The discovery of ancient tortoise and turtle fossils had led scientists to conclude that parts of the Andes were less than a mile in altitude about 13 million years ago.

A five-foot-long shell from an extinct tortoise of the genus Chelonoidis was found near Quebrada Honda, Bolivia in Altiplano in west-central South America, where the Andes are at their widest. It is the most extensive area of high plateau on Earth outside of Tibet.

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The Andes were formed by subduction - a process in which one tectonic plate slips under another. But plenty of questions remain over how the mountains – among the tallest in the world - rose to their current elevation.

"We're trying to understand how tectonic plate activity and changing climate affected species diversity in the past," Darin Croft, an anatomy professor at Case Western Reserve University School of Medicine and a paleomammalogist and whose study appeared in the Journal of South American Earth Sciences, said in a statement.

"One way all this diversity we see in the South American tropics today was generated was through elevation,” he said. “Mountains create many different climates and ecosystems in a small area, which promotes speciation."

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Croft found the tortoise remains in an embankment after he missed a turn on a path near Quebrada Honda and was working his way back toward his regular research site. Croft and Federico Anaya, a member of the geological engineering faculty at Universidad Autónoma Tomás Friás in Posotí, later identified other, more fragmentary tortoise remains from other sites in the area.

After returning to the United States, Croft sent photographs and three-dimensional computer-generated images of the remains to Edwin Cadena, a turtle expert now at Yachay Tech University in Ecuador.

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Cadena identified the tortoise as a member of the same genus as the Galápagos tortoise, Chelonoidis. He identified the extinct freshwater turtle as belonging to the genus Acanthochelys, whose surviving members occur throughout much of tropical South America.

The ancient tortoise and aquatic turtle most likely would have had physiological requirements like their modern relatives, which generally live at altitudes of up to about 1,640 feet. They can't thrive or reproduce at much higher elevations because of the cooler temperatures, Croft said.

Now, the researchers are trying to bolster their theory with additional fossils. They found fossil remains of a large snake in the same rock layer as the turtles. Those bones are currently being examined by Croft and his colleagues.