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Paleontologist John Scannella observed: “It is hard to walk out into the Hell Creek Formation and not stumble upon a Triceratops weathering out of a hillside. ” Forty-seven complete or partial skulls were discovered in just that area during the decade 2000–2010.  Specimens representing life stages from hatchling to adult have been found.  The function of the frills and three distinctive facial horns has long inspired debate. Traditionally these have been viewed as defensive weapons against predators.
More recent theories, noting the presence of blood vessels in the skull bones of ceratopsids, find it more probable that these features were primarily used in identification, courtship and dominance displays, much like the antlers and horns of modern reindeer, mountain goats, or rhinoceros beetles.
 The theory finds additional support if Torosaurus represents the mature form ofTriceratops, as this would mean the frill also developed holes (fenestrae) as individuals reached maturity, rendering the structure more useful for display than defense.