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Triceratops and Torosaurus dinosaurs ‘two species, not one’

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BBC Online:
A study has rejected claims that Triceratops and the lesser-known Torosaurus are one and the same type of dinosaur.
Research published in 2010 suggested the two-horned animals represented merely different growth stages, with Torosaurus the adult and Triceratops the youngster.
But researchers at Yale University say the fossils do not support the theory.
Details are published in the journal Plos One. Skulls attributed to Triceratops and Torosaurus The skull of Torosaurus (below) has a longer holed frill compared to that of Triceratops (top) Nicholas Longrich and Daniel Field, of Yale University, looked at 35 specimens ascribed to both species and concluded they represented two distinct creatures. "We looked at a bunch of changes in the skulls as the animals age and used a programme to arrange the skulls from youngest to oldest," explained Dr Longrich to BBC News.
"What we found is there are young Torosaurus individuals and very old Triceratops individuals and that's inconsistent with Torosaurus being an adult Triceratops."
Skulls attributed to the Torosaurus boast a longer frill with large holes while Triceratops has a smaller solid frill.
Dr Longrich argues that if these were the same animal, they would also expect to find transitional specimens in which the skull is morphing between the two skull types.
"We reviewed the evidence and there was no evidence for anything between Torosaurus and Triceratops. There are dozens and dozens of skulls and I think if those transitional forms really existed we would have found them."
Where the researchers say they could not find conclusive evidence for two species was the geographical distribution of the fossil evidence.
Fossils attributed to both species are found exclusively in North America, and although there are some sites where only one proposed genus has been found, the evidence could also be consistent with the single-species theory.