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Background  Recent assessments of morphological changes in the frill during ontogeny hypothesized that the late Maastrichtian horned dinosaur  Torosaurus  represents the “old adult” of  Triceratops , although acceptance of this finding has been disputed on several lines of evidence.    Methodology/Principal Findings  Examining the cranial morphology of 28 skulls in lateral view and 36 squamosals of  Nedoceratops hatcheri ,  Triceratops  spp. and  Torosaurus  spp. by means of landmark-based geometric morphometrics, we compared ontogenetic trajectories among these taxa. Principal Component Analysis and cluster analysis confirmed different cranial morphologies.  Torosaurus  shape space is well separated from  Triceratops , whereas  Triceratops horridus  and  Triceratops prorsus  partially overlap within  Triceratops  shape space. Linear regressions between shape and size suggest different ontogenetic trajectories among these taxa. Results support the “traditional” taxonomic status of  Torosaurus . We hypothesize that ontogeny drives cranial morphology with different patterns between  Torosaurus  and  Triceratops .    Conclusions/Significance    Torosaurus  is a distinct and valid taxon. Whether looking at entire skulls, skulls without the frill, frills alone, or squamosals,  Torosaurus  has different morphologies and distinct allometric trajectories compared to  Triceratops . This new approach confirms the taxonomic status of  Torosaurus  as well as the comparatively low diversity of ceratopsids at the end of the Maastrichtian in North America.

Is Torosaurus Triceratops? Geometric Morphometric Evidence of Late Maastrichtian Ceratopsid Dinosaurs