Five hundred million years ago, a larva this small swam in the oceans of the Cambrian Period, its tiny form supporting a brain, digestive organs, and nerve cells that would resound through the lineage of arthropods. Now, this larva, smaller than a fingernail, has appeared from stone, and for the first time in history, it is possible to examine its ancient neural system due to advanced imaging science.
The fossil, discovered in the Yu’anshan Formation in China, represents an early arthropod ancestor and dates from the era when forms of life were diversifying at a rapid rate among animals. The significance of this fossil, however, is the preservation of the soft tissues, something that rarely, if ever, happens. Typically, only the hard tissues, such as exoskeletons, can be preserved, as the brain and the organs have the tendency to decay. The “almost perfect preservation” was, however, the work of nature, as described by co-author Katherine Dobson.
The unearthing of these secrets needed technology that is no less impressive than the fossil itself. Scientists used synchrotron X-ray tomographic analysis, which is a non-invasive technique involving the use of powerful X-ray beams that can traverse a rocky sample and build a virtual reconstruction of the sample’s inner structures in three dimensions. Such techniques, also known as virtual palaeohistology, have been developed over the years in the Japan SPring-8 synchrotron radiation research facility’s beamlines BL20B2 and BL28B2, where micrometer-resolution analysis is possible over a wide image area without damaging the sample, which might be irreplaceably valuable. For this larva fossil, the analysis produced images of the entire brain, the circulation system, and nerve tissue up to the limbs and eyeballs.
One of the most intriguing discoveries was the presence of the protocerebrum, a brain area responsible for the integration of sensory information and movement in living arthropods. The appearance of this brain area in a so ancient organism is a strong indicator of the neural evolution in these organisms. Similar analyses in other arthropod organisms like Leanchoilia, from the Cambrian period, have revealed even more ancient brain regions like the prosocerebrum, which is conserved in the form of a carbon film and is associated with ancient eyes.
Such detailed neural preservation is a taphonomic achievement that is rarely possible, Alex Hastings explained. Soft-bodied fossils commonly fossilize when quickly buried in low-oxygen conditions, sometimes with the aid of sediments chock-full of minerals. In an environment rich in iron, as it is at Kaili and Chengjiang, iron-rich carbon films may substitute for the original soft-tissue morphology, which would explain the preservation of the ventral nerve cords and the bead-string ganglia and spaghetti nerve fibers in the fuxianhuiids, Hastings continued in his description of these remarkable fossils.
In an evolutionary sense, the larvae’s body connects the major transitions in arthropod evolution. The early stem groups, such as the gilled lobopods and radiodonts, tended towards simpler nervous systems, whereas later crown groups more advanced arthropods showed greater diversification in their sensory and motor systems. The emergence of the protocerebrum in this case argues for more complex nervous systems developing well before current models suggest.
The imaging solution further highlights the importance of technology in reshaping paleobiology. Conventionally, fossil preparation has been potentially harmful to fossilized remains, particularly those that contain fragile body parts. Synchrotron tomography, in this case, overcomes this limitation because thousands of high-definition images are created digitally in three-dimensional volumes, so that any organ system could be viewed from any perspective, a detail that would be invisible in a compressed fossil like those found in Burgess shale sites because their bodies were flattened. According to co-lead author and paleontologist Emma Long, three-dimensional preservation offers a framework for understanding flattened fossils that in turn allows us to better grasp their complex organ systems.
For biologists who study the evolution of life, this discovery is not merely an interesting aside it’s instead a significant data point that informs the reasoning behind the success of arthropods. The ability to have segmented bodies, jointed limbs, and adaptability through modularity has enabled arthropods to occupy all but the most extreme environments on the planet. The origins of the protocerebrum suggest that advanced processing of the senses was part of the recipe for success all along.
Ultimately, this fossil, no larger than a fingernail, is at once a relic and a revelation. It holds within its calcified tissues the history of the evolution of the nervous system through unpredictable biochemistries and with an engineer’s keenness of detail. It is through the small brain that the broader narrative of arthropod evolution comes into focus: from the Cambrian oceans to the present-day insects and lobsters that live abundant lives.
