A caramel coloured horseshoe crab is shown on a white background.

Modern horseshoe crabs look very similar to ancient fossils, but actually belong to a separate linage. However, it turns out that their brains are nearly identical ©The Trustees of the Natural History Museum, London

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310-million-year-old fossil shows how little horseshoe crab brains have changed

Horseshoe crabs are often called living fossils due to their superficial resemblance to fossils dating back hundreds of millions of years.

A new beautifully preserved fossil of a horseshoe crab has now revealed that their brains have hardly changed since at least the Carboniferous Period.  

Despite their name horseshoe crabs are not actually crabs, but are relatives of scorpions, spiders and mites that first evolved some 480 million years ago. During this period, known as the Ordovician, life in the oceans was rapidly diversify while animals were yet to even make it on to land.

While the hard exoskeleton of these creatures means that the remains of ancient horseshoe crabs are not exactly rare, as is often the case when it comes to fossils, preserved soft tissue is much harder to come by.   

A new fossil dating to 310 million year ago from the Mazon Creek deposit of Illinois is helping to change this, as it shows the internal organs of a horseshoe crab preserved in exquisite detail, including its brain.

Dr Greg Edgecombe is a Merit Researcher at the Museum who studies the evolutionary history of arthropods. Along with colleagues in Australia and the USA, he has helped to describe this extraordinary new fossil find in a paper published in the journal Geology.

'Among the hundreds of fossils of Euproops, the horseshoe crab we investigated, one stood out for showing soft anatomy, and we were able to match its form to what we see in the nervous system of the living horseshoe crab Limulus,' explains Greg.   

It is actually this incredible lack of change between the ancient and modern horseshoe crabs that allowed the researchers to identify this feature in the first place.

An ant trapped in golden-yellow amber.

Invertebrates trapped in amber are vital for our understanding of this group, but are limited by how old they can be ©The Trustees of the Natural History Museum, London

A skewed history of life

The process of fossilisation is more likely to occur for the parts of animal and plant bodies which are hard and robust, such as bones, teeth and shells. This means that there is a bias in the fossil record against many boneless invertebrates, as well as the softer parts of animals like their organs and flesh.

Plants and invertebrates which have been trapped in amber have been one way of peering back at these ancient organisms' history, as the sticky tree sap entombs the individuals in near perfect 3D, while the Burgess Shale formation from North America has revealed a whole wealth of early invertebrates preserved in exquisite detail. 

But the amber fossils typically date back as far as the Triassic, around 230 million years ago, while the Burgess Shale fossils reveal what was happening some 508 million years ago when animal life was first starting to diverge in major ways.

This creates a huge gap in our knowledge of invertebrate evolutionary history of some 300 million years.

'So, even though we can see arthropod brains from a wide timespan, there's actually really big gaps in between,' co-author John Paterson, a palaeontologist at the University of New England told Cosmos Magazine. 'What we've done with our new fossil discovery is fill one of those gaps.'

This new fossil also shows how the soft tissue of invertebrates can be preserved in an entirely new way.

It formed after an unfortunate horseshoe crab living in a shallow sea in what is now Illinois was encased in the mineral siderite. Not only did this mineral cover the crab externally, but also wrapped itself around many of the soft internal organs before they could decompose.

The fossiled horseshoe crab show that white brain naturally highlighed against the brown-grey rock.

The brain of the ancient horseshoe crab stands out from the rest of the fossil due to the way in which it was preserved ©Bicknell et al. 2021

When the soft tissue did eventually rot away, the internal cast of these features was then later filled with a clay mineral called kaolinite. This second mineral is lighter in colour, meaning that hundreds of millions of years later the horseshoe crab's brain was effectively highlighted to those who knew what they were looking at.

Ancient brain

The brain structure of the ancient crab is almost identical to that of living species. In fact, it is this extraordinary similarity that meant the researchers could be confident that what they were looking at was indeed the brain and other parts of the nervous system.

The branching structure shows where nerves split off to link up with the eyes and legs, as well as the central hole in which the feeding tube, or oesophagus, would have passed through.

'Now we know that both of two main lineages of horseshoe crabs – the wholly extinct group that our fossil belongs to and the group that includes the four living species ­– have the same basic neuroanatomy,' adds Greg.  

Despite horseshoe crabs often being referred to as living fossils, they have changed quite a bit in their external anatomy since they first evolved. But it seems that once the group of invertebrates had ended up with a brain structure that worked, there was little need to change it over the next 310 million years.