(CNN) – Similar sets of footprints discovered in Africa and South America reveal that dinosaurs once traveled along a common type of track 120 million years ago, before the two continents separated, according to new research.
Paleontologists have discovered more than 260 dinosaur footprints from the Early Cretaceous in Brazil and Cameroon, now separated by more than 6,000 kilometers on opposite sides of the Atlantic Ocean.
The footprints are similar in age, shape and geological context, according to Louis L. Jacobs, a paleontologist at Southern Texas Methodist University (SMU) and lead author of a study describing the footprints published this Monday by the New Mexico Museum of Natural History and Science.
Most of the fossilized footprints were made by three-toed theropod dinosaurs, while a few likely belonged to four-legged, woody sauropods with long necks and tails or to ornithischians, which had bird-like pelvic structures, said study co-author Diana P. Vineyard, a research associate at SMU.
The footprints tell a story of how the movements of huge land masses created ideal conditions for dinosaurs before supercontinents broke apart into the seven continents we know today.
The footprints were preserved in mud and silt along ancient rivers and lakes that once existed on the supercontinent Gondwana, which broke away from the larger landmass of Pangaea, Jacobs said.
“One of the youngest and tightest geological connections between Africa and South America was the northeastern bend of Brazil nestled against what is now the coast of Cameroon along the Gulf of Guinea,” Jacobs said. “The two continents were continuous along that narrow stretch, so animals on both sides of that connection could potentially move across it.”
Africa and South America began to separate about 140 million years ago. The separation created cracks in the Earth's crust, and as the tectonic plates beneath South America and Africa moved apart, magma from the Earth's mantle created new oceanic crust. Over time, the South Atlantic Ocean filled the space between the two continents.
But before this gradual change occurred, different types of basins formed as the Earth's surface moved apart. Rivers flowed into the basins, forming lakes, Jacobs explained.
The study's authors found evidence of what is known as a half-graben basin in the Borborema region of northeastern Brazil and a similar one in the Koum Basin of northern Cameroon.
“A mid-rift valley is an elongated basin that forms when the Earth’s surface is pulled apart by a fault on one side such that the valley floor slopes downward toward the fault along which movement is occurring,” Jacobs said by email. “If you hold your hand out in front of you and tilt your fingers downward, representing movement along the fault, rivers will flow down the valley and deposit sediment, and sediment will be eroded from the high side of the valley.”
In both basins, researchers found dinosaur footprints, ancient river and lake sediments, and fossilized pollen.
“Plants fed herbivores and supported a food chain,” he said. “Muddy sediments left by rivers and lakes contain dinosaur footprints, including those of carnivores, documenting that these river valleys may have provided specific pathways for life to travel across continents 120 million years ago.”
While dinosaur fossils can provide unique insights into the types of animals that roamed the planet millions of years ago, their footprints provide other windows into the past.
“Dinosaur footprints are not rare, but unlike bones that are often found, footprints are evidence of dinosaur behavior, how they walked, ran or otherwise, who they walked with, what environment they walked through, what direction they were going and where they were when they did so,” Jacobs explains.
It's difficult to know which specific dinosaur species traveled through the basins, but they paint a broader picture of the ancient climate and how different types of animals thrived in the environment created by continental rifting.
“If your dog and a coyote walk through the same marsh, you might know that two canine critters walked through there that look very similar, but you won’t be able to tell if they are different species. The same is true for dinosaur tracks,” Jacobs explains. “All animals have home ranges. All animals expand their home ranges. All animals exploit resources as needed based on availability, often related to seasonality. Herbivores follow nutritious plants; carnivores follow their herbivorous food.”
At that time, rainfall levels helped create a tropical rainforest-like environment with abundant vegetation. Animals arrived in the basins from both present-day Africa and South America, causing their populations to mix.
“Imagine an open, lush, vegetated basin for herbivores and carnivores below,” said study co-author Lawrence Flynn, associate director of the American School for Prehistoric Research and coordinator of laboratory security within Harvard University’s Department of Human Evolutionary Biology. “If there is no one in the ‘new’ territory, the animals will disperse across it, as there is no competition.”
Later, once the continents drifted apart, this disruption likely caused a break in genetic continuity, a key driver of evolution, Jacobs said.
The Cameroon dinosaur tracks were first discovered in the late 1980s, and Jacobs reported on them at the First International Symposium on Dinosaur Tracks and Traces, convened by paleontologist Martin Lockley, in 1986.
Jacobs then became friends with the study's author, Ismar de Souza Carvalho, now a professor in the Department of Geology at the Federal University of Rio de Janeiro. Jacobs studied dinosaur movements from the African side, while Carvalho did so from the Brazilian side.
As research into the African and South American basins continued in the following decades, Jacobs and Carvalho and their colleagues reviewed fieldwork and existing and new research to analyze where there were overlaps. The new study is published in honor of Lockley, who devoted his career to the study of dinosaur footprints.
“We wanted to bring together new and evolving geological and paleontological evidence to tell a more specific story of where, why and when dispersals between continents occurred,” Jacobs said.
“One of the beauties of this Earth is that any of us can see that Africa and South America used to fit together like puzzle pieces. It is easy to conceive that in a connected world, animals, including dinosaurs, could and would likely move from one place to another.”
