Feeding behaviour and bone utilization by theropod dinosaurs. 2009. D. W. E. Hone and O. W. M. Rauhut. Lethaia online, 3 August 2009
Tyrannosaurus are often seen as the perfect 'killing machines' with extremely powerful bites, which were able to bring down even the largest possible prey," says. "But the very few fossils that reflect the hunt of predatory dinosaurs on large herbivores tell a tale of failure – the prey either got away, or both prey and predator were killed."
On the other hand, the also extremely sparse cases of direct evidence for the diet of predatory dinosaurs – stomach contents and coprolites – show that juveniles or much smaller prey species were ingested and the latter were swallowed whole.
Rauhut and Hone propose as a hypothesis that large predatory dinosaurs only as an exception attacked other large dinosaurs, but mainly fed on juveniles. "Even modern predators prefer old and sick animals or unexperienced young individuals," states Hone. "These are an easy prey to bring down and the risk of injury for the predator is much lower. This strategy was probably the same in dinosaurs."
Crocodiles, the closest living relatives of dinosaurs, have extremely strong acids in their stomachs. They can completely dissolve the poorly ossified bones of young animals which adds important nutrients to the reptiles' diet. The fossil finds of juvenile dinosaurs that have been swallowed whole by theropods support the idea that dinosaurs might have profited from this as well.
Missing fossils, though, lend even more plausibility: "Finds of dinosaur nesting sites indicate that they contained large numbers of eggs which should have resulted in a high number of offspring," says Rauhut. "But little of this is reflected in the fossil record: Juvenile dinosaurs are surprisingly rare – maybe because many of them have been eaten by predators. Hopefully there will soon be more evidence to help us really understand the theropods' hunting behavior." link