Were the dinosaurs already dying out?

One of the best documented mass-extinction events in our planet’s history happened about 66 million years ago when a town-sized asteroid travelling at 20 kilometres per second slammed into what is now Mexico’s Yucatán Peninsula. The ensuing dust cloud blocked out the Sun for years and plunged the planet into the equivalent of a nuclear winter, changing the climate, halting plant growth and toppling the food chain. With the exception of their bird descendents, dinosaurs disappear from the fossil record at this point, suggesting the impact was linked to their demise. But was this the cataclysm that wiped out a thriving population, or were they already on shaky ground by the time this happened and it was merely the final straw that broke the Camarasaurus’s back? As she explains to Chris Smith, Bethany Allen is grappling with this big question at ETH Zurich…

Bethany - If dinosaurs were already declining before this time, maybe it doesn't matter that the asteroid struck, they would've gone extinct anyway. But actually if they were still diversifying and we were getting lots more dinosaurs at this time, potentially, if that asteroid hadn't struck, then that group could have still been around, maybe even today. I wanted to look at this question in a bit more detail using some new methods that have not been applied to this question before to see if we could actually figure this out.

Chris - So what would you look for then as the giveaway that either they were still diversifying and therefore still flourishing, or conversely, they were in decline by the time that mass impact happened?

Bethany - The approach that we used was to try and model the evolutionary process that describes how many different types of dinosaurs we had immediately before this mass extinction event. And in order to do that, we set up these models to describe the major processes underneath evolution, which are origination - this is a process where species divide and become multiple other species - and then compared that with extinction: when we lose species. When we have more origination, where we have species diversifying, if that's higher than the number of extinctions that we are having, then we know that dinosaurs were still diversifying. But conversely, if we're seeing more extinction, then dinosaurs were in decline and might eventually have become completely extinct.

Chris - Does the rate at which, and the fact they are diversifying, say that things are good or does it say that things are bad? Because you could argue, if they're under really severe pressure, there's a driver to diversify?

Bethany - Absolutely, and that's a great question that a lot of people are really interested in investigating in more detail. Because often what we think happens when a speciation takes place is that there'll be one population of dinosaurs, and for some reason that population splits. That's the process that precedes this speciation event. That can happen because dinosaurs in this population choose to eat different things, or they choose to mate with some other dinosaurs, but not others. This can lead to a split in the population. This can create more diversity. But then also when this population divides, the total population size is split between the two. This can also create some extinction risk if these population sizes become too small. So it can be quite a difficult time in evolution.

Chris - How did you get at this then, and what did you actually find when you started to piece it together in this way? What seems to be the picture that emerges?

Bethany - We know that the main piece of evidence that we have for understanding how much dinosaur diversity we had was the fossil record. These fossils, we can identify them and count the number of different species that we think that we had during the Cretaceous in the lead up to this mass extinction event. However, we know also that the fossil record is incredibly incomplete. Only a tiny fraction of all of the animals that have ever been alive in Earth's history actually make it into the fossil record, so we use two different evolutionary models. In one of the models, the fossil record as we see it, was treated as a series of fossilisation events. Each fossil that we included in our model was treated as independent. Then, in the other model, instead we treated the fossil record as one big process. And this enabled us to look at fossilisation as a rate. Our intention was that between these two models, we would be able to see the impact of how we can actually view the fossil record on what we estimated dinosaur diversity to be. And actually what we found was that these two models came out with completely different results. When we think about the fossil record as a series of individual fossils, we estimated the dinosaurs were probably already in decline before the Cretaceous, perhaps only by a small amount, but there was more extinction than speciation. However, when we think about the fossil record as a process, as a rate of the number of samples of dinosaurs that we have through time, instead we estimated much more uncertainty in the number of species that we had at the end of the Cretaceous. But actually this number was probably still growing and dinosaurs were still speciating and diversifying.

Chris - Where does this leave us then, Bethany? Because it seems that while it's insightful, what you've found, it doesn't actually answer our question. It creates more uncertainty, not less.

Bethany - Absolutely. It seems somewhat dissatisfying that our major result is we don't know, but we are hoping that actually the fact that we have mechanisms behind these two models that we can now point to and say, this is the root of the problem, hopefully means that going forwards there's more that we can learn about dinosaur evolution. I think one thing that we definitely did reveal is the fact that how we view the fossil record is incredibly important, and that means that actually the more that we can understand about the processes that create fossils, and also the more fossils that we can discover and add to the amount of information that we have, hopefully that will mean that in the future we can really get to the bottom of this problem.

Chris - I suppose, nevertheless, it also does keep both theories in play, doesn't it? Which means we keep on thinking, we keep asking questions, but what could be the mechanism if it wasn't just the giant impact that did for the dinosaurs? What other mechanisms ahead of that could also account for a decline if there already was one?

Bethany - So there have already been some studies that have been published talking about potential changes in climate that were happening around this time. There has been a suggestion that perhaps this meant that the particular types of environments that dinosaurs were living in were becoming less common. When there is less area for these dinosaurs to live in, this forces the population sizes to shrink and can ultimately lead to extinction.