The remains of two baby dinosaurs have been found inside the fossil of a 75-million-year-old tyrannosaur, shedding new light on the diet of the ancient predators.

According to a study published in the journal Science Advances, the hind limbs of two small bird-like dinosaurs called citipes were found beneath the rib cage of a juvenile gorgosaurus. The dinosaur is considered to be a close relation of the Tyrannosaurus rex.

The researchers behind the study say it suggests the younger gorgosaurus preyed on smaller dinosaurs. Earlier discoveries have sauggested the adult gorgosaurus attacked and ate very large plant-eating dinosaurs which lived in herds.

Dr Darla Zelenitsky, one of the lead scientists in the study, told the BBC the discovery is “solid evidence that tyrannosaurs drastically changed their diet as they grew up”.

She said: “We now know that these teenage (tyrannosaurs) hunted small, young dinosaurs. These smaller, immature tyrannosaurs were probably not ready to jump into a group of horned dinosaurs, where the adults weighed thousands of kilograms.”

The fossil was originally discovered in Alberta Badlands, Canada, in 2009, but was entombed in rock and took years to be prepared for study. The initial discovery was made by staff at Alberta’s Royal Tyrell Museum of Palaeontology, who spotted small toe bones protruding from the rib cage.

Dr Francois Therrien, the other lead scientist in the study, told the BBC: “The rock within the ribcage was removed to expose what was hidden inside.

“And lo and behold – the complete hind legs of two baby dinosaurs, both under a year old.”