Tuesday, 8 March 2016

Exceptional Fossils Provide Insight Into Stem Group Arthropod Neuroanatomy

A remarkable fossil of Chengjiangocaris where the nervous system is preserved
The discovery of the Maotianshan Shales in China has provided palaeontologists with a wealth of specimens of Cambrian organisms which are preserved in perfect detail. Combined with advanced new imaging techniques which can be used to map the chemical composition of fossils, they have given incredible new insight into the soft body anatomy of these long extinct organisms.

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
The fossils are spectacular, preserving great detail of the external anatomy as a pink tinge on a paler yellow shale, often with the internal anatomy of the gut. The element of the nervous system readily apparent was a long nerve chord running along the length of the body, replete with bead-like ganglia. Each ganglia was associated with the control of a pair of limbs. Closer examination showed that the ganglia were composed of many fibres, each five thousands of a millimetre in length.

'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
What is particularly interesting is how the neuroanatomy of Chengjiangocaris compares to other arthropod groups. The phylogeny of arthropods and their relatives has recently changed.

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.