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Some Bipedal Dinosaurs May Have “Wagged” Their Tails

Computer simulation of extinct theropod Coelophysis running at maximum speed, with “wagging” tail. Grey tiles = 50 cm, played at 0.2× actual speed. | Bishop et al., Sci. Adv. 2021; 7: eabi7348

Contrary to the notion that bipedal dinosaurs' tails simply counterbalanced the weight of their heads, novel 3-D gait simulations reveal that the tail likely played a more dynamic role in dinosaur locomotion, according to a new study published in the September 24 issue of Science Advances.

The findings suggest that the tail of Coelophysis bauri, a well-known Triassic species, regulated the creature's angular momentum and efficiency by "wagging" to the side as the dinosaur walked and ran — similar to how humans swing their arms as they stroll. The authors infer that this mechanism may have existed in many other bipedal, non-avian dinosaurs.

"People had previously pontificated about what dinosaur tails were used for, but for the first time we have moved beyond mere speculation and have rigorously demonstrated how the tail functioned and what benefit it conferred to the dinosaur," said Peter Bishop, a postdoctoral researcher in evolutionary biomechanics at Royal Veterinary College in London at the time of this research and the first author of the study.

Bishop suggests that the team's simulations may pave the way for developing simulations of other extinct species and could help animators create more accurate portrayals of dinosaurs in movies and television documentaries. The findings may also help engineers solve biomechanical challenges as they develop bioinspired robotics.

"For example, engineers are now looking at how adding a tail to your robot can improve its maneuverability or stability, looking to the swinging tails of the modern cheetah for inspiration," said Bishop. "[C. bauri] and bipedal dinosaurs may offer new hints for how to improve things further."

Beyond Fossils and Footprints

Scientists have found it challenging to piece together how extinct species moved based on fossilized bones and footprints. Previous simulations of walking and running in non-avian dinosaurs have usually treated the tail as a rigid extension of the pelvis. Bishop and colleagues began their own simulation-based research as part of a larger project led by John Hutchinson, a professor of evolutionary biomechanics at Royal Veterinary College. They wanted to know whether the different ways that dinosaurs and other species moved was correlated with how well they survived the end-Triassic mass extinction.

"When we began our study we were more interested in understanding the animal as a whole, and how different anatomies relate to different performance abilities, such as maximum running speed," said Bishop. "But we saw some very intriguing results with our simulations early on, revolving around the tail, and so decided to focus more on that to see what was going on."

The team decided to focus their research around Coelophysis bauri, one of the earliest theropod dinosaurs, which were carnivorous and walked on two legs. The species, which lived about 210 million years ago, has features such as long hindlimbs and a long, heavy tail that more or less represent a body shape and size ancestral for dinosaurs as a whole. C. bauri is also better known than other early bipedal dinosaurs, with numerous skeletons to inform scientists about its anatomy.

Simulating Dino Locomotion

To analyze how C. bauri moved about, Bishop and colleagues drew from new advances in computational biomechanics to develop gait 3-D simulations, borrowing from methods originally developed for applications in medicine and aeronautics, including NASA. The technique they used works via a digital musculoskeletal model that mathematically relates the size and shape of various body segments, accounting for forces exerted on the bones by muscles during movement and the pushing force generated by interactions between the feet and the ground. Simulations based on the model would then work within constraints such as the laws of physics to identify the optimal gait to, for example, maximize running speed.

C. bauri actively swung its tail during locomotion. | Bishop et al., Sci. Adv. 2021; 7: eabi7348

"We're using new numerical methods that tremendously increase the speed at which our simulations are able to be run," said Bishop. "Previous studies had to rely on expensive supercomputers, which still take many thousands of CPU hours to solve. Our simulations, in contrast, all solved within half an hour using a single core on a standard laptop."

"Ours is the first study to model a dinosaur, or any extinct animal, in full three dimensions, including the neck, back and tail as multiple, separate components," added Bishop. "Not only is this more biologically realistic, but it allowed us for the first time to examine how motions of different parts of the body are coordinated with one another to effect locomotion."

First, the researchers developed a framework to simulate muscle-driven locomotion in theropod dinosaurs, which they initially tested using a virtual musculoskeletal model of a tinamou bird — a reluctant flyer that prefers to walk or run. The tinamou served as a worthy model animal because it belongs to the primitive palaeognath group of birds, making it a better representative of the ancestral qualities of birds than more recently evolved species.

After determining that the simulations strongly resembled observations of the tinamou's real-life gait, Bishop and colleagues applied the framework to a musculoskeletal model of C. bauri.

A Dino That "Wagged" As It Walked

Unexpectedly, the simulations revealed that the dinosaur made pronounced movements with its neck and its long, heavy tail as it moved, supporting the idea that the tail regulated its whole-body angular momentum around the vertical axis. The tail "wagged" to the left whenever the dinosaur's left hindlimb retracted backwards and "wagged" to the right when its right hindlimb retracted backwards.

When the researchers forced the tail to "wag" in the opposite pattern, the dinosaur had to adopt a modified, more energetically costly movement pattern, revealing the importance of the tail's movement for minimizing muscular effort. Similarly, when the researchers removed the tail in further simulations, the simulated dinosaur had to apply 18% more muscle effort as it moved.

"The instant I saw the results of the first simulation, I said to myself, 'well this is interesting, I didn't see that coming,'" said Bishop. "The reason being that no one had ever really thought about dinosaur tail motions in straight-line, level walking and running before, so we had implicitly assumed that the tail would have just stuck out the end of the pelvis like a static counterbalance. To see it moving, and wagging in synchrony with the hindlegs, was very exciting."