An image comparing the different beetle morphologies as they relate to fighting mode compared to Walliserops. © Alan Gishlick

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Analysis of ‘pronged’ Trilobite tridents reveal the oldest known example of a sexual combat weapon

Scientists have revealed the oldest example of a sexual combat weapon similar to the structures found in modern day Dynastinae (rhinoceros) beetles. 

The discovery and analysis of a trilobite fossil Walliserops trifurcatus which displayed a four tined trident suggested that these structures may have been used in combat and competition for mates such as male ‘jousting’ to gain female favours.

A previous hypothesis had predicted the trident was used for feeding. However, the four tined individual had grown to full maturity and adult size. Malformations in a variety of living organisms show that a malformation in a feeding structure would have likely hindered the survival of the individual. This supported the theory that the trident may have played a sexually selected function instead. 

Prof Richard Fortey FRS, Scientific Associate at the Natural History Museum and co-author of the paper 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.’ 

Trilobites are a diverse extinct group within Arthropoda, a phylum shared by beetles and horseshoe crabs amongst others. The species lived during the Devonian, a period of significant adaptive radiation. Examples of sexually selected traits can be found in many living and fossil animal taxa and involves a process where traits are selected because they enhance fertilisation success. 

The individual W.trifurcatus fossil was found in Morocco. Scientists used morphometric comparisons of specimens including the unique fossil to map and compare the structures to the similar morphologies found in Dynastinae beetles. Results showed that the trident shares a similar structure and similar function to those used in sexual competition and combat making this the oldest example of its kind. 

Prof Richard Fortey says, ‘The evolution of sexually motivated competition in animals is hundreds of millions of years older than we thought.’ 

There are four known species of Walliserops trilobite, and each bears a trident with specific morphology distinctive to its species. This study lends evidence to the idea that trilobites may have been strongly sexually dimorphic and sheds vital information into the species morphology, life behaviours and evolutionary history. 

‘Trilobite tridents demonstrate sexual combat 400 Mya’ published by PNAS. 

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