The fearsome predator that once terrorised North America was the tyrannosaurus rex. The researchers believe the beast may not have been the only tyrannosaurus.
Experts studying remains thought to belong to T rex suggest they show evidence of three different species.
The lead author of the research, Gregory Paul, who was a dinosaur specialist on the film, said the findings had multiple implications, noting that previously experts had studied the growth of the T rex using remains from different rock layers.
He said that it may not be a good idea to look at different species.
More than one tyrannosaurus species has evolved over millions of years on Earth, as has been found for other dinosaurs who lived at the same time, such as triceratops.
In the journal Evolutionary Biology, Paul and colleagues report how previous work has shown that bones designated as being from T rex vary in terms of their stout build orrobustness, and different bones had one or two pairs.
Paul and colleagues looked at the length and circumference of the thigh bones in about two-thirds of the 37 T rex specimen they studied.
The team says their findings suggest that the thigh bones are not made for individual variation.
The sample we have of tyrannosaurus, the variation of the femur is greater than all other tyrannosaurids combined over 10m years of evolution.
The team say that the variation does not appear to be related to the size of the specimen or the age of the animal when it died.
Fossils with more gracile bones were only found in higher layers of silt, and these and more robust fossils were only found in lower layers, with only one incisor-like tooth in the lower jaw.
The team says they can't rule out other explanations for the findings, but they think the specimen found in the lower layers is from the species they call the tyrant lizard emperor.
Prof Steve Brusatte, a palaeontologist at the University of Edinburgh, was not involved in the work.
There is some variation in the fossils of T rex, so it's tempting to divide it into different species. He said that the variation is not indicative of a meaningful biological separation of distinct species that can be defined based on clear, explicit, consistent differences.
Prof Thomas Carr, a T rex expert at Carthage College in the US, said the definitions of the different species put forward by the team were vague, and the findings were at odds with his own work in which he analysed variations in 1,850 different features. If that signal was in the data, I would have picked it up.
The ability of researchers to reproduce the results of the study was one of the concerns.
Paul said it would be decades before there were enough museum specimen to do a statistical analysis. He said the variations were significant.
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