Fossil Arthropods, Early Brains: Inferring Cerebral Complexity from Preserved Sensilla  

Apart from a few remarkable fossils from Chengjiang and the Burgess Shale, no others suggest early organization of ganglia and nerve cords.  However, knowledge about the degree to which Lower Cambrian arthropods were equipped with brains is crucial to the recent field of neurophylogenetics, which uses a palette of neuronal characters for reconstructing arthropod relationships. 

The use of brain structures for inferring affinities within the Arthropoda has an honourable history.  Originally employed by Nils Holmgren in 1916, it was the first analysis of its kind to insist on a sister group relationship between the malacostracan crustaceans and insects.  A more refined approach, using parsimony analysis of some hundred or so brain characters from species representing all major arthropod groups, supports Holmgren's original contention.  However, a malacostracan-like ancestor to the Insecta is strongly disputed by molecular phylogenetics, which proposes an entomostracan-like ancestry. 

Observations of traces of cerebral ganglia in the Burgess Shale taxon Waptia fieldensis suggest a substantial brain supplied by short-stalked compound eyes and by paired uniramous head appendages equipped with two kinds of sensilla: short brush like extensions, reminiscent of aesthetascs, and longer setae, reminiscent of either mechanosensory structures or mixed mechano- and chemosensory sensilla found on present-day malcostracan antennules.  It is of considerable interest whether the crustaceomorph fauna show a loss or reduction of the second pair of head appendages – a feature typifying the insects and an unrelated group, the Myriapoda – as early as the mid-Cambrian.  Waptia may suggest such a reduction.  Further, the sensory structures on its carapace, abdomen and telson suggest that its sensory systems were as elaborate as those of certain extant malacostracans, particularly basal groups like the phyllocarids.  The degree to which central ganglia might have been elaborated, even in the case of trilobites and other arthropods that have gone extinct can, to some degree, be inferred from sensory and central innervation patterns of extant species.