The world was once a very different place. Several times during Earth’s history, plate tectonics have assembled and fragmented various supercontinents, enormous swaths of interconnected land surrounded by one or two gargantuan oceans. The most famous of these is undoubtedly Pangaea, which was born 300 million years ago, and began to self-destruct 175 million years ago.
The break-up of Pangaea occurred right in the middle of the Jurassic period, when dinosaurs ruled the world. Far from holding on to one piece of land, the dinosaurs began to spread out across these continent-sized fragments.
By scouring through the Paleobiology Database, which has cataloged every single dinosaur fossil ever found, researchers have managed to visually map how evolutionarily connected families of dinosaurs remained as they migrated across the world. Most intriguingly of all, the new study, published in the Journal of Biogeography, has uncovered an enigmatic mass exodus from what is now the European continent.
“This is a curious result that has no concrete explanation,” Dr. Alex Dunhill, a biogeographer from the University of Leeds and lead author of the study, said in a statement. “It might be a real migratory pattern or it may be an artefact of the incomplete and sporadic nature of the dinosaur fossil record.”
During the fragmentation of Pangaea, the world we know today began to take shape. Huge volcanism created by the splitting of continents warmed the world, so much so that even Antarctica became covered in conifer forests. By the time the non-avian dinosaurs went extinct 66 million years ago, they could be found all across the planet, from the emerging Australian continent to the nascent South American landmass.
By tracking the appearance of dinosaurian fossils across the world, this team of researchers managed to reveal what the movement of similar dinosaur families was like, from their emergence right until they met their end.
The first thing they noticed was that, despite landmasses becoming increasingly separated by water, the intercontinental exchange of dinosaur groups continued right up until they died out. This means they must have found a way to travel across huge, newly-formed oceans.
“We presume that temporary land bridges formed due to changes in sea levels, temporarily reconnecting the continents,” Dunhill added. “Such massive structures – spanning, for example, from Indo-Madagascar to Australia – may be hard to imagine. But over the timescales that we are talking about, it is perfectly feasible.”
The interconnectedness of dinosaurian families across the world. The thickness of the lines shows how many closely related families are shared between different land masses, during the Late Triassic (a), Early Jurassic (b), Mid Jurassic (c), Late Jurassic (d), Early Cretaceous (e), and early Mid-Cretaceous (f). Modified from Dunhill et al./Journal of Biogeography
Ornithischians and theropods, in comparison to the often-bulky sauropodomorphs, were able to move across the splintering world more easily – they tended to be smaller and more able to traverse across narrow seaways, either by swimming or even floating on buoyant masses of vegetation. Flight-able (avian) dinosaurs, which of course became contemporary birds, unsurprisingly show relatively high interconnectedness across the world.
Overall, though, the number of family “networks” – closely related dinosaur families found on separate land masses – decreased along with the increasing fragmentation of the world. A mass migration away from Europe is seen in the Late Jurassic (161 to 146 million years ago), but as the oceans proliferated, the number of networks decrease and the dinosaur families begin to evolve into distinctly different beasts.
Around 72 million years, the tail-end of the Cretaceous period, only North America and Asia have somewhat related dinosaur family groups on them. These networks all but disappeared 6 million years later as the asteroid impact propelled them to extinction.
Image in text: The break-up of Pangaea. Tbower/Wikimedia Commons; Public Domain