Findings in title quoted from the linked summary^1 and peer-reviewed journal article.^2

From the linked summary^1 released 16 Jan. 2023 by the U.K.’s Natural History Museum:

>Fighting for mates may be a behaviour that dates back over 400 million years.

>A unique specimen suggests that male Walliserops trilobites fought each other using trident-like structures to win the opportunity to mate with females.

>Co-author Professor Richard Fortey OBE FRS, who is a Scientific Associate at the Museum, says, “The extraordinary Devonian trilobite Walliserops carried a unique, giant trident on its head, the purpose of which has long been a mystery.”

>“We now believe that it was used for jousting between males striving for dominance. The evolution of sexually motivated competition in animals is hundreds of millions of years older than we thought.”

> 

>The researchers analysed the shape of the Walliserops tridents and compared them to the weapons of beetles to try and understand how they might have been used. They found the trident shape was most similar to beetles that try to tip over their opponents with shovel-like weapons.

>When the trilobites were alive 400 million years ago, it is believed they used their tridents to prod at each other before attempting to get underneath their rival and turn them over.

>While any trilobites that did get flipped were not necessarily stuck, the amount of time it could have taken to right themselves would have allowed victorious males the opportunity to mate with females.

>The trilobite at the centre of this study stands out from many other Walliserops specimens held in museums because of its unique trident.

>Instead of having three points, or tines, it is the only known individual to have grown four. The tines are all broadly equivalent in size and there is no sign of injury, which suggests it was born with four as a result of genetic mutation.

>Even more important than the four-tined trident itself is the fact that the specimen is fully grown. By making it to adulthood, it shows that the feature that makes it different from other trilobites didn't have a significant impact on its chances of survival.

^1 Unique trilobite trident could be the oldest evidence of male sexual combat, 16 Jan. 2023, https://www.nhm.ac.uk/discover/news/2023/january/unique-trilobite-trident-could-be-oldest-evidence-male-sexual-combat.html

^2 Gishlick A. and Fortey R. Trilobite tridents demonstrate sexual combat at 400 Mya. Proceedings of the National Academy of Sciences, 2023. 120 (4) e2119970120. https://doi.org/10.1073/pnas.2119970120