A research team, including a University of Guelph scientist, says the bones were contained entirely within the skin of some of the largest dinosaurs.
They may have stored minerals to help the massive creatures survive and bear their young during tough times.
The bones, shaped like footballs sliced lengthwise, were found in two sauropod dinosaurs — an adult and a juvenile — from Madagascar.
The study suggests that the long-necked plant-eaters used hollow "skin bones" called osteoderms to store minerals needed to maintain their huge skeletons and to large clutches of eggs.
In modern times, these "skin bones" appear in animals such as alligators and armadillos.
Sediments found around the dinosaurs' fossils showed the environment was highly seasonal and semi-arid. Periodic droughts caused massive die-offs, the researchers said.
Matthew Vickaryous, a biomedical scientist at the University of Guelph, was one of the authors of the paper published in Nature Communications.
A researcher at the Ontario Veterinary College, Vickaryous studies how skeletons develop, regenerate and evolve.
He helped interpret the results of CT scans and fossilized tissue cores taken from the dinosaurs.
"Our findings suggest that osteoderms provided an internal source of calcium and phosphorus when environmental and physiological conditions were stressful," he said.
The bones — about the size of a gym bag in the adult — are the largest osteoderms ever identified, the researchers said.
The adult's bone was hollow, which suggests extensive bone remodelling, added Vickaryous, but the juvenile specimen was solid.
That suggests that osteoderms became more important mineral stores as the animals grew, he said.
Osteoderms were common among armoured dinosaurs. Stegosaurs had bony back plates and tail spikes, and ankylosaurs sported heavily armoured bodies and bony tail clubs.
But these kind of bones were rare among sauropod dinosaurs, the researchers said.