Rare fossils preserving the brains of creatures living more than half a billion years ago shed new light on the evolution of arthropods.
Exquisitely preserved fossils left behind by creatures living more than half a billion years ago reveal in great detail identical structures that researchers have long hypothesized must have contributed to the archetypal brain that has been inherited by all arthropods. Arthropods are the most diverse and species-rich taxonomic group of animals and include insects, crustaceans, spiders and scorpions, as well as other, less familiar lineages such as millipedes and centipedes.
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 on August 19, 2021, 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.
Reconstruction of the brain and segmental nervous system showing the forward eye pair extending from the prosocerebrum, the sideward eyes from the protocerebrum, and four segmental ganglia. Farther back, within the trunk, each segment is equipped with a pair of ganglia that together are linked by a nerve cord extending the length of the body. The blue shaded areas indicate preserved gut tissue. Scale bar represents 2 millimeters. Credit: Nicholas Strausfeld
“The Kaili fossils open a window for us to glimpse the body plan evolution of animals that lived more than half a billion years ago,” said the paper’s first author, Tian Lan of the Guizhou Research Center for Palaeobiology at Guizhou University in China. “For the first time, we now know that arthropod fossils of the Kaili formation have the potential to preserve neural tissue that show us the primitive brain of the early stem arthropod existing at the dawn of the animal world.”
“Nervous systems, as other soft tissues, are difficult to fossilize,” added co-author Pedro Martinez of the Universitat de Barcelona and Institut Catalá in Barcelona, Spain. “This makes the study of the early evolution of neural systems a challenging task.”
The fossils also shed new light on the evolutionary origin of two separate visual systems in arthropod evolution: pairs of front-facing eyes or sideward looking eyes, the descendants of which are still present in species living today.