Between 230 million and 66 million years ago, dinosaurs plodded across the supercontinent Pangea, and migrated from Europe to other parts of the world. Now, by gathering and comparing all the data about their fossils, paleontologists have been able to visually map the dinosaurs' migration during the time they ruled the Earth.
The researchers used "network theory" in a new way to see how different dinosaur fossils were connected.
"A network is just as you imagine it being; it's a series of points which are your entities that you want to investigate," said study lead author Alex Dunhill, a paleobiologist at the University of Leeds, in the United Kingdom. "And then you look at how they interact or are connected together, by simply drawing lines between them." [Photos: One of the World's Biggest Dinosaurs Discovered]
The team chose continents as points and then drew connecting lines if the same types of dinosaurs were found on two or more continents.
"We can then use some really simple maths to look at how the level of connectivity and the strength of the connection changes through time," Dunhill told Live Science. "It's something that's used really commonly in computing."
For example, network theory is used all over the internet, which is basically one giant network itself. Things like Facebook friends and Twitter interactions can all be calculated and mapped by network theory.
Dinos on the move
The researchers looked at what happened when Pangea (sometimes spelled Pangaea) broke up into smaller continents in the Triassic period, which is when dinosaurs first evolved. By the end of the Cretaceous, about 65.5 million years ago, the continents had broken up and drifted, almost to the positions we know today. High sea levels during this era also meant that some land masses appeared to be completely isolated, the researchers said. Using the fossil data, the scientists mapped where the dinosaurs trekked as the supercontinent was becoming fractured.
"One thing we actually find is that even though the migration of dinosaur groups slows down, it doesn't completely stop," Dunhill said. "We're still getting the movement of dinosaur groups between major continental land masses, even when the continents appear to be really isolated." [In Images: How North America Grew as a Continent]
In other words, dinosaur families cropped up on continents even when they were completely separate from their original areas. Dunhill said this conclusion had been reached in previous studies using different methods, so the researchers were sure they were looking at the correct historical movements.
Dinosaurs may have been able to move across continents, and between islands, by the formation of temporary land bridges, which could have formed because of fluctuating sea levels during the Cretaceous era, Dunhill said.
To make the mapping exercise more manageable, the researchers separated the dinosaurs by type: the sauropodomorphs, which are huge, long-necked plant-eaters like the Diplodocus and Brachiosaurus; the theropods that include all the carnivorous dinosaurs like the Tyrannosaurus rex; and the ornithischians, which include all other plant-eaters, such as the Triceratops and Stegosaurus.
"One thing we found was that sauropodomorphs tend to be less mobile, particularly [compared to] the theropods," Dunhill said. "These were really big animals, and probably less likely to swim, and less likely to be able to get across sea waves than some of the other smaller dinosaurs."
The theropod family also includes birds, and although they probably weren't great at flying, Dunhill said they were probably mobile enough to be able to still disperse across narrow sea ways.
But figuring out whether the results show real patterns of dinosaur migrations — or whether the findings simply reflect limitations in the fossil record — has been challenging. [Paleo-Art: Dinosaurs Come to Life in Stunning Illustrations]
"The fossil record is incomplete and biased in quite a severe way, and the terrestrial vertebrate fossil record is incredibly patchy," Dunhill said. "The main problem we tried to overcome was working out if these were true biological patterns of dinosaur movement or just that we've got a varying quality of fossil records through time."
Europe has been sampled for fossils for more than 250 years, and North America and Asia have strong records of fossils. However, other parts of the world, such as Australia, Africa and Antarctica, have a poor history of digging up and documenting fossils, the researchers said.
To combat this, the researchers removed some of the areas where the fossil record isn't as strong from the analysis, and ran it again to see if the overall patterns changed through time. When they did this, they found that there was a decline in connectivity, meaning there were fewer connections between the dinosaur families across the world (thus they weren't as widespread). Using all the data showed more lines of connections, which showed the families were distributed further away, giving the impression that they travelled more distance.
Out of Europe
But what caused the dinosaurs to flee? Instead of a natural disaster happening in Europe that prompted the animals' migration, Dunhill said the dinosaurs' exit could have two possible explanations.
"There's a biological possible explanation where Europe had been isolated for a while, had a burst of speciation, and then re-connections occurred with the rest of the world," he said. "Then, these new groups of dinosaurs that have evolved in Europe have then radiated out and expanded their geographic ranges."
The other explanation, he admits, is a little less exciting.
"It may just be an artifact of this patchy fossil record, and that maybe Europe has a really good fossil record throughout all this time period and other areas don't," Dunhill said. "It's always really difficult to distinguish between the two."
Dunhill says that more data is needed to really know what the dinosaurs were up to during that period, but the next stages of the research will involve integrating dinosaur phylogeny into the networks, and looking at relationships between the different groups.
The study's findings were published April 25 in the Journal of Biogeography.