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Identical sets of footprints discovered in Africa and South America show that dinosaurs once traveled along a kind of highway 120 million years ago before the two continents separated, according to new research.
Paleontologists have found more than 260 dinosaur footprints from the Early Cretaceous period in Brazil and Cameroon, now more than 3,700 miles (6,000 kilometers) apart on opposite sides of the Atlantic Ocean.
The footprints are similar in age, shape and geological context, said Louis L. Jacobs, a paleontologist at Southern Methodist University in Texas and lead author of a study describing the tracks published Monday. New Mexico Museum of Natural History and Science.
Most of the fossilized prints were made by three-toed theropod dinosaurs, while a few may have belonged to large sauropod dinosaurs with four legs, long necks and tails, or ornithischians, which had bird-like pelvic structures, said study co-author Diana B. Crawford.
Vinyard, Research Associate at SMU.
The tracks tell the story of how the movements of massive land masses created the perfect conditions for dinosaurs before the 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 land mass of Pangaea, Jacobs said.
“One of the smallest and narrowest geological connections between Africa and South America was the elbow of northeastern Brazil adjacent to what is now the coast of Cameroon along the Gulf of Guinea,” Jacobs said. “The two continents were connected along this narrow stretch, so animals on either side of this connection could move across it.”
Africa and South America began to separate from each other 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 in 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 happened, different types of basins formed as the Earth’s surface separated. Rivers fed the basins, forming lakes, Jacobs said.
The study authors found evidence of a so-called semi-graben basin in the Borborma region of northeastern Brazil and a similar basin in the Koum Basin of northern Cameroon.
“A half-valley is a rectangular basin formed by the separation of the Earth’s surface with a fault forming on one side so that the valley floor slopes down toward the fault along which movement occurs,” Jacobs said via email. “Put your hand out in front of you. Tilt your fingers downward, representing movement along the fault. Rivers will flow down the valley, depositing sediment, and sediment will erode from the high side of the valley.”
In both basins, researchers found dinosaur tracks, ancient river and lake sediments, and fossilized pollen grains.
“The plants fed the herbivores and supported the food chain. The muddy sediments left behind by the rivers and lakes contain dinosaur footprints, including those of carnivores, which documents that these river valleys were able to provide specific pathways for life to move across continents 120 million years ago,” he said.
While dinosaur fossils can offer unique insights into the types of animals that roamed the planet millions of years ago, their footprints provide other windows into the past.
“Dinosaur tracks are not uncommon, but unlike the bones that are commonly found, footprints are evidence of dinosaur behavior, how they walked or ran or whatever, who they were walking with, what environment they were walking through, the direction they were walking in, and where they were when they were doing it,” Jacobs said.
It is difficult to pinpoint the specific types of dinosaurs that traveled along the basins, but they represent a larger picture of the ancient climate and how different types of animals thrived in the environment created by continental rifting.
“If your dog and a fox walk through the same mudflat, you might know that two dogs walked there, and that they look very similar, but you might not be able to tell if they are different species. The same is true of a dinosaur track,” Jacobs says. “All animals have home ranges. All animals expand their ranges. All animals exploit resources as necessary based on availability, often with seasonality. Herbivores follow nutrient-rich plants; carnivores follow vegetation.”
At that time, rainfall levels helped create an environment resembling a tropical rainforest with abundant vegetation. Animals came to the basins from present-day Africa and South America, causing their populations to mix.
“Imagine an open, lush pond with plants for herbivores and carnivores,” said Lawrence Flynn, a study co-author and associate director of the American School of Prehistoric Research and laboratory safety coordinator in the Department of Human Evolutionary Biology at Harvard University. “If there is no one on the ‘new’ grass, the animals will spread out, in the absence of competition.”
Later, once the continents had moved apart, this disruption likely would have interrupted genetic continuity, a major driver of evolution, Jacobs said.
Dinosaur tracks were first discovered in Cameroon in the late 1980s, and Jacobs reported on them at the First International Symposium on Dinosaur Tracks, convened by paleontologist Martin Lockley in 1986.
Jacobs then became friends with study author Ismar de Souza Carvalho, now a professor in the geology department at the Federal University of Rio de Janeiro. Jacobs was studying dinosaur movements from the African side, while Carvalho was studying them from the Brazilian side.
As research continued in basins in Africa and South America in the decades that followed, Jacobs, Carvalho, and their colleagues reviewed existing and new fieldwork and research to analyze the similarities. The new study is published in honor of Lockley, who devoted his career to studying dinosaur footprints.
“We wanted to combine new and advanced geological and fossil evidence to tell a more definitive story about where, why and when dispersals between continents occurred,” Jacobs said.
“One of the advantages of this land is that any of us can see that Africa and South America were once connected like pieces of a puzzle. It is easy to imagine that in a connected world, animals, including dinosaurs, might have been able to move from one place to another.”
