If you’re a dinosaur, or almost any other kind of vertebrate, the femur is probably the largest and most durable bone in your body. If palaeontologists find fossilized remains of one of your bones in 100 million years, it will probably be your femur. And you can tell a lot from looking at a femur—they will usually give you a pretty good approximation of a human being’s proportions, for example. Or a dinosaur’s.
So when local farmers alerted Argentinian palaeontologists Jose Luis Carballido and Diego Pol to a very large fossilized femur roughly 100 million years old, the scientists did the math and found that they’d discovered a new species of dinosaur. A very big new species of dinosaur.
Assuming initial calculations hold out, we’re talking about a 150,000-pound behemoth 20 meters (65 feet) tall—about the same height as the Great Sphinx of Giza—and 40 meters (130 feet) long. Its status as the largest land animal in history may be challenging to defend until more bones are found, but it will also be impossible to deny; the fossil is legitimate, the palaeontologists used standard calculations, and no evidence of a larger land animal has ever been found.