Trilobite fossil reveals how ancient arthropods mated

While a broken fossil isn't normally a cause for celebration, scientists researching trilobites have caught a lucky break.

There are over 20,000 species of the extinct trilobites from an extensive fossil record that spans 275 million years, but large parts of their behaviour remain a mystery.

However, a specimen of Olenoides serratus uncovered in Canada has now provided some of the first physical evidence of how these marine arthropods mated, offering new clues about their behaviour. 

A break in the fossil has revealed two pairs of unusual appendages that have been interpreted as claspers, which would have helped a male grab onto a female for fertilisation.

Professor Javier Ortega-Hernández, the co-author of the study describing the find, says, 'Traditionally, trilobites are looked at as examples of primitive animals. This discovery shows that they could actually display complex behaviours for reproduction, similar to what some of the animals that we have today are doing.

'It is really contributing towards a better understanding of the Cambrian environment as actually being a thriving, ecologically complex system, rather than a lesser version of the biosphere we have today.'

Dr Greg Edgecombe, a trilobite expert at the Museum who reviewed the paper, adds, 'These modified appendages give us a lot more insight into trilobite mating behaviour.

'Horseshoe crab males clasp the female to be in a dominant position relative to male competitors, and he will stay attached for days until she lays her eggs. 

'The claspers help the male to cling on and increase their chance of reproductive success, and it makes sense that trilobites would have a very similar reproductive strategy.'

The findings of the study were published in the journal Geology. 

What do fossils tell us about sex and reproduction?

Fossils can help scientists to identify many things about extinct organisms. Body fossils can provide evidence of what a species may have looked like, if an individual was suffering from disease, and how an organism grew.

Identifying behaviour, however, is much more difficult. Trace fossils, such as preserved footprints and poo, can help to identify how an animal might have moved or what it might have eaten, but other behaviours are harder to investigate.

Graduate student Sarah Losso, the study's lead author, explains, 'Occasionally, you will get fossil specimens that died and got preserved in the act of copulation, while some insects have been preserved during mating. However, short of that, it's hard to infer mating behaviours in extinct animals.'

It can also be difficult to tell apart the sex of fossilised animals, complicating any study of reproduction. For trilobites, differences in both size and morphology, such as the shape of the tail plate (called the pygidium), have been suggested as instances of sexual dimorphism.

However, these characteristics could also be representative of different species, especially if the different morphs don't consistently appear together across different sites.

In 2016, the discovery of fossilised trilobite eggs suggested that the presence of a brood pouch, where eggs and hatchlings can develop inside the parent's body for a time. 

While this is generally a feature of female arthropods, brood care is also a characteristic of male sea spiders, so this feature cannot be used to identify sex with total certainty. The new discovery of clasper-like appendages could help put trilobite sex beyond doubt for the first time.

'This study is important because the question of sexual dimorphism of trilobites has not been clearly answered,' Greg explains. 'This is the first case where trilobite sexual dimorphism has not been recognized in the exoskeleton or in body size, but rather in the morphology of the appendages.' 

How were the trilobite claspers identified?

While appendages in trilobites are not unknown, they are often not well-preserved. Unlike the trilobite exoskeleton, they are not reinforced with calcium carbonate which makes fossilisation less common. 

When they are preserved, the appendages are often concealed beneath the body, and would need to be specifically excavated from a fossil. In the O. serratus specimen used in this study, however, a break has removed part of the exoskeleton and revealed the appendages underneath.

'This specimen is particularly special as its exoskeleton has broken in a critical region,' Greg explains. 'This has exposed the appendage morphology at the back of the thorax and at the front of the pygidium, revealing two pairs of distinctive appendages which have been interpreted as pairs of claspers.

'This suggests the specimen is a male.'

The scientists ruled out the possibility that the appendages could be part way through regrowing, rather than small claspers, as their unique shape was shared by two successive pairs of segments on both sides of the body. This is unlikely to have occurred multiple times by chance.

Further evidence could be provided by finding more specimens with claspers. As O. serratus is a relatively common species, excavating the same segments of the body from other fossil specimens may help to prove the identity of these appendages.

'It's highly likely that more specimens do have these small appendages,' Greg says. 'Based on the similarity to claspers in horseshoe crabs and different groups of crustaceans, I think the interpretation that these are male claspers is pretty convincing. 

'It's a new instance of a part of the body that we didn't know could express sexual dimorphism in trilobites, and will certainly be explored further in the coming years.'