Presenting this week at the Society of Vertebrate Paleontology's meeting in Los Angeles, California, palaeontologist Jack Horner of the Museum of the Rockies in Bozeman, Montana, revealed that when he cut open the fossilized bones of dinosaurs in the museum’s collection and studied the layers of bone within, he found signs in most specimens that the animals were still growing at the time of their death.

In fossils labelled as juveniles, the outer bone layers contained canals that would once have held blood vessels, as well as large groups of osteocytes — cells that are important for bone formation. But the researchers were surprised to find similar signs of growth in adult fossils, because in most animals that are alive today, the skeleton tends to stop growing once adulthood is reached.

The key thing that Horner was looking for was arrested growth: closely packed bone layers bereft of osteocytes and blood vessels that are almost always found in skeletons that have finished growing. He did find them in a few fossils, indicating that dinosaur bones had the potential to eventually stop growing. But the vast majority of the bones he looked at did not have them.