Rare fossils preserving the brains of creatures living more than half a billion years ago shed new light on the evolution of arthropods such as insects and crustaceans.

The fossils, belonging to an arthropod known as Leanchoilia, confirm the presence — predicted by earlier studies in genetics and developmental biology of insect and spider embryos — of an extreme frontal domain of the brain that is not segmented and is invisible in modern adult arthropods. Despite being invisible, this frontal domain gives rise to several crucial neural centers in the adult arthropod brain, including stem cells that eventually provide centers involved in decision-making and memory. This frontal domain was hypothesized to be distinct from the forebrain, midbrain and hindbrain seen in living arthropods, and it was given the name prosocerebrum, with «proso» meaning «front.»

Described in a paper published today in the journal Current Biology, the fossils provide the first evidence of the existence of this discrete prosocerebral brain region, which has a legacy that shows up during the embryonic development of modern arthropods, according to paper lead author Nicholas Strausfeld, a Regents Professor of Neuroscience at the University of Arizona.

«The extraordinary fossils we describe are unlike anything that has been seen before,» Strausfeld said. «Two nervous systems, already unique because they are identically preserved, show that half a billion years ago this most anterior brain region was present and structurally distinct before the evolutionary appearance of the three segmental ganglia that denote the fore-, mid- and hindbrain.»

The term ganglion refers to a system of networks forming a nerve center that occurs in each segment of the nervous system of an arthropod. In living arthropods, the three ganglia that mark the three-part brain condensed together to form a solid mass, obscuring their evolutionary origin as segmented structures.

Fossils of Brain Tissue are Extremely Rare

Discovered in deposits of the Kaili formation — a geological formation in the Guizhou province of southwest China — the fossilized remains of Leanchoilia date back to the Cambrian period, about 508 million years ago. The Kaili fossils occur in sedimentary rock that has high concentrations of iron, the presence of which probably helped preserve soft tissue, which subsequently was replaced by carbon deposits.

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Materials provided by University of Arizona. Original written by Daniel Stolte. Note: Content may be edited for style and length.