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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsSexual dimorphism is a difference in appearance or anatomy associated with sex among members of the same species. Fossils can preserve patterns that suggest such differences, but a larger skeleton, a second body shape, or a fossil found near eggs does not by itself reveal which individuals were male or female. Paleontologists test whether the variation fits a sex-linked explanation and whether age, growth, preservation, geography, or other causes fit just as well.
What sexual dimorphism means
Sexual dimorphism refers to sex-associated differences in morphology or appearance within a species. The differences may involve overall size, proportions, bone shape, ornamentation, or soft-tissue features. It does not mean that every male and female can be distinguished by sight: the sexes may overlap, and a difference may be subtle or absent in the skeletal traits that fossilize.
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In living animals, researchers can sometimes establish sex independently through reproductive anatomy or other evidence, then compare traits between known-sex individuals. Fossils rarely preserve that kind of direct evidence. Scientists therefore usually begin with measured variation and ask whether sex is a plausible explanation, rather than simply labeling one form male and another female.
Why fossil sex is difficult to establish
Fossil samples are often small or incomplete, and specimens may come from different places, geological intervals, or populations. Those limitations can make ordinary variation look like two distinct groups. Differences may instead reflect growth, individual variation, geographic separation, taxonomic differences, injury, measurement error, or distortion during fossilization. The challenges are summarized in the review “Sexual dimorphism in dinosaurs” by Ludwig, Smith, and Ibrahim (2023).
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Soft tissues, behavior, and reproductive anatomy are rarely available to examine. Even when a fossil preserves a feature that seems reproductive, it may identify a particular biological state rather than the sex of every individual in the species.
How paleontologists test a proposed difference
1. Define the trait and the sample
Researchers first specify what appears to differ—for example, body size, the proportions of a bone, its curvature, or an ornament. They also assess whether the specimens are comparable: do they belong to the same species, and do they come from a population and time period that can reasonably be analyzed together?
2. Measure variation across specimens
Measurements may be linear dimensions, ratios, or three-dimensional landmarks used to describe shape. Geometric morphometrics can test whether shapes differ and, in some analyses, whether a pattern remains after accounting for size. This matters because a larger bone is not automatically a different shape, and a shape difference that persists independently of size is a distinct observation—not yet proof of a sex difference.
3. Compare with living relatives, cautiously
Birds and crocodilians provide living archosaur comparisons in which sex can be established independently. They can help researchers judge whether a proposed fossil trait has a plausible sex-associated counterpart. But resemblance does not prove that an extinct animal used the trait in the same way or that it was linked to sex in the same direction.
4. Test other explanations
Before interpreting apparent groups as sexes, researchers consider whether the pattern could reflect differences in age or maturity, body size, individual variation, pathology, place or time, preservation, or taxonomy. Specimens from separate sites or ages may not represent one population; damaged or distorted bones may also create misleading measurements.
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5. Choose statistical methods that fit the evidence
Researchers have used comparisons of means and medians, coefficients of variation, mixture models, shape ordinations, and effect-size approaches. These methods answer different questions and rely on assumptions about sample size, distributions, missing data, and sex ratios. A statistical result is only as informative as its data and assumptions.
Rehg and Leigh’s 1999 comparison tested three methods for estimating size dimorphism using known-sex body-mass data from 42 primate species. None was especially accurate overall, though the simple means method performed relatively well. The study is a reminder that methods developed to infer sex differences from fossil samples can be uncertain even when evaluated against living species with known sexes: “Estimating sexual dimorphism and size differences in the fossil record: a test of methods”.
What statistical findings can—and cannot—show
No detected difference does not prove there was none
Jordan C. Mallon’s 2017 analysis re-examined quantitative claims for nine dinosaur taxa. Using tests of normality, unimodality, and mixture models, it found no statistical evidence for the proposed dimorphism in the available data. Mallon cautioned that this does not establish that dinosaurs lacked dimorphism: the samples may simply be too limited or unsuitable for detecting it. Read the 2017 analysis in Paleobiology.
A detected difference does not automatically mean sexual dimorphism
Even a clear statistical separation between specimens could arise from age, geography, preservation, or taxonomic differences rather than sex. Saitta and Stockdale’s 2020 framework recommends assessing likely secondary sexual traits, testing alternatives, and weighing effect sizes and multiple lines of evidence instead of relying on a binary significant-or-not-significant result. Their effect-size framework addresses non-avian dinosaurs and other extinct taxa.
Methods can miss sex differences, too
Motani’s 2021 reanalysis examined published data for 139 sexually dimorphic living species. In that study’s dataset, an earlier method correctly recognized 5% of the species; a modified method reached 50%, and shape-based re-sexing with a misclassification rate below 0.05 was possible for about 32% of the species. These are results for that particular reanalysis of living species and its methods—not a universal success rate for sexing fossils. Motani’s findings underscore that shape-based estimates depend heavily on method and that unknown sex for fossil individuals creates an additional obstacle. See “Sex estimation from morphology in living animals and dinosaurs.”
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What eggs, nests, and medullary bone reveal
Medullary bone is temporary tissue associated with providing calcium during egg production in birds. Its presence in a dinosaur fossil may support a reproductive interpretation, but it is not a general test for identifying females: it is associated with a limited reproductive state, so its absence does not establish that an animal was male.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Likewise, a skeleton found with eggs is not automatically female. Nest or egg associations can be informative, but parental care is not necessarily exclusive to females; comparisons with living birds suggest males may have participated in care in some dinosaur species. The 2023 review discusses both the potential and limits of reproductive evidence: Ludwig, Smith, and Ibrahim, “Sexual dimorphism in dinosaurs.”
Case study: ornithomimosaurs at Angeac-Charente
A fossil bed at Angeac-Charente in southwestern France contains at least 61 ornithomosaurs thought to have died in a mass-mortality event, according to the 2023 review by Ludwig, Smith, and Ibrahim. A same-place, same-time assemblage offers a more coherent comparison than specimens gathered from unrelated layers and localities.
Researchers used three-dimensional geometric morphometrics on the best-preserved hindlimb bones and found subtle differences in femur shape, especially curvature, that were independent of bone size. They interpreted the pattern in light of comparable sex-associated variation among living archosaurs. The analysis did not establish which femur form represented which sex, and further work on growth variation was identified as necessary. It illustrates the distinction between detecting structured variation and assigning sex: the review’s account of the study.
How to evaluate a claim about two fossil forms
When researchers propose that two apparent morphs represent males and females, these questions help separate a plausible hypothesis from a demonstrated conclusion:
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- Age and growth: Are the specimens at comparable stages of maturity?
- Size and shape: Does the shape difference remain after size is accounted for, or is the claim based only on one group being larger?
- Place and time: Do the fossils come from the same assemblage and geological interval?
- Preservation: Could deformation, breakage, or reconstruction have changed the feature being measured?
- Biological plausibility: Is the trait a plausible sex-associated feature, and is there independent evidence linking it to sex?
- Method and uncertainty: Are the method’s assumptions, effect sizes, and uncertainty explained—not just whether a result passed a significance threshold?
What a careful conclusion looks like
Sexual dimorphism is biologically plausible and may have occurred in some extinct species, but recognizing it in fossils is difficult. A size gap or two clusters can raise a hypothesis; convincing identification requires a suitable sample, careful measurement, tests of competing explanations, and ideally independent evidence about sex. Published analyses differ because they examine different datasets and use different methods and evidentiary standards. The strongest conclusions are therefore case-specific: a study may detect a pattern without showing that it is sexual, or may fail to detect a difference without proving that none existed.
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