Dinosaur fossils reveal how birds got their distinctive walks

For decades, the world has been fascinated by the short arms of Tyrannosaurus rex. As it turns out, their legs are pretty interesting too.

A study published in the journal Nature reveals that some of the most famous theropods, a group of extinct carnivorous dinosaurs, could only move their legs in a very limited way.

Dinosaurs like T. rex had rigid joints and straight bones that restricted how they could move, meaning the knees of these animals were essentially acting like a hinge.

Things only began to change in smaller theropods, like Deinonychus, whose dynamic lifestyles lent themselves to having more flexible legs. One of the two lower leg bones, the fibula, became detached from the ankle and gradually shrank, allowing these animals to twist and move their legs in new ways.

This process eventually led to the diverse range of legs and walking styles seen in living birds, which are all descended from extinct dinosaurs. Dr Bhart-Anjan Bhullar, who co-authored the research, says that these changes have been overlooked for many years.

“The fibula is, in general, the more diminutive of the two lower leg bones, and often neglected in the study of vertebrate form and function,” Bhart-Anjan says. “But evolution acts on all parts of the body, great and small, so structures that have been ignored are often gold mines for new insights and untold tales.”

“We found that the very features that appeared in early dinosaurs to stiffen the leg ended up being co-opted in birds and their close relatives to mobilise the knee joint in a unique and extreme way. Over and again, we see that evolution operates by repurposing existing structures and functions, often in surprising and unpredictable ways.”

How bird legs evolved

At first glance, it might seem like birds have very similar legs to ours. The obvious difference is that their knee joint appears to be the other way around.

In fact, this isn’t their knee joint at all – it’s actually the bird’s ankle. Most of the upper leg is normally buried under their plumage, leaving the lower leg, the ankle joint and the foot protruding out from underneath.

This basic body plan has its origins hundreds of millions of years ago during the Mesozoic Era, which spans the Triassic, Jurassic and Cretaceous Periods. This was the time of the dinosaurs, which saw them rise from small reptiles to a diverse group spanning the entire world.

One of the key features of the dinosaurs is that they walked upright. Rather than having their legs out to the side, like in other reptiles, dinosaurs have their legs underneath their body. This required a variety of changes in the shape of their bones, including the tibia and fibula that make up the lower leg.

Until recently, it was generally thought that the important changes were happening in the tibia, and those in the fibula were just a side effect of the leg’s evolution. The researchers behind the current study weren’t so sure, so decided to look at the fibula more closely.

They compared x-ray videos of a walking iguana, crocodile and guineafowl to see how their fibulas moved during motion, and used them to create 3D videos. This showed that the much shorter bird fibula was able to cope with much greater movement than either reptile, being able to twist by over 100 degrees.

Dr Armita Manafzadeh, the study’s lead author, explains that this allows birds to live a different lifestyle from their distant relatives.

“The reduction in the size of the fibula means it can move completely differently from that of other living reptiles,” Armita says. “It’s why their knees are uniquely able to spin, allowing them to navigate their world more effectively.”

“They use that mobility to turn and manoeuvre on the ground, but we suspect they’re also using it in mating displays, prey gathering, and moving about tree branches.”

Changes in the dinosaur leg

Having found what change had taken place, the researchers now had to find out when it happened. To do so, they went back through the fossil record, looking at a variety of different animals who were on the path to birds.

In Marasuchus, a close relative of dinosaurs, the researchers found that the tibia and fibula generally moved together. This was partly due to the ankle getting less mobile, forcing the bones to follow each other closely.

This became more pronounced in large theropods like Allosaurus and T. rex. These dinosaurs developed a structure known as the fibular crest, which helped to lock their lower leg bones together.

The hinge-like knees of these dinosaurs eventually gave way to more flexible structures in Deinonychus, a relative of Velociraptor. By the time of Ichythornis around 90 million years ago, some dinosaurs already had bird-like knees that allowed them much greater mobility.

“It’s important to remember that, while the knees of extinct theropods such as Allosaurus would have been restricted to virtually hinge-like motion, this doesn’t necessarily mean that such dinosaurs were ‘bad’ at manoeuvring,” Armita adds. “Instead, they had a different strategy, relying on rotation at their hips to navigate the world in 3D.”

“As animals on the bird-line increasingly adopted a crouched limb posture, this strategy became less effective. Instead, the size reduction of the fibular unlocked a completely novel solution for controlling how they moved.”

The researchers plan to compare more living animals to their ancient relatives, so they can get deeper insights into the evolution of their distinct ways of moving.