310-Million Year Old Fossil Sheds Light on Horseshoe Crab Brains

The word fossil is synonymous with harder structures like bones or shells. It is extremely rare to find fossilized remains of softer tissue like the brain or muscles since they are prone to rapid decay. The unavailability of softer tissue makes it difficult for scientists to identify changes due to evolution. A recent paper analyzed a rare 310-million-year-old brain fossil of a horseshoe crab to make new revelations about how fossils are formed while also studying their evolution.

Soft tissue remains are usually found in amber inclusions, where an entire organism is exceptionally preserved by a sheer stroke of luck. But, they aren't the only source. Burgess Shale-type deposits, named after the fossil-bearing deposit, in British Columbia in Canada, are sources of soft-tissue fossils that are far older than amber inclusions. Scientists estimate them to be around 500 million years ago, the era termed as the Cambrian Period.

Researchers Russel Bicknell and John Paterson at The University of New England in Australia, along with a team of researchers from the UK and US, analyzed a horseshoe crab fossil that was found in the Mazon Creek in Illinois, USA. Fossil deposits from Mazon Creek have been found to be encased in mineral deposits of iron carbonate, called siderites.

During the fossilization process, as the siderites began encasing this horseshoe crab, the softer tissue-like brain began to decay. As the decay continued, the cavity left out by the decay was occupied by a clay mineral, kaolinite, which created a mold of the brain. Since siderites are darker in color while kaolinite is white, it made it easier to spot the different mineral deposits in the fossilization process. This demonstration of brain (or rather, brain mold) preservation is the first of its kind. Scientists were not aware of such preservation before this discovery. 

The ability to capture the brain morphology in the fossil, Euproops danae, allowed the researchers to compare it to the modern-day horseshoe crab. Euproops are now extinct having evolved over the millions of years, into their modern-day counterparts.  

"The fossil’s central nervous system is closely comparable to that of living horseshoe crabs and match up in their arrangement of nerves to the eyes and appendages. It also shows the same central opening for the esophagus to pass through," said John Paterson, Professor of Earth Sciences at UNE. "This is quite remarkable, given the substantial morphological and ecological diversification that has taken place in the group over the intervening 310 million years."

Soft-tissue fossils provide a glimpse into the life and life-forms that existed millions of years ago and improve our understanding of evolution.