When we think of dinosaurs, we tend to think of those that walked on two legs, like Tyrannosaurus rex and Velociraptor, and those that walked on four legs, like Diplodocus and Apatosaurus. But besides the way they walked, what were the main differences between the two? Let’s take a look. 

Evolution at work

The earliest dinosaurs were all bipeds, meaning they walked on two legs. They were relatively small - about a metre long - and walked on their toes (a little like cats do). This, combined with a long stride, meant they could move pretty fast and travel longer distances than many of today’s reptiles, like lizards and crocodiles. It wasn’t until the mass extinction at the end of the Triassic period that dinosaurs began to grow much larger and walk on four legs. 

Two vs four legs

Theropods - dinosaurs that were largely meat-eating and had lightweight, hollow bones - walked exclusively on two legs. Most of these dinosaurs had sharp, curved teeth and claws. They were the most diverse group of dinosaurs and ranged in size from the tiny Microraptor to the huge Tyrannosaurus rex. They were also quick on their feet - even T. rex could run at speeds of up to 25mph. During the early Jurassic, dramatic changes in the Earth’s climate meant that some dinosaurs began to get much larger and heavier. This meant that they had to shift to walking on four legs. The dinosaurs that made these shifts were largely plant-eaters, and one theory suggests that because the tough, fibrous plants they were eating needed longer to digest, they developed huge stomachs and intestines that couldn’t be supported by a two-legged stance. 

Sauropods like Diplodocus also grew long necks that allowed them to eat without having to move very much, so they could use less energy for walking and more for digestion. That means they were much slower than theropods, with top speeds of between 5-15mph depending on their size and weight. Because they were so large they were rarely preyed upon, meaning they could afford to move slowly. 

Armour and weapons

Some dinosaurs - like the thyreophorans (Stegosaurus and Ankylosaurus, for example) and the ceratopsians (such as Triceratops) - developed armour and built-in weapons along with their heavier bodies and large stomachs. These added further weight and gave them denser bones, meaning a four-legged stance was better for them. 

What did they share?

Both two-legged theropods and four-legged sauropods shared bones that were hollow and filled with air, similar to today’s birds. This made their skeletons both light and strong - which is why sauropods’ necks could get so long. These light, air-filled bones also worked with their lungs to make breathing super efficient, meaning they could take in oxygen when breathing both in and out, as opposed to only when breathing in, like humans. 

Discover dinosaurs in action

The Dinosaur World England season is back, with shows playing at the Troubadour Wembley Park Theatre from 28 July - 6 September 2026. Don’t miss out on this roarsome family adventure where you’ll meet a host of different dinosaurs - check out the Dinosaur World prices and book your dinosaur tickets today!