From about 161 to 66 million years ago, a group of large, feathered predators called abelisaurs roamed the Earth. Their extremely small forelimbs contrasted dramatically with their powerful legs, which they used to chase down their prey before snapping them up in their jaws, chomping on them with their razor-sharp teeth.

While clearly fearsome, their true sizes were actually poorly constrained. A new study published in the journal PeerJ has revealed that, based on a long-lost femur bone, these beasts could grow up to 9 meters (29.5 feet) in length, and potentially weigh up to 2 tonnes (2.2 tons).

The length of a dinosaur’s femur bone, located in its upper leg, provides paleontologists with a way to estimate both how long and how heavy its owner could be. Alfio Alessandro Chiarenza, a graduate from Imperial College London and the lead author of this new study, stumbled across an unidentified one gathering dust in a museum drawer in Palermo, Italy, and decided to identify what dinosaur it used to belong to.

^{The size of an abelisaur compared to a lowly human snack. David Bonadonna/Imperial College London}

The bone was originally found in a 95-million-year-old rock formation from North Africa. Back then, the region was full of mangrove swamps and rivers, unlike the more arid environment present today. The region was home to a plethora of creatures, including several large predators like Spinosaurus. However, Chiarenza determined that the femur belonged to another meat-eating monster called an abelisaur.

In fact, this abelisaur is one of the largest ever discovered. “Smaller abelisaur fossils have been previously found by paleontologists, but this find shows how truly huge these flesh eating predators had become,” said Chiarenza in a statement.

This find does raise a perplexing issue, though: How did two huge apex predators like Spinosaurus – the largest carnivorous dinosaur of all time – and this abelisaur co-exist in the same region? One possible answer lies in their hunting habits.

Spinosaurus was a fish-eating beast that lived near and in the more aquatic parts of the environment. Abelisaurs were inland hunters, chasing their prey down over solid ground. So perhaps the two groups of dinosaurs would have never crossed paths.

^{Spinosaurus lived in the same environment, so how did the two beasts not hunt each other into extinction? Linda Bucklin/Shutterstock}

Far from just Spinosaurus, however, fossil evidence from this region suggests that this abelisaur would have lived alongside four other additional meat-eating titans – surely they all couldn’t have lived side by side in a harmonious existence? This mystery is named Stromer’s Riddle, and it has baffled paleontologists ever since the eponymous researcher first found this fossil bed 1912.

This new study suggests that the fossil bed may have been deceiving. The authors think that the harsh and rapidly changing environment in the region may have, over time, caused a lot of disturbance and reorganization within the bed.

This may have mixed up the dinosaur fossil fragments, giving the impression that these gargantuan hunters lived in the same place. In reality, they may have lived quite far away from each other.