Some observations on the sensory organization of the crustaceomorph Waptia fieldensis Walcott

Observations of traces interpreted as cerebral ganglia in the Burgess Shale taxon Waptia fieldensis Walcott, suggest a substantial brain supplied by turreted compound eyes and a pair of uniramous head appendages equipped with three kinds of sensilla: short brush like extensions, reminiscent of asthetascs, and two species of longer setae, reminiscent of either mechanosensory structures or mixed mechano- and chemosensory sensilla typical of extant malacostracan crustaceans. Of considerable interest is the question whether already in the mid-Cambrian, stem crustaceomorph arthropods show a loss or reduction of the second pair of head appendages, a feature typifying the Insecta and the distantly related Myriapoda. W. fieldensis is suggestive of such reduction. Further, the sensory structures on its legs and telson suggest that sensory systems of this species were as elaborate as those of certain extant malacostracans, such as the phyllo-carids. The degree to which central ganglia might have been elaborated in arthropods that have gone extinct can, to some degree, be inferred from sensory distributions in fossils and comparisons with extant taxa.