Male and female members of one species can differ in size, ornamentation or body proportions. In living animals, such sexual dimorphism can be tested by observing reproductive behaviour and comparing large samples. Fossils rarely preserve those observations. A difference between two dinosaur skeletons may reflect sex, but it may also reflect age, individual variation, a second species or distortion during burial.
What evidence would be convincing?
A strong case requires several well-preserved individuals from the same species and comparable geological setting. Researchers can measure many bones and test whether the sample separates into consistent groups after accounting for growth and body size. If one sex is proposed to have a larger crest or more robust limb, the feature should recur across individuals rather than rest on a single striking specimen.
Even a bimodal pattern does not automatically identify males and females. The sample could contain two species or different age classes. Fossil beds may also overrepresent one age, season or habitat. Statistical methods help assess patterns, but they cannot restore information the fossils do not preserve.
Age and species can imitate sex differences
Many dinosaurs changed shape as they grew. Horns, skull crests, limb proportions and bone texture could develop at different rates. A small individual and a large adult may therefore look different without belonging to separate sexes. Growth series and bone histology can help estimate maturity, though preservation and sampling make those estimates uncertain.
Taxonomic diversity is another complication. Several similar species can occur in the same formation, while incomplete skeletons may be difficult to distinguish. If researchers combine multiple species into one sample, the result can resemble two anatomical morphs. Reliable identification has to precede a sexual-dimorphism analysis.
A population-level ornithomimosaur case
A 2023 study examined ornithomimosaur femora from the Angeac-Charente site in France. The bones were similar in size and came from the same locality, yet measurements formed two groups. The authors interpreted this pattern as consistent with sexual dimorphism, a potentially useful result because it draws on multiple specimens rather than one presumed male and one presumed female.
The study did not determine which group was male or female. A two-group pattern is evidence to investigate, not a direct label attached to each femur. Further specimens and independent anatomical or reproductive evidence would strengthen the interpretation. The result is specific to the sampled animals and should not be transferred automatically to other ornithomimosaurs or dinosaurs.
Reproductive tissue is rare and specific
Medullary bone is a temporary calcium-rich tissue that living female birds form before laying eggs. It has been reported in a small number of dinosaur fossils, including a Tyrannosaurus rex specimen. If correctly identified, this tissue indicates a reproductively active female at the time of death. It does not reveal the appearance of males, the average body size of each sex or whether a visible ornament was sex-specific.
Researchers must distinguish medullary bone from disease, ordinary bone growth and mineral alteration. Its value lies in identifying reproductive physiology in a particular individual, not in providing a universal sex test for all dinosaurs.
Why famous claims remain debated
Large crests, horns and size differences have often been interpreted as displays used by one sex. Such functions are biologically plausible, and display may have shaped dinosaur anatomy. But a crest can also identify a species, help recognition, regulate heat or serve several roles. Without a population sample and independent evidence, naming one morph “male” and another “female” goes beyond what the bones establish.
The responsible conclusion is measured: sexual dimorphism likely occurred in some dinosaur populations, as it does in many living vertebrates, and one ornithomimosaur sample offers a testable case. For most taxa, the sex of an individual skeleton remains unknown. Fossil differences are observations; their cause must be argued separately.
Frequently asked questions
Can palaeontologists tell whether a dinosaur fossil is male or female?
Usually not. Sex is rarely preserved directly, and skeletal differences can also reflect age, individual variation, species or taphonomic distortion.
What is sexual dimorphism?
It is a consistent difference in anatomy or appearance between males and females of one species, beyond differences caused by age or ordinary individual variation.
Did a 2023 study identify male and female ornithomimosaurs?
It found two groups of femoral measurements at Angeac-Charente that were consistent with dimorphism. The study could not assign either group to males or females.
Does medullary bone prove that a dinosaur was female?
A securely identified medullary bone can indicate a reproductively active female in a particular specimen. It does not show how the sexes looked on average.