At first glance, the term ‘neuropaleontology’ may seem like an oxymoron. The neuro- prefix means something related to the nervous system, and paleontology is the scientific study of what fossilized animal and plant remains tell us about the evolution of life on Earth. Brain tissue is soft and wet, however. It usually begins to decompose minutes after death, and rarely, if ever, leaves any trace in the fossil record.

Or so we’ve always thought. The recent discovery of preserved brain tissue in a small shrimp-like creature that lived some 520 million years ago challenges this conventional wisdom, however. It provides the most convincing evidence yet that brains can indeed fossilize, as well as valuable insights into how complex brains evolved, and how the arthropods are related to one another.

In 2012, neuroanatomist Nicholas Strausfeld and colleagues at the Natural History Museum in London described the fossilized remains of Fuxianhuia protensa, an ancestral arthropod that lived during the Cambrian period. This specimen was excavated from the Chengjiang Shales, a major fossil site in the Yunnan Province of southwestern China, and has an exceptionally well preserved brain and optic lobes, making it the oldest known fossilized nervous tissue.

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Paleontologists questioned the findings, however, arguing that they were experimental artifacts, or that the tissue was fossilized under very rare and special circumstances that likely made it a one-off event. But Strausfeld and his colleagues now report that seven other recently discovered Fuxianhuia specimens also have remnants of nervous tissue, and describe a set of experiments that recreate the conditions in which it may have been preserved.

The researchers examined the seven new specimens with a scanning electron microscope, revealing a common neural architecture which is preserved, to a greater or lesser extent, in all of them – three distinct segments of brain tissue, plus the optic lobes and optic tracts, which led from the eyestalks to the front of the brain. Chemical analyses of the specimens also revealed that the nervous tissue is preserved as a flat carbon film, sometimes overlaid with pyrite crystals.

Facebook Twitter Pinterest Schematic reconstruction of Fuxianhuia protensa, showing blood vessels in red, nervous tissue in blue, and gut in green. Image: Nicholas Strausfeld

They argue that in order for brain tissue to be fossilized it must have been buried rapidly in underwater mudslides, in the same way as other exceptionally well preserved fossils from similar Cambrian deposits: rapid burial in fine-grained sediments, followed by more deposition, would have sealed off the fossil bed from scavengers. Low oxygen levels would have suppressed any microbial activity and prevented the entombed carcasses from decaying.

In a second stage of the process, pressure from the overlying mud would compress the tissue and squeeze water out of it while allowing it to maintain its structural integrity. A prerequisite for this is that the tissue would have to be extremely dense to withstand the pressure – and we know, from comparative studies, that arthropods have the densest nervous tissue of all living creatures.

Although it’s impossible to reproduce the fossilization process exactly, the researchers ran a series of experiments designed to mimic the conditions under which they believe it took place. First, they placed live sandworms in seawater, covered them with slurries of clay, and left them to solidify. A month or more later, they cracked the clay open with a chisel, to find that the worms’ nervous tissue had remained intact. Then they buried live cockroach brains in wet clay and, upon examining them 10 days later, found them flattened into a paper-thin sheet, but similarly preserved.

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The neuroanatomy of Fuxianhuia provides new clues into the evolutionary history of the arthropods, a huge and diverse group of organisms that includes millions of arachnid, insect, and crustacean species. Some researchers believe that insects evolved from an ancient arthropod with a complex brain, which also gave rise to the crustaceans, but most point to a clam-like creature with simpler brain anatomy as their ancestor, such that their brains became increasingly complex during the course of evolution.

When Strausfeld and his colleagues first examined Fuxianhuia, however, they were surprised to discover that it had a complex brain consisting of three fused segments with a rich supply of blood vessels. This brain organization closely resembles that of extant insects, suggesting that the brains of certain arthropod species, such as the brine shrimp, regressed to less complex nervous systems as they evolved. The human brain is also partitioned into segments, most prominently during embryonic development, so it seems the basic ground plan for all nervous systems was laid down more than half a billion years ago, and has remained unchanged ever since.

References

Edgecombe, G. D., et al. (2015). Unlocking the early fossil record of the arthropod central nervous system. Phil. Trans. R. Soc. B. DOI: 10.1098/rstb.2015.0038 [Full text]

Ma, X., et al. (2015). Preservational Pathways of Corresponding Brains of a Cambrian Euarthropod. Curr. Biol. DOI: 10.1016/j.cub.2015.09.063 [Full text]