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Scientists have unearthed in a Sahara Desert oasis in Egypt fossils of a long-necked, four-legged, school bus-sized dinosaur that lived roughly 80 million years ago, a discovery that sheds light on a mysterious time period in the history of dinosaurs in Africa.

Researchers said on Monday the plant-eating Cretaceous Period dinosaur, named Mansourasaurus shahinae, was nearly 33 feet long and weighed 5.5 tons and was a member of a group called titanosaurs that included Earth's largest-ever land animals. Like many titanosaurs, Mansourasaurus boasted bony plates called osteoderms embedded in its skin.

Artist's life reconstruction of the titanosaurian dinosaur Mansourasaurus shahinae on a coastline in what is now the Western Desert of Egypt. Andrew McAfee / Carnegie Museum of Natural History via Reuters

Mansourasaurus, which lived near the shore of the ancient ocean that preceded the Mediterranean Sea, is one of the very few dinosaurs known from the last 15 million years of the Mesozoic Era, or age of dinosaurs, on mainland Africa. Madagascar had a separate geologic history.

Its remains, found at the Dakhla Oasis in central Egypt, are the most complete of any mainland African land vertebrate during an even larger time span, the roughly 30 million years before the dinosaur mass extinction 66 million years ago, said paleontologist Hesham Sallam of Egypt's Mansoura University, who led the study published in the journal Nature Ecology and Evolution.

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The scientists recovered parts of its skull, lower jaw, neck and back vertebrae, ribs, shoulder and forelimb, back foot and osteoderms.

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A lot of Africa is covered in grasslands, savannas and rain forests that obscure underlying rock where fossils may be found, said postdoctoral researcher Eric Gorscak of the Field Museum in Chicago, who was formerly at Ohio University.

While as massive as a bull African elephant, Mansourasaurus was modestly sized next to titanosaur cousins such as South America's Argentinosaurus, Dreadnoughtus and Patagotitan and Africa's Paralititan, some exceeding 100 feet long.

"Mansourasaurus, though a big animal by today’s standards, was a pipsqueak compared to some other titanosaurs," said paleontologist Matt Lamanna of the Carnegie Museum of Natural History in Pittsburgh.

Lamanna told USA TODAY that the well-preserved dinosaur was a "Holy Grail."

"This was the Holy Grail — a well-preserved dinosaur from the end of the age of dinosaurs in Africa — that we paleontologists had been searching for for a long, long time," he said.

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The researchers determined Mansourasaurus was more closely related to European and Asian titanosaurs than to those from elsewhere in Africa and other Southern Hemisphere land masses including South America formerly joined in a super-continent called Gondwana.

"This, in turn, demonstrates for the first time that at least some dinosaurs could move between North Africa and southern Europe at the end of the Mesozoic, and runs counter to long-standing hypotheses that have argued that Africa’s dinosaur faunas were isolated from others during this time," Lamanna said.