The study into Triceratops fossils from the US shows that the predator first slew its prey, then wrenched off its head in order to get to nutrient-rich neck meat hidden by body armour.
Tyrannosaurus was well known to feed on the Triceratops but until now few knew how it got around the herbivore's tough neck frill.
The study looked for an answer by examining bite-scarred Triceratops bones, the 'Daily Mail' reported.
Researchers led by Denver Fowler at the Museum of the Rockies in Bozeman studied specimens from Montana's Hell Creek Formation which showed the characteristic Tyrannosaurus bite marks.
Most of the 18 specimens they found with the marks were skulls, none of which showed any kind of healing - indicating that bites were inflicted on dead animals as they were eaten.
Researchers were intrigued to see that many extensive puncture and pull marks were actually on the neck frills of the specimens.
This seemed to make no sense. Made up mostly of bone and keratin, the neck frill would not have offered much nutrition to a hungry dinosaur, Fowler said.
The team discovered deep parallel grooves which revealed the Tyrannosaurus was probably actually using its teeth and jaws to pull on the frill in an effort to reposition its prey.
They suggested that the frills were the only barrier stopping the predator from sinking its teeth into Triceratops nutrient-rich neck muscles.
The study was reported in the journal Nature.
