ABSTRACT
The higher phylogeny and classification of the Ostracoda have always been topics of contention and this has hampered inclusion of this group in general analyses of crustacean or arthropod phylogeny. Also, the inclusion of hard part characters in earlier attempts to investigate ostracod phylogeny has introduced a large degree of homoplasy in the resulting trees. Here, we present an analysis of the phylogeny of the extant ostracod subfamilies, using nearly exclusively morphological soft part characters. The homologies of these limb characters in a crustacean context are extensively discussed. The resulting maximum parsimony and distance analyses show a good resolution of the phylogeny of the myodocope subfamilies, well-supported by bootstrap values, but in neither of the two analyses are the podocope phylogenies supported by bootstrap statistics. We test the hypothesis that this discrepancy in resolution between the two subclasses is due to a difference in evolutionary tempo (punctuated in Podocopa, gradual in Myodocopa) by comparing mean distances, average branch lengths and by applying relative rate tests. To the best of our knowledge, this is the first time ever that Li’s relative rate tests have been applied to a morphological data matrix. None of the tests yields significant differences in past evolutionary tempo within the two groups. However, there is a significant difference in mean relative distance between the lineages leading up to the two subclasses. Puncioidea, here considered to constitute Recent representatives of the otherwise Palaeozoic Palaeocopida, have significantly different evolutionary rates from other groups, but cluster within the Podocopida. The slowdown in evolution indicates that this lineage might have experienced, and might still be experiencing, period(s) of morphological stasis.
