Sexually dimorphic sea urchins: identifying the sexes in Pentechinus mirabilis

The sexes of most sea urchin species are externally indistinguishable from one another. Where gross sexual dimorphism does exist, it is invariably associated with nonplanktotrophy. In the most extreme examples, female morphology has become modified to create depressions on the test where offspring may be sheltered throughout development. Since a number of the features used for taxonomic classification are lost on fossilization, it can be difficult to recognize fossil individuals as the dimorphic males and females of the same species. In addition, diagnoses of dimorphic taxa commonly concentrate on the striking female traits, hindering identification of the simpler male form. Here, we present an approach based on morphology and crystallographic determination of developmental mode that allows the recognition of the males of marsupiate echinoid taxa. Males of the brooding temnopleurid Pentechinus mirabilis from the Oligocene of Victoria, Australia are described for the first time.