Trilobites used trident-like horns to fight over mates like stags

Trident-like horns on the heads of some trilobites were probably used in fights over mates. This hypothesised behaviour is the oldest example of sexual combat that has been identified in the fossil record.

“Extraordinary structures in organisms cry out for functional explanations,” says Alan Gishlick at Bloomsburg University in Pennsylvania.

Previously, palaeontologists had suggested that the tines on Walliserops, a trilobite that lived around 400 million years ago, could have been used as a defence against ancient nautilus that were hungry for these marine invertebrates. The prongs could grow to more than 25 millimetres long, nearly the size of the rest of the animal.

But Gishlick and his colleague Richard Fortey at the Natural History Museum in London have come to a different conclusion, after studying an unusual specimen of Walliserops with four tines instead of three.

The four-pronged trilobite stuck out to Gishlick because it was comparable in size to other adult Walliserops, indicating that it had the expected lifespan for its species. This appeared to be evidence against the trident being a defensive weapon, as such an abnormality in a defensive structure might have made the trilobite more vulnerable.

Structures used in competition for mates are less critical to survival. “We know there is a high degree of tolerance for malformation in structure related to sexual selection because they only affect mating,” says Gishlick.

The researchers looked for more evidence in modern Japanese rhinoceros beetles (Trypoxylus dichotomus), which have similar structures jutting from their heads. Male beetles often have variations or abnormalities in horn shape as those structures are used in courtship contests with other males rather than as a defence against predators.

This is also the case for deer and wild sheep, with their horns having more to do with face-offs between each other than pushing back predators.

While the sex of the fossil trilobites is difficult to discern, the similarities between Walliserops and the rhinoceros beetles led Gishlick and Fortey to suspect the trident-bearing Walliserops were males.

“It is amazing to see that such complex behaviours appeared very early in the course of evolution and have endured to the present day,” says Jean Vannier at the University of Lyon, France, who wasn’t involved in the study.

“Anything that enables us to better understand past life and test our hypotheses as rigorously as possible is crucial to understand evolution of form and function,” says Gishlick.