The first animals were incredibly small, but once they evolved (around 800 million years ago) it didn’t take them long to achieve giant body sizes, says Will Newton.
These prehistoric giants weren’t just confined to the oceans where they were supported by the buoyancy of water, they lived on land too, leaving footprints so large that an average-sized human could comfortably lay inside of one.
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A lot of prehistory’s most infamous giants, such as T.rex, lived during the Mesozoic Era, a period of time that lasted from 252 to 66 million years ago and is best known as the ‘Age of Reptiles’.
This is the period of time that encompassed the rise, reign, and fall of the dinosaurs, as well as their distant cousins the pterosaurs and the marine reptiles. While these were some of the largest animals to ever live, they’re not quite the biggest.
What’s the largest animal that has ever lived?
Believe it or not, the largest animal to ever exist is actually alive today - the blue whale, or Balaenoptera musculus. This 29.9m-long, 199-ton leviathan appeared in the Early Pleistocene (around 1.5 million years ago) and is product of an evolutionary trend towards gigantism in whales.
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As a group, whales can trace their evolutionary roots back to a fox-sized mammal that had four legs and lived on land. This small, unassuming animal is known as Indohyus and it looked and behaved a lot like a pig, only it’s thought to have spent a lot of its time in water.
After Indohyus came animals like Pakicetus, Ambulocetus, and Basilosaurus. These animals record the stepwise acquisition of adaptations for a fully aquatic lifestyle, such as flippers, pad-shaped tail fins, and - eventually - giant body sizes.
From these early whales, baleen whales evolved. This group is characterised by the huge, bristly plates that they use to filter plankton, krill, and small fish from the water column. These plates are made of keratin - the same thing that makes up our hair and fingernails - and they hang from the upper jaws of baleen whales in place of teeth.
This way of feeding is incredibly efficient and allows baleen whales to consume enormous amounts of food, which enables them to grow to humongous sizes. The blue whale is the culmination of millions of years of evolution and serves as a prime example of how the solution to ecological success underwater is often size.
That said, size isn’t everything and bigger isn’t always better - at least in the animal kingdom. At times of environmental unrest, it’s large animals that often face the most ecological pressure and, as a result, are most at risk of extinction. This is because large animals tend to live longer, reproduce less frequently, and - evolutionarily speaking - take longer to adapt to a rapidly changing environment.
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Despite large animals’ vulnerability to extinction, there exists an evolutionary theory that states that groups of animals get bigger over time. This is known as Cope’s Rule and it has been used to explain why mammals, in particular, have dramatically increased in size since their small, humble beginnings in the first half of the Mesozoic Era.
What is Cope’s Rule and how does it work?
Cope’s Rule is named after the American palaeontologist Edward Drinker Cope and postulates that animal lineages tend to increase in body size over evolutionary time, with directional selection acting on an organisms’ size more so than it does on other morphological traits.
To put it simply, Cope’s Rule suggests that larger body sizes are associated with increased evolutionary fitness, meaning larger animals are more likely to survive, reproduce, and pass on their giant genes to the next generation. This, over the course of millions of years, produces larger and larger animals.
While Cope’s Rule has been discussed for more than a century, it’s still poorly understood and evolutionary biologists are split on whether or not it is a valid theory. There are many cases that support Cope’s Rule; horses, for example.
This group's earliest members were no bigger than housecats but over time they increased in size to the point that today’s draught horses, which exceed heights of 2m and weigh more than a ton, are amongst the largest horses that have ever lived.
The same evolutionary trend towards gigantism is seen in many other groups of mammals, such as whales, dogs, and camels, as well as several extinct groups of reptiles, such as pterosaurs and ceratopsian dinosaurs - the group that includes the well-known horned dinosaur, Triceratops.
However, there are many cases that don’t support Cope’s Rule, suggesting it doesn’t hold true in all groups of animals, or at all taxonomic levels. Some have even suggested that Cope’s Rule may simply be a psychological and/or a statistical artefact - the result of singling out lineages that display the trend we want to see and erroneously focusing on extremes that intrigue us, rather than examining full ranges of variation.
What are the benefits of being big?
Regardless of whether or not Cope’s Rule is a valid theory, there are many advantages to being big and therefore reasons why certain groups of animals may have followed an evolutionary trend towards gigantism.
The major benefits of being big are increased defence against predation and - at least in carnivorous groups - increased predation success. Another benefit is a more varied diet. Typically, the larger an animal is, the greater its range of acceptable foods. This helps large animals survive periods of climatic upheaval when there may be a shortage of certain foods.
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However, this resistance to climatic variation and weather extremes is only beneficial to a point; during times of rapid change, larger animals are more susceptible to extinction, largely due to the fact that they have a much longer generation time than smaller animals and therefore a slower rate of evolution.
Other benefits of being big include better insulation against the cold - large animals have a small surface area to volume ratio, so they lose heat a lot slower than small animals that have a high surface area to volume ratio. On average, large animals also live longer and are more intelligent than small animals.
What are the downsides of being big?
There are, of course, some downsides to being big, chiefly the fact that large animals need to eat more than small animals do to sustain themselves. They also need more water and access to larger territories. This dependency on limited resources is often what keeps the total population of large animals relatively small, which in turn makes them more susceptible to extinction.
As animals get bigger, they also tend to produce fewer offspring. For example, the average elephant will only give birth to four or five calves during the course of its life, while the average rat will produce roughly 10 pups per litter and may have as many as 12 litters per year. The offspring of larger animals also require more parental investment and take years, sometimes decades to fully develop.
Is it better for animals to be large or small?
In general, it’s better to be bigger, but there comes a point when animals get so big that extinction is almost inevitable. It’s widely thought that extinction events are what keep large animals in check and stop certain groups from achieving gargantuan proportions. Yes, evolution may favour large animals, but extinction favours the small and counterbalances this gradual increase in body size that we see over time.
While extinction events may have wiped out groups of large animals before they got too big and somewhat stabilised the distribution of animal size over time, there were several periods in prehistory when animals were certainly a lot larger, on average, than they are today. The best example of such a period is the Cretaceous (140 to 66 million years ago) - a time when dinosaurs ruled over the land and grew larger than houses.
Why were dinosaurs so large?
When we think of large, prehistoric animals, we often think of dinosaurs - and for good reason. From 201 to 66 million years ago, they were the largest, most dominant animals on land, dwarfing our comparatively puny mammalian ancestors and consigning them to the ecological shadows.
The largest group of dinosaurs are the aptly-named titanosaurs, the last-surviving sub-group of a diverse group of herbivorous dinosaurs known as the sauropods. They lived during the Cretaceous and are known from fossils found on all seven continents, including Antarctica. Like other sauropods, titanosaurs had incredibly long necks, barrel-shaped bodies, and columnar legs, but they were almost twice as tall and more than four times as heavy.
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Argentinosaurus is widely regarded as the largest titanosaur, not to mention the largest animal to ever walk on land. It measured 40m from head to tail, stood taller than a five-story building (at around 20m), and weighed in at a whopping 80 tons. To fuel such a giant body, some think Argentinosaurus may have had to eat roughly 850kg of plants per day. Its close relatives Puertasaurus and Patagotitan were almost just as big.
How dinosaurs grew so large has puzzled researchers for more than a century. It’s often said that dinosaurs were able to achieve giant sizes because of higher oxygen levels and lower gravity during the Mesozoic Era, but these theories have been largely debunked.
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Instead, researchers think dinosaurs possessed a combination of unique adaptations that helped support their massive bulk. A particularly important adaptation is their upright, pillar-like stance. This stance is not only capable of supporting lots of weight, it’s also more energy efficient than the sprawling stance of their distant relatives, lizards and crocodiles.
As a group, dinosaurs are also known for having relatively light and hollow bones. Other animals have solid bones, making them a lot heavier relative to their size. If dinosaurs had solid bones, like those of mammals, they’d have never been able to reach the sizes they did - gravity would have brought the likes of Argentinosaurus crashing down to the ground.
Another adaptation unique to dinosaurs (and their descendants birds) is their incredibly efficient respiratory system. The hollow bones of dinosaurs contained air sacs that kept them light and served as temporary stores of oxygen. The combination of these air sacs and their lungs meant that dinosaurs were supplied with oxygen when they breathed in and when they breathed out. This boosted their metabolisms, which in turn allowed them to grow larger.
It’s these adaptations that ultimately allowed dinosaurs to become giants. While not all dinosaurs were giants (some, such as Microraptor, were no bigger than sparrows) they were a lot larger than today’s animals, on average. According to a relatively recent study, most non-avian dinosaurs weighed roughly 3.5 tons, which is about as heavy as a modern hippopotamus.
Why aren’t there many large animals alive today
If we compare today’s snapshot in time to prehistoric times, then there are far fewer large animals; but this is somewhat of a false dichotomy. In doing this, we’re comparing a single global ecosystem to thousands of global ecosystems lumped together.
The Mesozoic Era was indeed a time of giants, but most of these giants didn’t live alongside one another. Take T.rex for example, it lived closer in time to the invention of the iPhone than it did to Stegosaurus, another giant dinosaur.
Like today, most prehistoric periods had their giant representatives, as well as diminutive representatives and all the other sizes in between. The reason we think prehistoric animals were so big may simply be a matter of perception. Instead of viewing prehistory as a collection of different periods, each with its own distinct animals, we tend to think of it as one extended period when giants lived together en masse.
That said, there are fewer large animals alive today than there were just 12,000 years ago - a span of time that, geologically speaking, is shorter than the blink of an eye. This is the period that marks the end of the last ice age and the subsequent extinction of many large mammals, including wooly mammoths, sabretooth tigers, and giant sloths.
Some continents were worse hit than others by this extinction event, but most recorded heavy losses of megafauna - a term used to describe animals that weigh more than 45kg. The causes of this extinction event are heavily debated, but the consensus is that a combination of rapid climate change and predation pressure from humans wiped out many groups of megafauna. The result is today’s comparatively low density of large terrestrial mammals, most of which are confined to Africa.
While the largest animal to ever exist may be alive today (the blue whale), it’s fair to say that, on land at least, we’re living through an unusual period where the largest animals are significantly smaller than their counterparts from just a few thousand years ago. It’s quite alarming to think that we may have had something to do with this.
Will animals get bigger in the future?
There’s certainly a possibility that we could see the return of some large animals in the future, though it’s unlikely that we’ll see giants like Argentinosaurus ever again. This is because they, and other titanosaurs, had a very specific set of adaptations that no modern groups of animals possess.
It seems more likely that, before we see animals get bigger, we will witness the disappearance of what few large animals we have left. There’s growing evidence to suggest that we’re currently undergoing a sixth mass extinction and one that may be even more destructive than the asteroid-induced event that wiped out the dinosaurs.
As mentioned above, extinction events don’t typically favour large animals. This in mind, instead of wondering what giants may evolve in the future, it’s perhaps a better idea to channel our energy into protecting the giants alive today - they’re going to need all of the help they can get if the grim forecasts are correct.