Did plants go extinct alongside the dinosaurs?

If you ask almost anyone what “mass extinction” means to them, they’ll invariably talk about animals. But plants are certainly not immune and the signs are that many individual species have been wiped out by events like the meteor impact that did for the dinosaurs. Speaking with Chris Smith, Peter Wilf is at Penn State University…

Peter - Well, there's a really grand picture here, Chris, because we have the last great mass extinction that's famously known as the dinosaur extinction. An asteroid the size of San Francisco traveling 20 times the speed of a bullet crashed into the earth, crashed into Mexico, actually, 66 million years ago. And it caused the extinction of about 70% of species on the planet, both on the land and in the seas. One of the most dramatic events in all of earth history. And it set the stage for our modern world, the rise of mammal dominance and the rise of flowering plant dominance. What's much less understood is exactly how the plants responded to this, the plants that make the environment that we live in. Today, we're starting to understand how this event transformed life on earth forever.

Chris - What would it have been like? How would the environment have responded in the short term as well as longer term?

Peter - The asteroid would've excavated a hole in the earth's crest, maybe more than 10 kilometres deep. It would've ejected a plume high up into the atmosphere, maybe even into outer space. It would've generated massive shock waves, enormous tsunamis, some of the largest probably that ever occurred in earth history that swamped low lying areas around the world. But this was only the beginning of what went wrong at that time. The soot that was blood up into the high atmosphere blocked the sun and shut down photosynthesis and froze the planet so it would've frozen all of our lovely tropical plants. And for a period of six months to two years, there was no photosynthesis and it was freezing everywhere. Even in the tropics.

Chris - How did any plant survive that then, given the extreme conditions? Obviously there are some that are well adapted to extreme cold and they possibly would've fared better, but those tropical plants that you mentioned, why weren't they just wiped out completely?

Peter - Well, plants are just so much more resourceful than we give them credit for, and many species of plants were wiped out completely. But plants have a variety of mechanisms. They have seed banks, for example. They have underground components, roots, for example, to re-sprout. We know that this asteroid didn't kill everything because we're here today. We have a biodiverse world still today, and what we see is an incredible transformation of the types of forests before and after. So the effects were very profound. My co-author Mónica Carvalho put together a beautiful study published in Science a couple years ago now, where they looked at more than 45,000 fossil pollen grains and maybe 5,000 fossil leaves. And they showed that the birth of the tropical rainforest immediately followed the death of the dinosaurs. So the most diverse biome on earth didn't exist before this happened. And this is when we see flowering plants taking over the tropical rainforest, making large fruits, making large seeds, supporting arboreal life, arboreal mammals, all kinds of life driving terrestrial diversity. So even in the wake of the disaster, there were some big changes that led to the tropical rainforest being where most of the species on earth are located today. So we have both disaster and rebirth. It's quite a story

Chris - If one thinks about the tree of life and specifically plants on it, where you've got a stem and then branches and then twigs and, and eventually leaves, how far back down the tree of life did the asteroid prune?

Peter - That's a great question, and it really goes to the heart of some of the debates that we're having. They're very short answer is this extinction couldn't have pruned all the way down because as many of our colleagues have observed, maybe no plant families went extinct at this extinction. So that sounds like what kind of an extinction is that? But at the same time, everywhere we look, Colorado, Montana, North Dakota, Argentina, Columbia, we see more than half of the species growing extinct. What this means is that there is a rediversification, if you like, from the stems that you're mentioning. Why can't you kill off a plant family? Well, it's very hard. By the end of the Cretaceous, the flowering plants were already very, very diverse. Each family had hundreds to thousands to 10 thousands of species in it. And those species had many, many, many individuals. So what we're showing here is that this discussion about well, no plant families went extinct. You, it sort of whistles past the graveyard of all those species that went extinct. And it's, you know, it's species that matter today that we're trying to conserve the if, if we're gonna estimate very rough ballpark that the dinosaur extinction killed 50% of the plant species. You know, that's about what we're looking at today. If you look at the estimates of how many species of plants are vulnerable to extinction right now, it's about 40%. It's a very comparable number. So one of the things that we're reviewing here is this aspect that contrasting this observation of plant families not going extinct with the reality that a large numbers of species go extinct and the species really do matter.