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

Leonardo Maiorino,Paolo Piras,Andrew A Farke,Tassos Kotsakis,Richard J Butler

doi:10.1371/journal.pone.0081608

Abstract

BackgroundRecent 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 FindingsExamining 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.

Highlights

Triceratops, Torosaurus, and the problematic Nedoceratops represent some of the historically first-known horned dinosaurs (Ceratopsidae, Chasmosaurinae), discovered in western North America at the end of nineteenth century [1,2], as well as the temporally latest ceratopsids, from the terminal Cretaceous [3,4,5,6,7,8]
Scannella and Horner [8] reviewed the taxonomic status of Nedoceratops hatcheri, correctly noting that at least some of supposed autapomorphies were found within Triceratops, and reaffirming Nedoceratops as a transitional ontogenetic morph
In this paper we explore the cranial morphologies of specimens previously referred to Triceratops, Nedoceratops and Torosaurus (27 skulls, including Nedoceratops hatcheri), using landmark-based geometric morphometrics in order to investigate cranial shape changes and infer ontogenetic trajectories

Summary

Introduction

Triceratops, Torosaurus, and the problematic Nedoceratops represent some of the historically first-known horned dinosaurs (Ceratopsidae, Chasmosaurinae), discovered in western North America at the end of nineteenth century [1,2], as well as the temporally latest ceratopsids, from the terminal Cretaceous (late Maastrichtian) [3,4,5,6,7,8]. Whereas Scannella and Horner argued that Nedoceratops hatcheri (known from a single skull from the Lance Formation of Wyoming) represented a transitional ontogenetic stage between Triceratops and Torosaurus morphotypes, Farke [7] hypothesized that Nedoceratops displayed autapomorhic features distinguishing it from Triceratops and Torosaurus specimens. 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. Whether looking at entire skulls, skulls without the frill, frills alone, or squamosals, Torosaurus has different morphologies and distinct allometric trajectories compared to Triceratops

Methods

Results

Conclusion