No Triceratops in the Evanston Formation...probably.
Skye McDavid, December 11, 2022
Various sources list a record of Triceratops, sometimes specifically Triceratops horridus, in the Evanston Formation in Western Wyoming. This includes the Dinosauria's hugely important, but flawed Dinosaur Distribution section. I'm not convinced that's it's really there.
Every source I've found stating Triceratops is present in the Evanston Formation either directly or indirectly cites Rubey et al. (1961), which establishes a Late Cretaceous age for the Evanston Formation based primarily based on palynostratigraphy but also macroscopic fossils. One of these is a Ceratopsid jaw, which they refer to Triceratops cf. flabellatus. The problem is that Triceratops is not diagnosable based on the jaw alone. Further, Triceratops flabellatus, named by Marsh in 1889, was supposedly diagnosed based on features of the frill. It is now considered a synonym of Triceratops horridus, readily differentiated from the other valid species Triceratops prorsus by the length of the nasal horn. (Though this isn't the only difference.) The jaw is also only briefly mentioned in the paper and the only source provided is personal communication. No catalog number or institution is given, making it impossible to track down the specimen in question and reassess it.
I have no reason to question that there was a Ceratopsid in the Evanston formation. It's not impossible that the Ceratopsid in question is Triceratops. But based on what is currently published, we cannot say with confidence that there was Triceratops in the Evanston Formation.
Rubey WW, Oriel SS, Tracey JI Jr (1961) The Age of the Evanston Formation, Western Wyoming. United States Geological Survey Professional Paper 424-B:153-154 [PDF]
Weishampel DB et al. (2004) Dinosaur Distribution. In Weishampel DB, Dodson P, Osmólska H (eds) The Dinosauria. University of California Press.