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Why were dinosaurs so big? The secrets of titanosaurs' super size

Dinosaurs are the largest land animals that have ever lived. Titanosaurs such as Puertasaurus, Patagotitan and Argentinosaurus were the longest and heaviest of them all - far longer than a blue whale and perhaps as heavy as 10 or more elephants.

Watch our animation and read on to find out how dinosaurs were able to grow so big.

Super-sized sauropods: the titanosaurs

When we think of big dinosaurs, it's often famous examples like Spinosaurus and T. rex that spring to mind. But dinosaurs got much, much bigger.

If you're shouting 'What about Diplodocus and Brachiosaurus?!', you're on the right track. Sauropods such as these were enormous.

But the true champions were the titanosaurs, the last-surviving sauropod group. Patagotitan, for example, was nearly half as long again as Diplodocus, almost twice as tall and about four times heavier.

Artist's impression of the titanosaur Patagotitan

Patagotitan was around 37.5 metres long and weighed an estimated 57 tonnes, making it 50% longer and four times heavier than Diplodocus. © Sphenaphinae (CC BY-SA 4.0) via Wikimedia Commons

Our dinosaur expert Professor Paul Barrett says, 'Although the blue whale is famous for being the biggest animal that has ever lived, it cheats - it lives in water, which helps to support its weight.'

'Whales lack the bone structure and muscles to support their own weight and, if they beach, they die. By contrast, dinosaurs always held up their own weight.'

'Titanosaurs such as Puertasaurus, Patagotitan and Argentinosaurus are by far the largest animals to ever live on land.'

Find out more about 11 of the biggest dinosaurs and how scientists work out their size.

How do dinosaurs measure up to the largest land mammals?

African savanna elephants, Loxodonta africana, are the largest land animals alive today. Most weigh five to seven tonnes. The biggest titanosaurs were about 10 times heavier, maybe even more.

The largest known mammal that has ever lived on land - at least that can be reliably sized - is the extinct rhinoceros relative, Paraceratherium. It grew to about 7.4 metres long, just under 5 metres tall and weighed up to 20 tonnes. The prehistoric elephant Palaeoloxodon namadicus may have been slightly larger, but not much fossil material has been found, making an accurate body mass estimate difficult.

Paul says, 'Sauropods were the only group that were heavier than the largest land mammals.

'Interestingly, theropod dinosaurs such as Spinosaurus and T. rex were also the only animals to exceed the largest land-based mammalian predators in size.'

Artist's impression of a herd of running Compsognathus dinosaurs

Although dinosaurs were the largest land animals to ever live, not all dinosaurs were huge. Some were pretty tiny. For example, Compsognathus was only about 2.5 times as long as an average man's footprint. Bird-like dinosaurs and birds are smaller still. © Dotted Yeti/ Shutterstock

So why could some dinosaurs grow so large?

Various theories have been proposed to explain what enabled dinosaurs to reach such immense sizes - from low gravity to higher oxygen levels in the atmosphere. Most of these have been debunked.

For example, for Earth's gravity to decrease significantly, the planet would suddenly need to become much smaller in size. Instead, Earth grew in mass quickly in its early history - long before life evolved - and has been fairly constant in size ever since.

Likewise, oxygen levels in the Mesozoic Era weren't very different from what they are today. So this can't explain the enormous nature of some dinosaurs.

Scientists now think the reason dinosaurs were able to grow to such big sizes was down to a combination of characteristics. Read on or watch our animation to find out what these were.

Body shape

Upright stance

All dinosaurs had an upright, pillar-like stance. This could support a large body better than the sprawling stance of other reptiles such as lizards and crocodiles. It also allowed dinosaurs to use less energy to move.

Diagram showing the difference between an upright stance and a sprawling stance

With their legs positioned under their body rather than sticking out to the side, dinosaurs' weight was better supported. The biggest dinosaurs walked on four legs. Adapted from an image by Fred the Oyster, via Wikimedia Commons, licensed under CC BY-SA 4.0.

All dinosaurs shared this body shape advantage. But while two legs are good for running - and so all predatory dinosaurs were bipedal - four legs can support a much larger body.

This is why the biggest bipedal dinosaurs, such as Spinosaurus, Tyrannosaurus and Giganotosaurus, only weighed about seven tonnes. That's still massive, but the largest sauropods were around eight to ten times heavier. They walked on four legs.

Unique hands and feet

Sauropods had further features that made them more stable and capable of carrying hefty weight. Their wrists and ankles were less mobile, which made them stronger. Their hands and feet were also huge and padded, like those of elephants, which helped them spread their weight.

Wide hips

In addition, titanosaurs had wider hips than earlier sauropods, which made them even more stable. It also made more space for their sizeable stomach. A bigger belly meant they obtained more energy and nutrition from the plants they ate. Important stuff when you need to power such a big body!

Long necks

Gathering enough food using the least effort possible was something these giants were good at.

Sauropods had very long necks, which allowed them to stand still and stretch high, low and wide for the best plants around. Part of the reason elephants can grow so big is because their trunk lets them forage for food without moving much, in a similar way.

Extremely long necks also meant sauropods could pluck leaves from the tops of tall trees, which were out of reach to most other animals - much like giraffes do today.

Artist's impression of a Diplodocus dinosaur eating leaves high up in a tree

Sauropods' long necks gave them a big advantage, as they could eat from a wide area without moving and access food that was out of reach for other animals. © Catmando/ Shutterstock

Sauropods' long necks were beneficial for other reasons too, which we'll come to later.

One of the reasons sauropods were able to have such long necks was because they had relatively small heads. This was possible because they had fewer teeth. They swallowed without chewing.

Light bones

Another secret to dinosaurs' size lay inside their bones. Unlike most other reptiles and mammals, sauropods and theropods had some bones that were hollow and filled with air.

Other animals have solid bones, which means they are much heavier for their size. This is why the heaviest animals alive today - whales - can only live in water. It helps to support their mass, which can be as much as 180 tonnes in the case of blue whales, equal to 30 elephants.

Dinosaurs' air-filled bones were lighter than solid bones, but still strong. Sauropods' lighter neck bones were another reason their necks could be so long.

Artist's impression of Mamenchisaurus

Professor Paul Barrett and colleagues recently discovered that Mamenchisaurus sinocanadorum had a 15-metre-long neck, making it the longest necked dinosaur known. © Elenarts/ Shutterstock

Efficient, bird-like breathing

Dinosaurs' lighter bones were related to their bird-like breathing system.

The lighter bones of sauropods and theropods contained air sacs, which worked with their lungs to make their breathing very efficient.

Whereas humans and other mammals only receive oxygen when they breathe in, the combination of air sacs and lungs meant dinosaurs were supplied with oxygen even as they breathed out.

It's this same efficient breathing system that lets geese fly over the Himalayan mountains in Asia, where oxygen levels are low.

Without this ability to take up oxygen continuously, sauropods wouldn't have been able to grow so big or have such long necks. 

Three geese flying

Thanks to their efficient breathing system, bar-headed geese are able to fly over the Himalayas, despite the low oxygen levels there. Sauropods and theropods breathed in a similar way. © Imran Shah (CC BY-SA 2.0), via Flickr

Energy efficient

It wasn't just sauropods' breathing that was efficient. So was their metabolism - how food is converted into energy and eventually waste.

Although sauropods grew very quickly when they were young, their metabolism slowed and became more efficient as they got older, so they needed less food for their size than mammals.

Sauropods, and particularly titanosaurs, had very large stomachs. Food took longer to pass through it, giving the dinosaurs more time to digest the food and get nutrients out.

They were able to eat plants that other animals might struggle to digest. They probably ate conifer tree leaves and twigs and possibly even cones.

These larger dinosaurs also lost less energy as heat because they had less surface area relative to their body volume.

The sizeable advantages to being large

Being big had many benefits. The sheer size of titanosaurs and other large sauropods helped to protect them from predators.

They could also eat food that was out of reach to other animals. In times of famine or drought, they could survive for a while thanks to the fat and water stored in their bodies.

These dinosaurs were also suited to travelling long distances, helping them to find new sources of food, water and mates.

Artist's impression of five Argentinosaurus dinosaurs

A herd of Argentinosaurus titanosaurs roaming in search of food and water. © Catmando/ Shutterstock

Bigger isn't always better

As energy efficient as titanosaurs were, if food and water shortages went on for a long time, they would struggle to find enough to eat and drink. Larger animals need more food to survive than smaller ones.

Another disadvantage to being big is that it's easier to overheat. But titanosaurs and other sauropods had adaptations that helped them to keep cool.

Their lighter skeletons meant less power was needed to move their enormous bodies, so they produced less body heat when they moved than they would have if they had had solid bones.

Their long neck and tails increased the surface area through which they could lose heat. Elephants' large ears perform the same task.

Could land animals evolve to ginormous sizes again in the future?

Well, perhaps. But it's not likely they'd be as big as titanosaurs or other sauropods.

Paul explains, 'It's not impossible that any land animal around today could one day evolve to ginormous sizes like sauropod dinosaurs. But sauropods were able to grow so large because of a specific set of features that no modern animal has.'

Vulnerable to extinction

While we no longer have dinosaur giants roaming these lands, we are lucky to share the planet with our own colossal creatures, such as elephants, rhinoceroses, giraffes, polar bears, sharks and whales.

However, like titanosaurs, their size makes them more vulnerable to habitat loss, food shortages and climate change. It's important we all work together and act to make sure these modern-day giants don't just survive but thrive.