Dinosaurs dominated our planet not because of their massive size or fearsome teeth — but thanks to the way they walked

Dinosaurs may have taken over the planet and ruled for over 160 million years thanks to the way they walked, a new study suggests. By adapting to walk on both two and four legs, they diversified and outcompeted other organisms to become the dominant terrestrial vertebrates from the end of the Triassic (251.9 million to 201.3 million years ago) until their extinction around 66 million years ago during the Cretaceous period (145 million to 66 million years ago).

In a new study, published Feb. 7 in the journal Royal Society Open Science, researchers described how dinosaurs took over the world by filling ecological niches that emerged following a series of ecological collapses. Because dinosaurs walked on their hind legs, and later also on all fours, dinosaurs had a distinct advantage during a period that saw massive environmental changes. 

Dinosaurs were part of a group known as the Avemetatarsalia that evolved alongside a group of related reptiles, the Pseudosuchia, which includes the ancestors of modern crocodiles. The two groups appeared during the Triassic, in the wake of the Permian mass extinction 252 million years ago. 

The researchers used fossilized leg bones from 208 species of avemetatarsalians, pseudosuchians and their near relatives to determine their changes over time. 

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When they first appeared, pseudosuchians were the more diverse group. Some pseudosuchians walked on their hindlegs, but the majority retained a crawling habit. Dinosaurs were initially bipedal and could run, rather than just lumber along like their ancestors. Being able to move quickly enhanced their abilities to evade predators and catch prey, giving them an advantage during the drying climate of the Triassic. 

"There was severe pressure for food. Somehow the dinosaurs, which had been around in low numbers already for 20 million years, took off and the pseudosuchians did not," lead author Amy Shipley, a paleobiology masters student at the University of Bristol, said in a statement. "It's likely the early dinosaurs were good at water conservation, as many modern reptiles and birds are today. But our evidence shows that their greater adaptability in walking and running played a key part."

At the end of the Triassic, another mass extinction eliminated most pseudosuchians, with the exception of crocodylomorphs — which stayed on four legs. Some dinosaurs retained their upright, bipedal posture while others again dropped to all fours. This enabled them to diversify and fill a wide range of ecological niches.

"After the end-Triassic mass extinction, we get truly huge dinosaurs, over ten metres [33 feet] long, some with armour, many quadrupedal, but many still bipedal like their ancestors," study co-author Suresh Singh said in the statement. "The diversity of their posture and gait meant they were immensely adaptable, and this ensured strong success on Earth for so long."

While dinosaurs developed other features that helped them thrive— including the ability to regulate temperature with feathers and more efficient breathing mechanisms — the researchers concluded that their wide variety of locomotion strategies gave them a crucial edge.