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Brain fossils break new ground in palaeontology
Museum scientists use cutting-edge imaging techniques to unravel the secrets of 517-million-year-old animals, uncovering new evidence for brain fossilisation.
Using sophisticated equipment in the Museum's Imaging and Analysis Centre, the researchers found that the brains and nervous systems of seven specimens of Fuxianhuia protensa (an extinct, sea-dwelling arthropod) have been fossilised as thin layers of carbon for 517 million years.
It is one of very few cases in which multiple fossil specimens of the same Cambrian-age species have been found with brain and nerve tissue intact.
The research was carried out by Dr Greg Edgecombe and Dr Xiaoya Ma of the Natural History Museum and Professor Nicholas Strausfeld of the University of Arizona, who are leading exponents of neuropalaeontology - a budding field focusing on ancient fossil brains.
The F. protensa specimens reveal that ancient arthropods developed a brain and central nervous system similar to today's lobsters and crabs as early as 517 million years ago.
'Until we began to document fossilised brains, nerve cords and optic nerves from the Cambrian, neuroanatomists and palaeontologists did their studies in parallel but didn't really work with each other. They were dealing with completely different kinds of data,' says Dr Edgecombe.
'Now we have the possibility of tracing the early evolution of animals using information from the nervous systems of both fossil and living organisms.'
Believe it or not, the squiggly traces of black in this picture are actually areas of the F. protensa fossil's preserved neural tissue, scanning electron microscope images show
A rare find
Until recently, the fossilisation of neural tissue was believed to be highly unlikely or near impossible.
Even in specimens where complete exoskeletons have been preserved, examples of fossilised brains or nerve fibres are rare, and multiple lab experiments have shown that such tissue decays rapidly.
But researchers interested in combining palaeontology with the study of brains have been chipping away at this assumption.
In 2012, a team including Dr Edgecombe, Dr Ma and Prof Strausfeld released a study in the journal Nature describing a remarkably complete F. protensa specimen found in Chengjiang, southern China.
The arthropod's central nervous system, including the brain, was relatively well-preserved, and the specimen went down as the earliest known example of a fossilised brain.
Other palaeontologists were sceptical, however. Some suspected that the Chengjiang specimen would prove to be a one-off, or that the apparent neural tissue might actually represent a different anatomical system.
The researchers now have further fossil evidence for the fossilised brain tissue. They created this tracing of the preserved neural system of F. protensa by merging information from three different fossil specimens.
Squashed cockroach brains
But in two papers released this year, the team addressed these doubts directly.
The most recent, published in November 2015 in the Philosophical Transactions of the Royal Society B, included a couple of experiments supporting the idea that soft tissue can be preserved soon after death.
Prof Strausfeld used living specimens of Nereis virens (a marine sandworm that burrows in wet sand and mud) to show that nerve cords retain integrity even when entombed in clay sediment and compacted using brass weights.
Another experiment involved squashed cockroach brains (pictured below). The brains were sandwiched between layers of clay and left to dry for 10 days, demonstrating that compaction doesn't necessarily alter the order in which the different segments of the brain are arranged.
Together, these findings support the idea that brain and nerve tissue can be preserved if entombed in sediment soon after death, and that even wafer-thin traces of the brain or nerve cord can provide reliable data about the specimen.
The different stages of the cockroach brain-squashing experiment. The arrows on pictures 'a' and 'd' indicate that the squashing didn't radically alter the arrangement of the brain segments. © Phil. Trans. R. Soc B
Fossil pathways
In another study, published in Current Biology in October, the team detailed seven more Chengjiang F. protensa fossils with intact brains, refuting claims that the 2012 specimen would prove an exception.
Using the Museum's sophisticated scanning electron microscopy equipment in the Imaging and Analysis Centre, they confirmed the original finding that F. protensa had a relatively complex brain structure. It consisted of three parts, which sent nerves to the eyes, antennae, and a pair of post-antennal appendages.
Crucially, with the help of Museum electron microscopist Tomasz Goral, they were also able to work out how the F. protensa brain and nerve tissue were preserved.
This elemental analysis shows F. protensa neural tissue preserved as carbon in pink, and the iron layers in green. The analysis was carried out at the Museum using energy-dispersive X-ray spectroscopy.
Geochemical analysis revealed that the traces of brain were fossilised as thin films of carbon, covered in layers of pyrite (iron sulphide) that varied in thickness between the specimens.
The researchers say the arthropods were probably buried by a marine mudslide very soon after death, protecting the carcasses from potential scavengers. Such a rapid entombment in sediment would also have ensured a low level of oxygen, shielding the soft tissue from decay due to harmful bacteria.
'Evolutionary trees based on DNA sequences of living animals, and new finds of exceptionally preserved fossils from the Cambrian, both predict that arthropods diversified rapidly in the "Cambrian explosion", between 541 and around 515 million years ago,' says Dr Edgecombe.
'Now we have direct evidence that the main lineages of arthropods had already evolved some of the diagnostic characters of their nervous systems within about 20 million years of the first traces of arthropods in the fossil record.'