Cambrian Crustaceans and the Evolution of Arthropod Feeding Complexity  

In the modern world, crustaceans are taxonomically and ecologically dominant among aquatic arthropods, while their terrestrial descendants, the insects, account for the vast majority of animal diversity.  Insights into the origins of this success are provided by the opening of a new window onto early crustacean evolution.  An exceptionally preserved assemblage of organic-walled arthropod cuticle from the early Cambrian Mount Cap Formation (Northwest Territories, Canada) contains a diversity of sophisticated feeding devices, including mandibular grinding surfaces, co-planar filter plates, and several styles of substrate-scraping machinery.  Equivalent structures are known from various extant crustaceans and other mandibulate arthropods, where they represent adaptations for the precise handling of fine particulate food.  However, comparable structures are apparently absent from the millimetre-scale individuals preserved in Orsten-type assemblages, which have previously provided the only reliable record of crustaceans in the Cambrian.  I compare the appendage characters preserved among the Mount Cap and Orsten-type fossils in terms of morphology and functional significance, and discuss to what extent the differences are influenced by phylogeny, ontogeny, ecology, and taphonomy.  The implications for arthropod relationships and feeding complexity evolution are considered in the context of longer term patterns in the arthropod fossil record.