|A remarkable fossil of Chengjiangocaris where the nervous system is preserved|
This is particularly useful as their primitive nature means that the anatomy of their exoskeletons cannot always provide a conclusive means of identifying their place in the tree of life. Soft part anatomy, however, can be used to find previously unknown connections between fossil and extant groups. In addition to classification, the preservation of soft tissues has allowed palaeontologists to speculate in greater detail about the lifestyles of Cambrian organisms.
The preservation of the brain in Fuxianhuia protensa shows that these early arthropods were neurologically complex and capable of processing large quantities of sensory data. Fuxianhuia is not the only species to have had its nervous system preserved. Recently discovered specimens of Chengjiangocaris provide incredible new insight into the neuroanatomy of early arthropods.
'This is a unique glimpse into what the ancestral nervous system looked like,' said Dr Javier Ortega-Hernández from the University of Cambridge. 'It’s the most complete example of a central nervous system from the Cambrian period.'
|Top: a specimen of Chengjiangocaris. Bottom a magnified fluorescence |
microscopy image of the ventral nerve chord and its ganglia
'These delicate fibres displayed a highly regular distribution pattern, and so we wanted to figure out if they were made of the same material as the ganglia that form the nerve cord,' said Ortega-Hernández. 'Using fluorescence microscopy, we confirmed that the fibres were in fact individual nerves, fossilised as carbon films, offering an unprecedented level of detail.'
|Panarthopod phylogeny along with the priapulid outgroup|
Within the group panarthropoda ('all arthropods') the onychophorans are the most basal (primitive). These are the little known velvet worms. True arthropods are the euarthropoda which sit at the top of the phylogeny. In between are the tardigrades or the more commonly known, water bears. The closest relatives to the panarthropods are the priapulids, a group of worms.
Chengjiangocaris, as a stem group arthropod, possessed a nervous system similar to that of the priapulids and onlychophorans. This is characterised by a central nerve chord with other nerves branching off at regular intervals. Tardigrades and euarthropods, however, possess more streamlined nervous systems where many of these branches have been lost. This suggests that simplification has been a key trend in arthropod evolution. What is more, this discovery shows the importance of the fossil record in evolutionary biology. Molecular studies can demonstrate phylogenies and divergence times, but only the fossil record can show the characteristics of the stem groups of the tree of life.
'The more of these fossils we find, the more we will be able to understand how the nervous system and how early animals evolved,' said Ortega-Hernández.