A 240 million-year-old fossilized leg bone has yielded evidence of what might be one of the earliest known cases of cancer ever found.

Scientists in Germany detected a highly malignant tumor in the fossilized leg bone of a stem turtle, which lived during the Triassic Period when many new species emerged on Earth, including the earliest dinosaurs and the first primitive mammals.

The unlucky victim of this rare ancient bone cancer was Pappochelys rosinae, a shell-less, extinct ancestor of modern turtles. The only known Pappochelys fossils to be found so far surfaced in a quarry located in southwestern Germany and was unveiled to the world in 2015. While researchers celebrated the find for helping to fill out the modern turtle’s evolutionary timeline, ABC Science reported, one of its discoverers noticed something intriguing: an uneven growth running along one left femur, or thigh bone.

When a team of paleontologists and physicians in Germany analyzed the fossil with a micro CT scan, they concluded that it was a highly malignant bone tumor known as a periosteal osteosarcoma.

Not only that—the tumor in this prehistoric stem-turtle looked “almost exactly like osteosarcoma in humans,” as coauthor Patrick Asbach, a medical doctor and radiologist at Berlin’s Charité University of Medicine, told National Geographic.

According to the American Cancer Society, osteosarcoma in humans is the most common type of cancer that starts in the bones. It’s not a common cancer, but about 800 to 900 new cases are diagnosed in the United States each year, mostly among children and young adults. Osteosarcoma most often metastasizes to the lungs, but can also spread to the brain, organs or other bones.

Scientists use paleopathology, the study of ancient diseases in both humans and animals, to understand how diseases have changed over time in response to evolving pathogens and immune systems, as well as environmental conditions. But cancer cases, in particular, are exceptionally rare in the fossil record, as Asbach and his co-authors point out in the new study, published online in JAMA Oncology. Cancers tend to attack soft tissues, which are usually not preserved over the centuries.

The earliest known case of human cancer was also osteosarcoma, found in the 1.7 million-year-old fossil of an early human ancestor in Swartkrans cave in South Africa. Scientists have found several other extremely ancient cancers in fossilized fish and amphibians, but the new find represents the oldest known example of this type of cancer in an amniote, a group that includes mammals, birds and reptiles.

“It is almost obvious that ancient animals would have cancer, but it is so very rare that we find evidence of it,” study co-author Yara Haridy, a paleontologist at the Museum für Naturkunde in Berlin, told Science News.

Despite this rarity, she says, the discovery of a tumor in a Triassic-era animal suggests cancer is “a vulnerability to mutation deeply rooted in our DNA.”