The Rise of Angiosperms Following the K-Pg Mass Extinction Event

Wednesday - September 20 2023, 05:27 UTC - 1 year ago

The Cretaceous-Paleogene mass extinction (K-Pg) event was a major event which wiped out between 70-75% of species on Earth, including the non-avian dinosaurs. Although the majority of pre-K-Pg species went extinct, many families of angiosperms survived. How angiosperms thrived in different regions and the consequences of this on today’s species is a fascinating story.

If you looked up 66 million years ago you might have seen, for a split second, a bright light as a mountain-sized asteroid burned through the atmosphere and smashed into Earth. It was springtime and the literal end of an era, the Mesozoic.

If you somehow survived the initial impact, you would have witnessed the devastation that followed. Raging firestorms, megatsunamis, and a nuclear winter lasting months to years. The 180-million-year reign of non-avian dinosaurs was over in the blink of an eye, as well as at least 75% of the species who shared the planet with them.

Following this event, known as the Cretaceous-Paleogene mass extinction (K-Pg), a new dawn emerged for Earth. Ecosystems bounced back, but the life inhabiting them was different.

Many iconic pre-K-Pg species can only be seen in a museum. The formidable Tyrannosaurus rex, the Velociraptor, and the winged dragons of the Quetzalcoatlus genus could not survive the asteroid and are confined to deep history. But if you take a walk outside and smell the roses, you will be in the presence of ancient lineages that blossomed in the ashes of K-Pg.

Although the living species of roses are not the same ones that shared Earth with Tyrannosaurus rex, their lineage (family Rosaceae) originated tens of millions of years before the asteroid struck.

And the roses are an not unusual angiosperm (flowering plant) lineage in this regard. Fossils and genetic analysis suggest that the vast majority of angiosperm families originated before the asteroid.

Ancestors of the ornamental orchid, magnolia, and passionflower families, grass and potato families, the medicinal daisy family, and the herbal mint family all shared Earth with the dinosaurs. In fact, the explosive evolution of angiosperms into the roughly 290,000 species today may have been facilitated by K-Pg.

Angiosperms seemed to have taken advantage of the fresh start, similar to the early members of our own lineage, the mammals.

However, it’s not clear how they did it. Angiosperms, so fragile compared with dinosaurs, cannot fly or run to escape harsh conditions. They rely on sunlight for their existence, which was blotted out.

What Do We Know? .

Fossils in different regions tell different versions of events. It is clear there was high angiosperm turnover (species loss and resurgence) in the Amazon when the asteroid hit, and a decline in plant-eating insects in North America which suggests a loss of food plants. But other regions, such as Patagonia, show no pattern.

A study in 2015 analyzing angiosperm fossils of 257 genera (families typically contain multiple genera) found K-Pg had little effect on extinction rates. But this result is difficult to generalize across the 13,000 angiosperm genera.

My colleague Santiago Ramírez-Barahona, from the Universidad Nacional Autónoma de México, and I took a new approach to solving this confusion in a study we recently published in Biology Letters. We analyzed large angiosperm family trees, which previous work mapped from mutations in DNA sequences from 33,000-73,000 species.

This way of tree-thinking has laid the groundwork for major insights about the evolution of life on Earth, for instance, how both major and minor events shaped the tree's structure .

In our study, we looked for signatures of sharp population declines associated with the K-Pg event in 56 angiosperm family trees. We found these declines in over a third of the trees, but not in the others. Overall, some angiosperms were affected much more than others. The diversity in the responses clearly showed a pattern that ties with geographic differences.

The overall picture confirms that the asteroid impact had different effects on angiosperms in different regions. Understanding where this leaves us today requires further research, but it is clear that the K-Pg event left a lasting mark on how some angiosperm families evolved, although in an uneven fashion.

The living species today, such as roses, were testament to their extraordinary ability to survive and adapt in the harshest of conditions. Interesting to think that modern-day roses still bear some of the genetic traits left by the survivors of the asteroid impact 66 million years ago, reinforcing their place in the eternal story of life on Earth.