focused on reconstructing ancient climates and ecosystems in North America and East Africa and on developing new and improved palaeoclimate and palaeoecological proxies. His favourite dinosaur is one of the largest herbivores of all time - the Apatasaurus.