Torosaurus

Torosaurus (meaning "pierced lizard") was a large, quadrupedal, plant-eating Ceratopsian dinosaur with three horns on its large head. Torosaurus hatched from eggs and may have lived in herds.

Torosaurus lived during the Upper Cretaceous, about 70–65 million years ago, in what is now western North America. It was similar to Triceratops. Some had thought that Triceratops was a younger form of Torosaurus.Scannella J. and Horner J.R. 2010. "Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny ." Journal of Vertebrate Paleontology, 30(4): 1157 - 1168. Horner, Jack. TEDX Talks: "Shape-shifting Dinosaurs ". Nov 2011. Accessed 20 Nov 2012.

However, latest thinking is that the two are definitely different genera.<ref>Bowdler, Neil 2012. BBC News Science & Environment. Triceratops and Torosaurus dinosaurs 'two species, not one''' </ref> In 2011, 2012 and 2013, studies of outside features of known specimens decided that shape and development differences mean the two genera are not synonyms. The main problems in the "one genus" idea are:  Longrich N.R & Field D.J. 2012. Torosaurus is not Triceratops: ontogeny in chasmosaurine ceratopsids as a case study in dinosaur taxonomy. PLoS ONE 7 (2): e32623. Farke A.A. 2011. Anatomy and taxonomic status of the chasmosaurine ceratopsid Nedoceratops hatcheri from the Upper Cretaceous Lance Formation of Wyoming, U.S.A. PLoS ONE 6 (1): e16196. Maiorino L; Farke A.A. et al 2013. Is Torosaurus Triceratops? Geometric morphometric evidence of Late Maastrichtian Ceratopsid Dinosaurs. PLoS ONE 8(11): e81608. doi:10.1371/journal.pone.0081608
A lack of good transitional forms, 
The apparent existence of authentic Torosaurus'' subadults, different skull proportions independent of maturation
The hole formation at an adult stage is not part of a normal ceratopsian growth sequence.

 References 

category:Ceratopsids