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.2013 Nov 26;8(11):e81608.
doi: 10.1371/journal.pone.0081608. eCollection 2013.

Is torosaurus triceratops? Geometric morphometric evidence of late maastrichtian ceratopsid dinosaurs

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Is torosaurus triceratops? Geometric morphometric evidence of late maastrichtian ceratopsid dinosaurs

Leonardo Maiorino et al. PLoS One..

Abstract

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.

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Conflict of interest statement

Competing Interests:Andrew A. Farke is a PLOS ONE editorial board member. This does not alter the authors' adherence to all of the PLOS ONE policies on sharing data and materials.

Figures

Figure 1
Figure 1. Formalism for heterochronic processes and their morphological expressions.
Redrawn and Alberch et al. [15] and after Reilly et al. [18]. Paedomorphosis and peramorphosis can result from multiple perturbations of the three developmental parameters: rate, onset and offset. Each of the simple perturbation trajectories (shaded boxes) can be shifted by one, the other, or both of the other two parameters (circles and rectangles). a, the age at the onset of development; b, the age at offset of development; kr, the rate of development (i.e. the rate of change in shape); and ks, the rate of growth (i.e. the rate of change in size). See also Table 1.
Figure 2
Figure 2. Landmarks and semi-landmark configurations.
A, landmark configuration for skull in lateral view. B, landmark configuration for squamosal. C and D are subunits of skull configuration. Landmarks have identical definitions. Scale bars equal 10 cm. See Table S3 in File S1 for landmark definitions. The image of YPM 1822 is used under the courtesy of the Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA. All rights reserved.
Figure 3
Figure 3. Principal Component Analysis and linear regression between shape and size performed on skulls.
A, relationship between PC1 and PC2. B, morphological changes in skull shape associated with size. Points dimensions are proportional to specimen Centroid Size. The continuous line representsTorosaurus latus morphospace. The dotted line representsTriceratops prorsus morphospace, and the dashed line representsTriceratops horridus morphospace.
Figure 4
Figure 4. Principal Component Analysis and linear regression between shape and size performed on skulls without frill.
A, relationship between PC1 and PC2. B, morphological changes in skull without frill shape associated with size. Points dimensions are proportional to specimen Centroid Size. The continuous line representsTorosaurus latus morphospace. The dotted line representsTriceratops prorsus morphospace, and the dashed line representsTriceratops horridus morphospace.
Figure 5
Figure 5. Principal Component Analysis and linear regression between shape and size performed on frills.
A, relationship between PC1 and PC2. B, morphological changes in frill shape associated with size. Points dimensions are proportional to specimen Centroid Size. The continuous line representsTorosaurus latus morphospace. The dotted line representsTriceratops prorsus morphospace, and the dashed line representsTriceratops horridus morphospace.
Figure 6
Figure 6. Principal Component Analysis and linear regression between shape and size performed on squamosals.
A, relationship between PC1 and PC2. B, morphological changes in squamosal shape associated with size. Points dimensions are proportional to specimen Centroid Size. An asterisk indicates specimens with dubious species assignation. The continuous line representsTorosaurus latus morphospace. The dotted line representsTriceratops prorsus morphospace, and the dashed line representsTriceratops horridus morphospace.
Figure 7
Figure 7. UPGMA cluster analysis performed on the four samples.
A, UPGMA cluster analysis of skulls. B, UPGMA cluster analysis performed on average values of skull. C, UPGMA cluster analysis of skulls without frill. D, UPGMA cluster analysis performed on average values of skull without frill. E, UPGMA cluster analysis of frills. F, UPGMA cluster analysis performed on average values of frill. G, UPGMA cluster analysis of squamosals. H, UPGMA cluster analysis performed on average values of squamosal. Asterisks indicate specimens with dubious species assignation. Light blue indicatesTorosaurus latus specimens, black indicatesTriceratops prorsus specimens, grey indicatesTriceratops horridus specimens, red indicatesNedoceratops hatcheri and orange indicatesTorosaurus utahensis specimens.
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References

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