Scientists from the University of Leeds in the U.K. have been investigating a mass dinosaur migration out of Europe during the early Cretaceous period. According to a report from UPI, the recent study utilized “network theory” to map the various paths of dinosaur species throughout the Mesozoic Era.

Working together with scientists from the University of Bath, researchers pulled data from the dinosaur fossil record and created a visualization of the various paths of dinosaur families throughout the continent over time.

The study reveals that at the beginning of the Cretaceous period, roughly 125 to 100 million years ago, dinosaurs began leaving Europe in great numbers with no new dinosaurs entering the continent.

According to the study’s lead author, professor Alex Dunhill from the Leeds School of Earth and Environment, the reason behind the dinosaurs’ departure from Europe remains unclear. “This is a curious result that has no concrete explanation,” he said.

Dunhill suspects that there may have been some common factor driving the migration out of Europe, but acknowledges that gaps in the fossil record may be hiding an explanation from clear sight.

Perhaps one of the more plausible explanations is that the dinosaurs vacated Europe as the massive landmass known as Pangaea began to split. The divide of the “supercontinent” split apart huge numbers of species, isolating them for millions of years.

The dinosaurs in Europe may have been trapped there, but could have come upon an opportunity to escape as the landscape continued to change. According to Dunhill, “We presume that temporary land bridges formed due to changes in sea levels, temporarily reconnecting the continents.”

While the study demonstrates that the dinosaurs populating Europe made a massive exodus near the early Cretaceous period, it remains a mystery as to exactly what drove them out.

A press release from the University of Leeds describing the details of the study can be found here.