In an attempt to understand the evolution of great appendage arthropods, palaeontologists have uncovered an intact 520 million-year-old nervous system.

The ancient Chinese fossil underwent a CT scan when a team of palaeontologists, including the Natural History Museum's own, sought to understand the evolutionary transitions of a set of ancient early great appendage arthropods, which today include lobsters and millipedes. It's a field known as fossilised neuroanatomy, which Natural History Museum palaeontologist Greg Edgecombe says has been used here for the first time in this type of evolutionary study. The find is an important one, because the actual grouping of species into the great appendage arthropod category has been debated over the years.

The group is defined by the giant appendages protruding form its members' heads, but spans a wide variety of species. The anomalocaridid group of ancient marine creatures is accepted as part of the group. But how evolution could lead from here to such disparate-looking creatures as scorpions remained a mystery. 

The Chinese fossil provides the link. It is of a member of the alalcomenaeus genus, a genus which was originally classed as crustacean before studies of its giant appendage resulted in reclassification. The scan showed that the creature's brain, nerve cord and nervous tissue related to the eyes remained intact, and it was analysis of these parts that helped corroborate the team's assumption that ancient great appendage arthropods are in fact related to today's chelicerata, a group of arthropods that include horseshoe crabs, scorpions and spiders.

The fact that the nerve cord was intact all the way down eight of the fossil's 11 trunk segments was key in making the link, as it's the differentiation between segments and how they line up with antennae or limbs that helps differentiate between species and give clues as to how they evolved and which are closest related. This was made clear by analysing which part of the nervous system ran through which segment. 

"We had suspected from their external appearance that the great appendage arthropod was part of that group, and by identifying the nervous system, we now have greater evidence to show that indeed they are," said Edgecombe. "The Alalcomenaeus nervous system has features that only occur in chelicerates and closely resembles what we see in young horseshoe crabs. This supports the idea that horseshoe crabs and great appendage arthropods belong to the same group."

"It is very exciting to use new techniques to successfully reveal such a complete central nervous system from a 520-million-year old fossil, and in such detail," said Edgecombe's colleague Xiaoya Ma. "This research helps to open up further possibilities in the relatively new field that we call neuropalaeontology. It also shows how important museum specimens are in answering some of the big questions about evolution."

The study revealing the team's findings is published in the journal Nature this week.


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