What would be scarier, being chased down by a Tyrannosaurus, or being hunted by a Velociraptor? A groundbreaking new study in Scientific Reports has come up with an answer: Tyrannosaurs were far more suited than Velociraptors to chasing down prey. In fact, the enigmatic Nanotyrannus (“tiny tyrant”) may have been the most fearsome, well-adapted hunter of all the tyrannosaurs.

Unless you can see dinosaur tracks side by side, an incredibly rare find, estimates of a species' running speed can only be inferred from skeletal remains. These give researchers the ability to calculate how it walked and, with the aid of computer simulations, an estimate of how fast it could have run.

This new study takes a different approach. Instead of focusing on maximum running speeds, the authors have concocted a new method that estimates a dinosaur’s adaption to running. This measurement is known as its cursoriality, and takes into account two factors. The first is the body mass of the species – the heavier the beast, the more powerful legs it will need to run.

The second is the length of its lower leg compared to its upper leg. Faster animals, as a general rule, have longer lower legs than their upper legs. For example, cheetahs are faster than similarly-sized lions for this exact reason. The authors of this study took into account both these factors, creating an equation that described the species’ cursorial limb proportion score, or CLP.

^{The Nanotyrannus was the most fearsome predator of all, based on its CLP score. Persons & Currie, 2016/Scientific Reports}

The length of the dinosaur’s femur bone in its upper leg – an indicator of its body mass – was used to give an estimate of how long its lower leg should be. This was then compared to the fossilized evidence of its actual lower leg length. If its predicted lower leg length was greater than the actual value, then it would have a positive CLP score, indicating it was better adapted to running than a similarly-sized dinosaur with a smaller lower leg length.  

The Velociraptor, whose name means “swift thief,” may not have been as speedy as many thought. In fact, its turkey-sized frame and proportionally small lower leg length earned it a very low CLP score of -13.2, revealing that they are among the least adapted for running.

Previous studies have compared lions and cheetahs to Tyrannosaurus rex and Nanotyrannus lancensis, respectively, the latter of which is a controversial tyrannosaur that many believe is a misidentified juvenile T. rex. The smaller, more gracile, longer-legged N. lancensis was thought to be the “cheetah” to the larger, less cursorial T. rex “lion.”

This study appears to back up this idea: The T. rex gains an impressive CLP score of 11.5, but this pales in comparison to the N. lancensis’ 35.8, meaning this carnivore was likely an incredibly speedy, fearsome hunter. More importantly, however, based on its very distinct lower leg dimensions, this study suggests N. lancensis was indeed its own species.

“As far as I'm concerned, it was the scariest dinosaur,” said lead author Scott Persons, a paleontologist at the University of Alberta, in a statement. “No dinosaur was better adapted to chase you down.”