The origin and early evolution of chelicerate arthropods

Arthropods, the most diverse group of Metazoa today (and in geological history), provide critical data for analysing processes and patterns in evolution. Although the overall pattern of arthropod evolution is wellknown, detailed relationships within the m ajor taxa are far from resolved. The most important such group in the Palaeozoic is the Arachnomorpha, which includes the trilobites and chelicerates, and is represented in a number of exceptionally preserved fossil deposits. A nu mber of newly discovered examples remain to be described and, together with reinvestigation of some incompletely known forms, will allow a new assessment of early arthropod evolution. The best known and most diverse (c. 300 species) Palaeozoic chelicerate s are the eurypterids (sea scorpions). Many Silurian and Devonian examples are known, but earlier poorly understood remains from the Ordovician require urgent review. The affinities of the chasmataspids have long been disputed, but new material suggests t hat they were related to eurypterids. Similarity of body form, reproductory and respiratory palaeobiology supports a close relationship between scorpions and eurypterids but the implications of this for relationships among the rest of the arachnomorphs re main to be explored. Newly discovered Palaeozoic arthropods will allow a number of important questions about chelicerate relationships to be resolved.