A 400-million-year-old trilobite specimen has revealed new insights into the evolution of sexual combat weapons. The trilobite’s trident, usually a three-pronged fork thought to be used for feeding, had four prongs, leading researchers to believe it evolved for fighting, not for food.
“The extraordinary Devonian trilobite Walliserops carried a unique giant trident on its head, the purpose of which has long been a mystery,” said Professor Richard Fortey FRS, science associate at the Natural History Museum in London and co-author of a new paper in a statement sent to IFLScience. . “We now believe it was used for ‘jousting’ between males fighting for dominance.”
The trilobite fossil is a Walliserops trifurcatus specimen that had reached full maturity and adult size before dying and becoming part of the fossil record. He had a malformed trident fitted with four prongs instead of three, but despite this he was able to survive into adulthood.
If the trident was a crucial tool for feeding, a malformation should lead to an early death as the animal could not feed properly. Since this trilobite clearly did well with its unique trident, the researchers of the new paper speculated that it could have been used in sexual combat instead.
A morphometric comparison of the trilobite’s trident revealed that it shares similarities with the sexual combat arms of some beetle species. Image credit: Alan D. Gishlick
Anatomical weapons for fighting over potential mates are widespread throughout the animal kingdom, but perhaps the most comparable in the case of trident-wielding trilobites are the rhinoceros beetles of the subfamily Dynastinae. As their name suggests, these arthropods have horn-like protrusions on their heads which they use to joust competing males in an attempt to mark a mate.
The position and inflexibility of the Walliserops trident also seemed ill-suited to the feeding apparatus, but a comparison of his trident with the secondary sex characteristics of comparable living species within Dynastinae revealed similarities. It seems that the tridents of Walliserops the trilobites would have been in the right place to fight for females, which might explain how our malformed four-toothed trident specimen was able to thrive despite having a less conventional kit on its head.
With morphometric analysis supporting the hypothesis that the trilobite’s trident was used in sexual combat, Walliserops becomes the earliest known example of weapons of sexual combat known to science.
“The evolution of sexually motivated competition in animals is hundreds of millions of years older than we thought,” Fortey concluded.
The study was published in the journal PNAS.