INTERSEX SPECIMENS OF THE FOSSIL CRAB DAKOTICANCER OVERANUS FROM THE LATE CRETACEOUS PIERRE SHALE OF SOUTH DAKOTA, USA

Fossil decapod species vary in numbers of specimens and degree of preservation. A collection from the Late Cretaceous Pierre Shale in South Dakota has yielded a large number of specimens of Dakoticancer overanus from the vicinity of Mobridge, South Dakota that are very well preserved. Approximately eight hundred and forty specimens have been measured, and their sex has been determined. Eleven of them are recognized as having both male and female morphology. Upon statistical analysis, it is proposed that this collection, including the intersex individuals, came from a single population. Thus, the eleven intersex specimens do not belong to a different taxon.

Primary female sex characteristics consist of gonopores located on the coxae of the 3^rd pereiopod and spermatheca between sternal plates 7 and 8. Secondary female sex characteristics consist of gently sloping sternal plates to accommodate a wide pleon. Male primary sex characteristics consist of gonopores on the coxae of the 5^th pereiopod. Secondary sex characteristics consist of granulated sternal plates that are axially depressed three-quarters the distance from the distal edge into an ungranulated, steep slope to accommodate a narrow pleon. The intersex specimens mainly show female primary sex characteristics and secondary male sex characteristics. This form of intersex is seen in extant decapod species. However, 6 specimens show gonopores on the coxae of the 4^th pereiopod in addition to gonopores on the 3^rd coxae, which has not been reported in extant species. Two specimens exhibit secondary male characteristics (but with a missing pleon) and primary female sex characteristics (spermatheca noticeable). There are several explanations as to why intersex specimens are present within a population. One main reason is parasitic castration from acrothoracican barnacles. Parasitic castration is well documented in modern species and can metastasize in different ways but mainly by feminizing juvenile males. It is difficult to determine the exact cause of intersexuality in extinct fossil species because internal tissue is not available. The presence of gonopores on coxa 4 is especially difficult to understand because it has only been documented once before, by Bishop (1983) on a Dakoticancer overanus specimen from the same locality as this collection.