Sexual Selection in the Fossil Record

Many extinct animals carried conspicuous structures: crests, horns, plates, tusks or elaborate feathers. In living species, some traits influence access to mates or competition with rivals. Palaeontologists therefore ask whether sexual selection helped shape fossil ornaments. It is a useful hypothesis, but it is difficult to prove because a fossil preserves anatomy, not courtship.

What sexual selection means

Sexual selection is a form of natural selection in which differences in mating success affect which traits become more common. It can act through competition between individuals, through mate choice, or through both. These processes do not always produce the same anatomy. A structure used in combat may also be displayed; a bright feature may help with recognition, intimidation or species identification.

In extinct species, researchers cannot watch behaviour or easily assign fossil individuals to sexes. A large crest or horn is therefore not evidence of sexual selection by itself. Its function might have involved feeding, defence, heat exchange, sound production, recognition or several roles at once. The best explanations test alternatives rather than beginning with an attractive story.

How fossils can test the idea

One line of evidence is variation among specimens. If a structure changes predictably with growth, it may have developed at maturity. But growth stages can be mistaken for separate species, and adult variation is not automatically sexual dimorphism. Researchers compare bone texture, histology, size, shape and other anatomical features across multiple individuals.

Another test asks whether a structure was costly or mechanically useful. A large ornament may have required energy to grow or carried a locomotor cost. Biomechanical analysis can estimate how it might have resisted forces, while comparisons with related living animals suggest possible uses. These approaches narrow the possibilities; they rarely reveal the exact behaviour that occurred.

A study of Stegosaurus mjosi reported anatomical variation consistent with sexual dimorphism, but even its authors noted that assigning sex to fossil individuals remains uncertain. In fossil birds, medullary bone – a temporary tissue associated with eggshell production in living birds – can provide stronger sex-related evidence. Such exceptional clues help calibrate interpretations, yet they do not solve every species.

Display and survival can overlap

Researchers have proposed that some pterosaur and dinosaur crests served in social signalling or mate choice. A crest that looks extravagant to people may still have had several functions. In a 2012 review, mutual sexual selection, in which both sexes carry ornaments and choose mates, was considered for fossil ornithodirans. More recent analyses of early birds have tested whether differences in feathers and anatomy might relate to sex, but fossil samples remain limited.

These studies illustrate why “ornament” is a descriptive starting point rather than a final functional diagnosis. The same structure may help animals recognise members of their species and influence mate choice. It might also act in competition. Different roles are not mutually exclusive.

Keep confidence proportional to evidence

A strong fossil argument combines repeated specimens, a growth series, anatomical or histological evidence, sound phylogenetic comparisons and tests of competing functions. Even then, the conclusion should remain a hypothesis with a stated level of confidence. A single ornate skull cannot tell us whether males fought, females chose, both sexes displayed, or the feature served another purpose.

Fossils let scientists investigate sexual selection, but not by reading behaviour directly from shape. That distinction also matters when interpreting dinosaur horns and crests: anatomy is observed; its social meaning is inferred.