How do extinct marine megafauna measure up?

The modern marine megafauna - broadly meaning big ocean inhabitants - play important ecological roles and include many charismatic species that have drawn the attention of both the scientific community and the public. Just recently, for instance, we learned that migrating whales move thousands of tonnes of nitrogen around the planet. However, the extinct marine megafauna have never been assessed as a whole - in fact there’s not even an agreed definition of what constitutes extinct marine megafauna - and what were their biological and ecological patterns? This was the challenge that, as she explains to Chris Smith, the University of Zurich’s Kristina Kocakova’s set for herself…

Kristina - Large animals in the oceans, which we also call megafauna, are extremely important for these ecosystems. They do a lot of important roles in these oceans, they transport nutrients, they shape the populations of other animals, and so on. And in the modern oceans, we do have a definition for what constitutes marine megafauna, which is heavier than 45 kilograms.

But we actually never properly had a proper definition of marine megafauna of extinct animals, of the ones that we can find in the fossil record. So, the aim of this paper was to do a thorough search of literature and compile a database of all animals that are larger than one metre, and then make a decision whether this kind of value would be good as a definition for what constitutes marine megafauna of extinct animals.

Chris - How far back in time did you go?

Kristina - We went all the way back to the Cambrian, so that's the beginning, that's when the boom of life happened, which is more than 500 million years ago. We actually find our first instances of megafauna already at this time. So, we tried to cover the entire span of complex life on Earth.

Chris - Quite a tall order, that one, isn't it? How did size change over time? Because when we think back to the era of dinosaurs and things, we think of these enormous creatures, where life today is small in comparison. But immediately it strikes me that in the sea we still have today enormous creatures. Blue whales are absolutely huge. And has that always been the case, or has there been a shift over time?

Kristina - When we compiled this data, we've noticed that we always see increase in size over time. Animals do tend to get larger and the rate at which they do it can differ. And it also depends on what kind of group of animals it is, whether we're talking reptiles or invertebrates or mammals.
Some of the largest animals that we found were found in the more recent times, in this period called Neogene, which is from around 23 million years ago. And this is kind of when we started seeing, for example, the iconic megalodon, we started seeing some really large whales. But even though these animals could have reached sizes around 20 metres, they are still 10 metres shorter than the blue whale, which we see around today, which I think is incredible that we happen to live at the same time as the largest marine animal that ever lived.

Chris - The interesting transition that's also, I suppose, happened is that we've gone from an era of largely reptiles to an era where the really big animals are mammals now in the oceans, haven't we?

Kristina - Definitely. So, actually reptiles are the largest group of extinct large animals that we found. The largest amount of large reptiles that we found were identified from the Mesozoic, which is also called the age of reptiles.

And this is exactly the time where you would also be thinking about dinosaurs. We, of course, at the end of the Mesozoic had the bolide impact, which caused all dinosaurs to go extinct. And it also heavily affected animals in the marine realm, which included these large ichthyosaurus, which are these kind of fish-shaped reptiles that really dominated the oceans. And actually the largest animal that we found, not only in the Mesozoic, but overall was an ichthyosaur, which could reach 21 metres in length. In the following era, the radiation of mammals occurred and they started dominating the oceans, although it didn't happen immediately. It did take some time for these large whales and other mammals to spread out and reach the incredible sizes that they reach today.

Chris - Now you've got this, it's obviously a first step to establish a foundation for understanding more about what hasn't made it into the present era. What has emerged in terms of interesting findings and also what gaps has this revealed to us? Things we don't know that we really now need to go after to fill in the holes.

Kristina - This study kind of represents a good starting point just by providing that definition of what constitutes megafauna, what constitutes these impressively large animals, which then can be applied - if needed - in other studies. Another thing is that during the process of gathering information, like for example, the ecological roles. And so, for example, gathering the information of what kind of diet some of these animals had, we realised that for quite a few of them, we still do not have this kind of information. And this is quite crucial to be able to make proper conclusions explaining what was happening in the ecosystems.

We do have some kind of starting point in the study. We do have pretty good information about, for example, the majority of the megafauna were large animals that ate other animals. So, they weren't really filter feeders, unlike what we see today, which is really interesting.
But for many, many other aspects, there are still definitely gaps that need to be fulfilled.

Chris - One thing I often wonder when I sort of go down to the beach and I look out on the sea and realise that three quarters of our planet is covered in ocean. And I often think back to the time when life was in its infancy. Were those oceans absolutely replete and boiling with life? Or were they relatively underpopulated? Was it masses of empty water and the odd thing living in it? What was it like then?

Kristina - It really depends on which specific time in the history of life you look at. Of course, the Cambrian explosion is popular because that was the time where really life was booming. There was a lot of shallow areas, areas where life truly was replete. Whereas if you would go to times after some kind of mass extinctions, of course, life would be more rare.

Chris - So, if you had to go back in time and you were going to take a dip, because I often think, would something eat me the minute I got into one of these primitive oceans? What period would you go back to knowing you'd be safe to have a swim versus if you desperately wanted to get eaten, when would be the best time to go back to?
Kristina - I would say that for a human, I suppose Cambrian would be the best time because that was the era of invertebrates. And I'm not sure if some of these invertebrates would know what to do with a fleshy being such as myself if they found me in the water. Whereas if I would desperately want to get eaten, I would say like anytime when the megalodon was around as a safe bet.