A fossil trilobite with spines visible on its legs

A well-preserved set of appendages of Olenoides serratus from the Burgess Shale, one of the two trilobite species analysed in the recent study. You can see the delicate spines on the trilobite's pair of opposing limbs. Image by Sarah Losso

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Some trilobites crushed their prey to death with their legs

By studying modern horseshoe crabs, researchers have been able to build up a picture of how some extinct arthropods such as trilobites may have fed on hard-shelled prey.

Some trilobites probably used their legs to crush their prey before passing the food to their mouths.

Trilobites are arthropods that lived on Earth between 521 to 252 million years ago. To date there are more than 20,000 species recorded, making them one of the most successful groups of early arthropods to have evolved. Despite leaving a vast fossil record, very little is known about how they may have eaten.

It turns out that just like we use chopsticks to move food towards our mouth, some trilobites may have stabbed and crushed their prey with their legs as they ate.

Dr Greg Edgecombe, a researcher at the Museum who focuses on the evolutionary history of arthropods, has been investigating how trilobites fed in a new paper published in the Royal Society B.

'We used the horseshoe crab, a living arthropod that eats hard prey, to bring life to models of how two species of trilobite used their legs when feeding,' says Greg. 

The researchers found that one trilobite species, Redlichia rex, shows a similar leg structure and pattern of strain at its leg base as the horseshoe crab, indicating it used its legs to crush shelled creatures before eating them, a behaviour called durophagy. 

This feeding behaviour may have accelerated the 'arms race' between trilobite predators and their shelled prey. 

Trilobites scuttled around the seafloor 

'Some trilobites got their nutrients from the sediment, but mostly they were predators or scavengers. We can see trilobite trackways in the fossil record, and sometimes we find a fossil trilobite at the end of these trails,' says Greg. 

Trilobite exoskeletons were enriched with calcium carbonate and would have been hard and crunchy to eat. But exactly how the animals themselves ate is not fully understood.

A trilobite fossil against a white background

A trilobite fossil © The Trustees of the Natural History Museum, London

Using the horseshoe crab to model trilobite behaviour

Greg and his colleagues have turned to the horseshoe crab as a living example of a shell-crushing marine arthropod that they can use to compare with the extinct trilobites.

'The trilobite fossils we looked at are flat, like pancakes, because they are preserved in mudstone where there has been a degree of compaction,' says Greg. 'This means we don't get their three-dimensional form.' It's difficult to reconstruct how the living animals would have looked and behaved, which is where the horseshoe crab comes in.

A horseshoe crab feeds on prey such as mussels by grasping and cracking it with a section of their legs. Once the prey is broken open, its other legs move the flesh forward like a conveyor belt towards the mouth, shearing and slicing the meat as it passes. 

'We used tools employed by engineers to model strain distribution on all the horseshoe crab’s appendages as they eat, and we can then use this information to extrapolate to trilobite fossils,' says Greg. 

The researchers also looked at a fossil of Sidneyia inexpectans, another predatory arthropod from the 508-million-year-old Burgess Shale. 'By looking at S. inexpectans gut contents we know that this big arthropod ate trilobites and other prey with calcium carbonate shells,' explains Greg. 

They studied what part of the legs of the horseshoe crab and S. inexpectans were most subject to microstrain and how strain and stress are distributed across the leg. 

A horseshoe crab against a white background

A horseshoe crab © The Trustees of the Natural History Museum, London

Looking for signs of strain on trilobite legs

The team looked specifically at two exceptionally well-preserved Cambrian trilobite species. The first, Redlichia rex, comes from the Emu Bay Shale in South Australia while the second, Olenoides serratus, is from the Burgess Shale.

Redlichia rex, where 'rex' means 'king', was a big animal growing up to 25 centimetres long and was probably as terrifying a trilobite as its name implies. Its appendages show microstrain patterns along their strong-spined edge similar to those seen in the legs of horseshoe crabs and S. inexpectans. This indicates that it too probably ate hard-shelled prey in a similar grasping and cracking technique. This finding is supported by the discovery of broken trilobite remains clustered as faecal waste which suggests that it may have had cannibalistic tendencies. 

The appendages of O. serratus, however, showed a different pattern. It had low microstrain values for its appendages, suggesting that this species wasn't well-adapted to crush hard prey and probably preferred to eat softer creatures, using its slender spines to pick up and manipulate flesh.

Four diagrams of the legs of four arthropods

Models of the front legs of the four arthropods in this study, the blue areas are the parts of the leg that grasped and possibly crushed prey. Top left, a horseshoe crab, Limulus polyphemus. Bottom left, Sidneyia inexpectans. Top right, Redlichia rex. Bottom right, Olenoides serratus 

The colour key to the diagram of arthropod body parts

An evolutionary relationship that's in the past

There is still uncertainty around the evolutionary relationship between trilobites and horseshoe crabs which makes direct comparisons difficult.

'The further you get away from something in terms of its evolutionary relationships, the riskier your assumptions for things like mechanical strain, muscle function and muscle identity are,' says Greg.

'We've used computer models and fleshed them out using a living arthropod, the horseshoe crab, so we can say something about their 3-D shape and volume. We have scaled the trilobite volume to the horseshoe crab make the mechanics work, so that's a caveat you have to acknowledge'.

Greg says the next step is to examine sea scorpion fossils held at the Museum. Sea scorpions are another group of extinct arthropods that are more closely related to horseshoe crabs than the trilobites. Scanning these fossils would help refine the assumptions we make about muscles and exoskeletal strain in extinct arthropods. 

'We have fabulous sea scorpions from Silurian deposits in the Baltic in the collections, and we are able to uncover their appendages by dissolving rock to recover the three-dimensional fossils,' says Greg. 

This will allow Greg and his colleagues to carry out similar work that they have done with the trilobites to see how the sea scorpions would also have fed.