Protoceratops andrewsi is often presented as a dinosaur whose skulls reveal male–female differences. A 2015 analysis of 29 skulls found that the proposed male and female groups broadly overlapped. The study weakens that particular skull-based sexing claim; it does not show that the species had no sexual dimorphism.
Can we easily distinguish male and female Protoceratops?
Not from the skull traits tested in the 2015 study. The older proposal treated differences in skull width above the eyes, nasal height, and frill width and height as signs of sex. But those categories were inferred from the traits themselves, rather than established by independent evidence that identified each fossil’s biological sex.
That distinction matters: specimens called “males” and “females” in this debate are more accurately described as hypothesized males and hypothesized females. The labels represent an earlier interpretation, not confirmed sex assignments.
What did the 2015 study test?
Published on May 7, 2015, the peer-reviewed study by Leonardo Maiorino, Andrew A. Farke, Tassos Kotsakis, and Paolo Piras examined photographs of 29 Protoceratops andrewsi skulls from the Late Cretaceous Djadokhta Formation of Mongolia. The fossils included juveniles, subadults, and adults. Researchers compared skull shape in both lateral (side) and dorsal (top) views using two-dimensional geometric morphometrics, a method that analyzes the positions and relationships of anatomical landmarks.
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They used several statistical approaches, including principal component analyses, nonparametric tests, cluster analysis, and regressions relating shape to size. Some earlier sex assignments were ambiguous; changing those assignments did not alter the overall findings. The study’s scope was cranial shape in this sample—not direct sex determination for each fossil, or a survey of every body region.
What did the skull comparisons show?
The hypothesized male and female groups broadly overlapped. The analyses found no clear overall difference in skull shape or size between them. One analysis did identify a possible difference in rostrum shape, but the authors treated it as tentative: cluster analysis did not separate the groups clearly, and specimens assigned to opposite groups could resemble one another.
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The authors’ conclusion was appropriately limited to their test: “The results of this work do not support the hypothesis of sexual dimorphism within P. andrewsi.” In context, that means their skull-shape analysis did not support the earlier proposed cranial distinctions as reliable indicators of sex.
Why growth can look like a sex difference
Skull shape changed with size in both views. Because the researchers did not have histological age estimates for the specimens, they used size as a proxy for age. That is a useful but imperfect way to assess development: a larger skull is not, by itself, a direct measurement of an individual’s age.
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As skull size increased, the study documented changes including a more developed and wider frill, a longer rostrum, a more pronounced nasal bump, a change in the angle of the external naris, and a relatively smaller orbit. Juvenile and adult-like skulls could therefore look substantially different even if the differences were caused by growth rather than sex.
The reported shape–size relationship was study-specific: the regression had R² = 0.226 for the lateral-view sample and R² = 0.126 for the dorsal-view sample. These values describe the relationship in the analyzed skulls; they are not measures of sexual dimorphism across the species.
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What the evidence does—and does not—settle
| Question | What the study supports |
|---|---|
| Do the proposed skull measurements clearly separate the hypothesized sexes? | No. The groups broadly overlap in overall shape and size. |
| Could some of the variation reflect growth? | Yes. Shape varied with size, and the study described several changes associated with skull development. |
| Does the study prove Protoceratops had no sexual dimorphism? | No. It tested cranial morphology in a sample of 29 skulls, not every trait or body region. |
| Could better age control or other anatomical evidence change the picture? | Possibly. The authors noted that a larger, more comprehensive sample and better control of growth stage could affect the conclusions; postcranial traits were not resolved by this skull study. |
The broader fossil record reportedly includes more than 100 well-preserved Protoceratops skulls and skeletons, but the study analyzed 29 skulls. A larger number of fossils in the record does not automatically answer the question: comparisons also depend on which specimens can be studied together and how reliably their growth stages can be assessed.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to read claims of sexual dimorphism in fossils
A striking contrast between a few fossils is not enough to establish that they represent males and females. In this case, the proposed sex categories were based on the same skull traits later used to distinguish the groups, while skull shape also changed with size. A careful interpretation therefore separates three possibilities: sex-linked differences, growth-related change, and variation among individuals.
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For Protoceratops, the 2015 analysis makes the older skull-based claim less convincing, but it does not close the question of sexual dimorphism. Stronger conclusions would require evidence that can distinguish sex from development and individual variation, potentially including other traits and a larger sample with better age control.
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