Arthropods have traditionally been divided into four classes: Trilobita, Chelicerata, Crustacea, and Uniramia. Most living and post-Cambrian fossil arthropods fall into one of these four classes. However, a number of arthropod fossils from the Cambrian Period have been described which do not obviously belong in any of these classes. Many of these arthropods have been found in the famous Middle Cambrian Burgess Shale, in the Canadian Rocky Mountains of British Columbia. Other "weird wonder" arthropods have been found in China, Australia, Russia, Poland, and the USA. (See Gould, 1989, for an overview.) Recent work (Wills et al. 1994) has shown that these arthropods are mostly in "stem clades"- that is, they are extinct relatives of living arthropods that lack some of the characters that typify groups with living members. The cladogram we present here is thus something of an oversimplification.
Debate as to which living phyla are the closest relatives of arthropods continues to rage. Arthropods were once considered to be close relatives of annelids, or segmented worms, but this is no longer universally accepted (Eernisse et al. 1992), and so far, no consensus has emerged from molecular studies. However, on both morphological and molecular grounds it seems that two groups that are sometimes given phylum rank, the Onychophora and the Tardigrada, are the closest living relatives of the arthropods. Some Vendian and Cambrian organisms that are not arthropods may have been even closer to living arthropods, although finding their precise position is difficult. For the purposes of this exhibit, these are classified as the "vendiamorphs" and the "anomalocaridiids." The cladogram used in this virtual exhibit is a simplification of that in Wills et al. (1994), with some additions based on Eernisse et al. (1992) and Waggoner (1996). It should be emphasized that arthropod phylogeny is often hotly debated, and a number of alternative hypotheses have been proposed.
Eernisse, D. J., J. S. Albert, and F. E. Anderson. 1992. Annelida and Arthropoda are not sister taxa: a phylogenetic analysis of spiralian metazoan morphology. Systematic Biology 41(3): 305-330.
Gould, S. J. 1989. Wonderful Life. W. W. Norton, New York.
Waggoner, B.M. 1996. Cladistic hypotheses for the relationships of arthropods and problematic taxa. Systematic Biology 42(2): in press.
Wills, M. A., D. E. G. Briggs, and R. A. Fortey. 1994. Disparity as an evolutionary index: a comparison of Cambrian and Recent arthropods. Paleobiology 20(2): 93-130.
Care to do more searching in the huge literature on arthropod phylogeny? This bibliography compiled by R. C. Brusca is a good place to start for the literature as of October 1996.