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. 2015 May 7;10(5):e0126464.
doi: 10.1371/journal.pone.0126464. eCollection 2015.

Males Resemble Females: Re-Evaluating Sexual Dimorphism in Protoceratops andrewsi (Neoceratopsia, Protoceratopsidae)

Leonardo Maiorino  1 , Andrew A Farke  2 , Tassos Kotsakis  1 , Paolo Piras  1
Affiliations

Males Resemble Females: Re-Evaluating Sexual Dimorphism in Protoceratops andrewsi (Neoceratopsia, Protoceratopsidae)

Leonardo Maiorino et al. PLoS One. .

Abstract

Background: Protoceratops andrewsi (Neoceratopsia, Protoceratopsidae) is a well-known dinosaur from the Upper Cretaceous of Mongolia. Some previous workers hypothesized sexual dimorphism in the cranial shape of this taxon, using qualitative and quantitative observations. In particular, width and height of the frill as well as the development of a nasal horn have been hypothesized as potentially sexually dimorphic.

Methodology/principal findings: Here, we reassess potential sexual dimorphism in skulls of Protoceratops andrewsi by applying two-dimensional geometric morphometrics to 29 skulls in lateral and dorsal views. Principal Component Analyses and nonparametric MANOVAs recover no clear separation between hypothetical "males" and "females" within the overall morphospace. Males and females thus possess similar overall cranial morphologies. No differences in size between "males" and "females" are recovered using nonparametric ANOVAs.

Conclusions/significance: Sexual dimorphism within Protoceratops andrewsi is not strongly supported by our results, as previously proposed by several authors. Anatomical traits such as height and width of the frill, and skull size thus may not be sexually dimorphic. Based on PCA for a data set focusing on the rostrum and associated ANOVA results, nasal horn height is the only feature with potential dimorphism. As a whole, most purported dimorphic variation is probably primarily the result of ontogenetic cranial shape changes as well as intraspecific cranial variation independent of sex.

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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

Fig 1
Fig 1. Hypothetical large “male” at left (AMNH 6438) and hypothetical large “female” at right (AMNH 6466).
Redrawn and modified after Brown and Schlaikjer [23]. Scale equals to 10 cm.
Fig 2
Fig 2. Linear measurements measured on skull.
Redrawn following the measurement standards originally in Dodson [25]. The numbers of measurements corresponds to those reported in Table B in S1 File.
Fig 3
Fig 3. Linear regressions of basal skull length (BSL) against the four sex-discriminant variables of the Protoceratops sample (see Table B in S1 File).
Black dot represents “male”, white dot represents “female”, gray dot represents juvenile, and red dot represents a sex-undetermined specimen. Scale of axes is logarithmic.
Fig 4
Fig 4. Landmark and semi-landmark configurations.
A, landmark configuration for skull in lateral view. B, landmark configuration for skull in dorsal view. C and D are subunits of the skull configuration. Landmarks have identical definitions. See Table C in S1 File for landmark definitions.
Fig 5
Fig 5. Principal Component Analysis performed on the linear measurements of skulls.
The continuous line represents “juvenile” morphospace. The dotted line represents “male” morphospace and the dashed line represents “female” morphospace. Asterisks indicate specimens shared with Dodson [25].
Fig 6
Fig 6. Principal Component Analysis performed on the skulls in lateral view.
The continuous line represents “juvenile” morphospace. The dotted line represents “male” morphospace and the dashed line represents “female” morphospace. Asterisks indicate specimens shared with Dodson [25].
Fig 7
Fig 7. Principal Component Analysis performed on skulls with the frill excluded.
The continuous line represents “juvenile” morphospace. The dotted line represents “male” morphospace and the dashed line represents “female” morphospace. Asterisks indicate specimens shared with Dodson [25].
Fig 8
Fig 8. Principal Component Analysis performed on the frills.
The continuous line represents “juvenile” morphospace. The dotted line represents “male” morphospace and the dashed line represents “female” morphospace. Asterisks indicate specimens shared with Dodson [25].
Fig 9
Fig 9. Principal Component Analysis performed on skulls in dorsal view.
The continuous line represents “juvenile” morphospace. The dotted line represents “male” morphospace and the dashed line represents “female” morphospace. Asterisks indicate specimens shared with Dodson [25].
Fig 10
Fig 10. UPGMA cluster analysis performed on the four samples.
A, UPGMA cluster analysis of skulls in lateral view. B, UPGMA cluster analysis performed on skulls in dorsal view. C, UPGMA cluster analysis of skulls without frill. D, UPGMA cluster analysis performed on the frills. Asterisks indicate specimens shared with Dodson [25]. Light blue indicates a “male”, pink indicates a “female”, grey indicates a juvenile.
Fig 11
Fig 11. Visualization of shape-size relationship via CCA analysis for cranial shape in lateral view and cranial shape changes associated with the increase of size.
Asterisks indicate specimens shared with Dodson [25].
Fig 12
Fig 12. Visualization of shape-size relationship via CCA analysis for cranial shape in dorsal view and relative cranial shape changes associated with the increase of size.
Asterisks indicate specimens shared with Dodson [25].
See this image and copyright information in PMC

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