Protoceratops andrewsi has often been presented as a classic case of male and female dinosaurs having different skulls. But the traits used to sort the fossils into those categories were themselves the proposed sex markers. A 2015 analysis of 29 skulls found broad overlap between the groups, while growth offered a strong alternative explanation for many of the differences.
Can we easily distinguish male and female Protoceratops?
Not from the skull features tested in the 2015 study. Researchers had proposed that males and females differed in skull width above the eyes, nasal height, and the width and height of the frill. Those were hypotheses based on measurements—not independently confirmed sex identifications.
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Leonardo Maiorino, Andrew A. Farke, Tassos Kotsakis, and Paolo Piras tested the proposed distinctions using geometric morphometrics, a method for comparing shape. They analyzed photographs of 29 Protoceratops andrewsi skulls in lateral and dorsal views. Their paper, published May 7, 2015, found no clear overall difference in skull shape or size between the specimens previously labeled “males” and “females.” The authors concluded that their results did not support the hypothesis of sexual dimorphism in P. andrewsi—in the context of this skull-shape analysis. Read the 2015 study in PLOS ONE.
Why the sex labels need quotation marks
The categories used for comparison came from earlier trait-based assignments, especially an approach proposed by Peter Dodson in 1975. Because the supposed differences helped define which fossils counted as male or female, those assignments were not independent proof of each specimen’s biological sex. The paper also notes that some assignments were ambiguous; changing the classification of those specimens did not alter the overall result.
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For that reason, the careful terms are “hypothesized males” and “hypothesized females.” A visible contrast between a few skulls cannot by itself establish two sex-specific forms, particularly if age or individual variation could also account for it.
Growth can change the same skull features
The fossils in the study ranged from juvenile to adult-like skulls. The researchers found that skull shape changed with size in both views, making growth a plausible explanation for much of the variation. They used size as a proxy for age because histological age estimates were unavailable for the specimens.
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- As skulls grew, the frill developed and widened.
- The rostrum lengthened, and the nasal bump became more pronounced.
- The external naris tilted, while the orbit became relatively smaller.
These changes matter because immature and mature animals can look different even without a sex-linked skull difference. In the study’s regressions, shape-size relationships had R² values of 0.226 for the lateral-view sample and 0.126 for the dorsal-view sample. Those figures describe the study’s particular samples and regressions; they are not measures of how sexually dimorphic the species was.
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What the study found—and what it did not
The proposed groups broadly overlapped in the analyses of overall skull shape and size. One analysis did report a possible difference in rostrum shape, but the authors treated it as tentative: cluster analysis did not produce clear separation, and specimens assigned to opposite groups could resemble one another.
This was a study of cranial morphology based on two-dimensional photographs, not a direct determination of the sex of each fossil. It therefore weakens the case for the particular skull-based criteria tested; it does not show that male and female Protoceratops were identical or that the species lacked sexual dimorphism altogether.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why this fossil example is still useful
Protoceratops is a neoceratopsian from Mongolia’s Late Cretaceous Djadokhta Formation. Maiorino and colleagues noted that more than 100 well-preserved skulls and skeletons were known from the broader fossil record, yet their analysis included 29 skulls. A substantial fossil record gives researchers material to compare, but it does not automatically solve the problem of identifying sex or separating sex differences from growth.
The broader lesson is methodological: a shape difference is not automatically a sex difference. Researchers need to consider age, variation among individuals, sample size, and whether the proposed groups are independently established. The study leaves open the possibility that dimorphism could appear in other traits, including the postcranial skeleton, or become clearer in a larger sample with better control of growth stage.
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