Professor Dr P Martin Sander of the University of Bonn’s Palaeontology Department discusses his research into the evolution of giant body size in animals and how this research can tell us more about the fascinating mystery of the giant ichthyosaur – a creature that went extinct around 201 million years ago.
How big did animals get, and what was the largest to ever roam the planet? Dr Sander’s new research indicates that the largest ever animal may have been the Triassic giant ichthyosaur.
Until now, fossils from the Late Triassic period have been scarce. Despite their size, giant ichthyosaur are often absent from the fossil record, with most remains often being fragments of much larger bones.
The origins of the ichthyosaurs start in the aftermath of the worst-ever mass extinction at the end of the Permian period over 250 million years ago. But where did they go? Were there disadvantages to achieving such giant body size?
Innovation News Network spoke to Dr Sander to further understand the evolution of giant animals and what environmental and evolutionary factors played a part in this.
What did giant ichthyosaurs look like and when did they live?
Ichthyosaurs were dinosaur-like creatures that lived between 250-90 million years ago, which overlaps with land-living dinosaurs. We can characterise their appearance from fossils that were first found in the UK and Germany.
They were fish-lizards, meaning they descended from lizard-like animals and returned to the sea. This process is known as secondary aquatic adaptation, which still happens today. However, the ichthyosaurs were the first to do this. They did what whales did later by becoming fully marine and cutting all connections with the land, as they were life-bearing creatures.
Our research focuses on the Triassic ichthyosaurs because they are much less well known than the familiar Jurassic ones – there haven’t been any Triassic ichthyosaurs discovered. Jurassic ichthyosaur looked like today’s dolphins; however, they were the lone survivors that killed off most of the Triassic ones about 200 million years ago. The Triassic ones were much more varied in body size, from small, fish-sized creatures to giant, whale-sized ones.
How are ichthyosaur related to other reptiles?
It’s difficult to understand what other reptiles are related to the ichthyosaur because marine reptiles didn’t appear on as fossils before the Triassic age, and there are no intermediates between any kind of land-living reptile and these fish lizards.
How they evolved is clearly understood in whale evolution, which happened 45 million years ago. We’re partially missing that in ichthyosaur history because it’s too far back. We only know how they appeared on Earth and their specific characteristics, going from egg-laying on land to evolve life-bearing in the ocean. But, like whales, ichthyosaurs had to surface to breathe.
Why do land animals return to the ocean to become giant ichthyosaur and whales?
This has been constant over the last 250,000,000 years and is still ongoing. A good example is the Californian sea otters, which are intermediates that live close to the coast and can go on land. There are resources in the ocean; somehow, mammals and reptiles are better at using those resources than fish.
Common knowledge has always believed that the blue whale is bigger than anything else. But now there are two challengers. One is these giant ichthyosaurs of the Late Triassic, but we have very little information about them. We only know they come from all corners of the Earth, but the fossil record from about 230 to 200 million years is minimal. This means that we cannot discount the possibility that there were blue whale-sized ichthyosaurs.
Giant animals have roamed Earth for millions of years. Why does this continue to rise?
Before we ask why gigantism arises, we have to define what giant means in terms of body size, but there is no unit to measure it with. Therefore, we use so-called proxies and other ways of describing size in science.
Size is measured most easily as mass, which is different from weight. Weight happens on land, and mass is independent of gravity. Mass is the most common proxy or descriptor used for size in biology. But length or height are also essential proxies.
Then comes the question: how heavy does the animal have to be considered giant? Seemingly, animals in the sea can get bigger than on land. Then, if we ask, when did the first giant animals arise? When do they show up on Earth? After the mass extinction event at the end of the Permian era, animals that weighed more than ten or 20 tonnes appeared in the ocean. So, from the Triassic onwards, animals with giant body sizes appeared in the sea. Before this period, giant animals were never seen, but we don’t know why.
Our research has revealed that the first giant animal that ever lived on the planet was a giant ichthyosaur. By any standard, it is a large animal with a two-metre-long skull, suggesting the creature’s vast size. But calculations based on other body parts indicate an animal nearly 18 m long with a mass of 45 tonnes.
Moreover, we can see how giant this ichthyosaur was by evaluating the last 250,000,000 years and using proxies to determine giant body sizes in today’s animals. Whale sharks, the largest fish today, weigh around ten tonnes and are about 15 metres long. Only the secondarily adapted animals, the land-living animals that return to the sea, appear bigger than the biggest fish.
If there are advantages to being larger, speaking in species and even speaking in individuals, life will go where it can – there are always advantages to larger body size.
What are the primary evolutionary advantages and disadvantages associated with the evolution of giant body size in animals?
This concept can be compared with economics, where larger companies enjoy certain advantages, and they also have disadvantages. The advantages are known as the economy of scale. If a company produces one million cars annually, it will likely be more efficient than one that produces 100,000 cars yearly.
Biological scaling is a fundamental concept in this kind of research because it asks how certain factors relate to size or, more specifically, mass. For example, how much muscle energy a giant animal has to use to travel through the ocean doesn’t increase as fast as its size.
This concept can be understood through the example of ships. It’s more energy-efficient to have giant ships than to have medium-sized vessels.
For animals, this effect of scaling is that metabolic rate and energy demand for staying alive do not increase as fast as body size. This means that the economy of scale in larger animals is more efficient. Another advantage of scaling and giant body size is the concept of sexual selection — where larger individuals of one sex, commonly males, will have higher reproductive success than smaller individuals, and thus, the larger animals will pass on their ‘largeness’ genes to their offspring.
There are always size limits, but the essential thing in biology is that the economy of scale and sexual selection will often favour larger sizes. If life can go there, then it will. In the end, the question boils down not to what the advantages of larger body sizes are, but where is the organism’s limit to body size?
One effect is that the ocean habitat has more resources and migration opportunities; marine creatures can explore this and have the chance to get bigger. A concept known as ‘island dwarfing’ demonstrates the opposite of this, which has seen elephants, for example, living on small islands get smaller.
The other reason for animals in the sea getting bigger than those on land is because there’s no gravity in the ocean. This means an animal’s skeleton or construction doesn’t have to support itself. For those two reasons, animals in the sea are expected to have a higher ceiling of body size than animals on land.
Do evolutionary trends or constraints apply to evolving giant body sizes in different animal groups?
We need to look further than land versus sea because if it simply came down to this, why are dinosaurs bigger than elephants? The biggest land-living dinosaurs were ten times bigger than the biggest elephants.
This is where biology comes in more precisely. For example, giant long-neck dinosaurs and tyrannosaurs had respiratory systems akin to a bird’s. This allowed them to have a much lighter skeleton. Various factors allow gigantism on land. However, the factors enabling it in the sea have not been researched.
That’s what we’re aiming to discover right now. First, we have to try and find out what are the most giant animals that do or have inhabited the ocean. Once we know that, then we can start looking for biological factors.
What ecological factors or conditions might have favoured the evolution of giant marine reptiles?
It’s been theorised that the Triassic ocean may have been warmer than the modern ocean. However, the argument goes the other way around when studying whale evolution. Larger-sized whales, such as blue whales, only evolved in the last few million years. This is generally attributed to the ocean having become more productive because of the cooling of the Earth.
There’s also evidence to suggest that giant body size occurs in more productive environments. The Perucetus, an early whale from Peru, has been statistically estimated to weigh between 80 and 350 tonnes. This could have been the biggest animal that has ever lived on Earth based on mass. It evolved on the Peruvian coast, which has a hugely productive environment because of ocean current upwelling.
How did giant ichthyosaur achieve and sustain such enormous body sizes?
I believe that to become large, either on land or in the sea, you must be warm-blooded. Looking at modern giants in the ocean, like whales and sharks, some sharks are large, but they’re nowhere near as big as whales. Therefore, giant ichthyosaur were likely warm-blooded. The same applies to creatures on land, by the way.
Sea turtles, crocodiles, and lizards, which are cold-blooded marine reptiles, are tiny compared to whales. This is because cold-blooded creatures can’t grow fast enough. If a creature needs to grow up to 20 tonnes, but it would take 500 years before it is adult and can reproduce, the chance of surviving for those 500 years is extremely low.
What role does oxygen availability play in the maximum size animals can achieve through evolution?
This boils down to the question: Is it nature or nurture? Is it intrinsic or extrinsic factors? We know that the atmosphere’s oxygen content has changed through geologic time. Three hundred million years ago, in late Carboniferous times, the oxygen content in the atmosphere was 30% — much higher than today’s content at 21%.
Oxygen content reached an incredible high but then decreased dramatically between 300 and 250 million years ago, at the time of the Great Extinction. At this time, oxygen content was around 18% — lower than today. It dropped even further towards the end of the Triassic, and then eventually, about 90 million years ago, it reached modern levels.
Based on plots of maximum body size and comparing it to atmospheric oxygen content, a hypothesis suggests that early dinosaurs, Tyrannosaurus, and long-neck dinosaurs had a competitive advantage in the thin air of the Late Triassic because they had bird lungs. One of the advantages of the bird lung is that it handles thin air much better than a mammal lung.
This principle would apply to whales and ichthyosaurs because all secondarily marine creatures are air-breathing. Giant ichthyosaur and whales would have depended on atmospheric oxygen, but again, maximum body size does not seem to track maximum oxygen content and vice versa because the giant ichthyosaurs of the Late Triassic existed during the period of the lowest oxygen. So, there is no indication that oxygen content plays a role in body size.
And what about temperature? Some sea surface temperatures were very high in the Triassic period, but this doesn’t apply to whales. 50 million years ago, when whales went into the sea, the water was much warmer than today, but whales became larger in cooler water.
I suspect neither oxygen nor water temperature really influenced the evolution of giant body size. This pattern repeats from what we’ve seen on land – environmental factors don’t determine size.
How has the evolution of giant body size influenced our understanding of Earth’s ancient ecosystems?
In palaeontology, we must understand modern ecosystems to understand ancient ecosystems and vice versa. Therefore, by studying fossil ecosystems, we can understand if modern ecosystems are in any way unique or if they’re simply reruns of what has happened before.
The basic pattern in evolution is the day-to-day Darwinian, gradual change of organisms. But then, if we throw in palaeontology or geology, we see geological agents resetting the course of evolutionary history, like exceptionally huge volcanic eruptions and giant meteorite impacts. If they’re big enough, you could hypothesise that a really giant meteorite impact could kill off life on Earth and end evolution.
That’s the general pattern, but after these extinction events, regular evolution will take over again and produce the same thing or a different thing, depending on the biology.
If Earth suffered another mass extinction event, could giant animals evolve again?
Life will always go to its limits – meaning giant animals could definitely evolve again. If whales were killed off in an extinction event, there would always be another warm-blooded animal that could reach the size of a whale or giant ichthyosaur due to warm blood allowing faster growth.
