Well, one thing we know for sure: If any folklore, belief, or ‘fact’ related to a fossil species sits around long enough, eventually someone will come along and study it. This usually involves reformulating the idea as one or more testable hypotheses, then attacking the hypotheses … much like Tyrannosaurus might or might not have attacked Triceratops, to see if it can be killed, or alternatively, has the mettle to survive for a while longer.
And thus, science progresses.
So now we have a paper entitled “Evidence of Combat in Triceratops” by Farke et al, just out in PLoS ONE.
But how do you answer the question at hand? A first approximation is to look at the dinosaur skulls and see if you can find the sort of poke mark or healed damage that indicates that they were head butting in life, or being attacked by something bigger, or whatever. But there are several problems with this method. For one thing, it is often very difficult to tell if a hole or mark on the bone is what one might think it is, rather than some other mark caused by some other thing that you have not thought of. Marks are, it turns out, very ambiguous in their causes. Even healed bone may be hard to identify with certainty, and the cause of the damage may be even less clear in such cases.
Over the last decade or so, taphonomists (the people who study these marks on bones) have come up with an axiom: It’s the pattern that matters, not the mark. A given mark will always be equivocal, but the pattern of marks across a series of bones from many individuals may be interpretable.
In the case of the present study, the researchers have come up with a rather ingenious approach. They ‘ve taken two distinctly different kinds of dinosaurs (each a different genus, but with mixed species) that have two different patterns of horns on the face. The Triceratops has two big horns coming out of its head and one smaller horn on the nose, and the Centrosaurus has two smaller horns coming out of its head and a larger horn on the nose.
If the marks are random and meaningless with respect to the dino’s behavior, then the pattern observed on one genus should be pretty similar to (or, more exactly, not statistically different from) the pattern observed on the other genus. If, however, the patterns are different, then perhaps meaning can be attributed to the pattern, in the sense that the physical differences between these genera may account for the difference in patterns of marks.
And they are in fact different.
We [statistically compared] lesions in Triceratops (which possesses two large brow horns and a smaller nasal horn) and the related ceratopsid Centrosaurus (with a large nasal horn and small brow horns) … The two taxa differ significantly in the occurrence of lesions on the squamosal bone of the frill … but not in other cranial bones …
This pattern is consistent with Triceratops using its horns in combat and the frill being adapted as a protective structure for this taxon. Lower pathology rates in Centrosaurus may indicate visual rather than physical use of cranial ornamentation in this genus, or a form of combat focused on the body rather than the head.
So there you have it. I would put it this way: The previous assertion that Triceratops engaed in some kind of combat or protection with its head is consistent with the pattern of evidence seen across several hundred fragments of bone, and Triceratops and Centrosaurus may have differed somewhat in this behavior.
Andrew A. Farke, Ewan D. S. Wolff, Darren H. Tanke (2009). Evidence of Combat in Triceratops PLoS ONE, 4 (1) DOI: 10.1371/journal.pone.0004252