Morphology and arrangement of meromes in Ischadites dixonensis, an Ordovician receptaculitid

Ischadites dixonensis (Miller and Gurley, 1896), from the Ordovician Galena Group of Illinois, is the oldest described North American ischaditid. The holotype is exceptionally well preserved and provides information on merome head morphology, articulation, and arrangement on the surface of the thallus. Meromes occur in whorls and form an equal number of dextral and sinistral parastichies. Each parastichy begins either at one of the heads surrounding the basal pole or at a triangulum, and extends to the margin of a clearly demarcated apical lacuna. The unusually complete morphological characterization that is now available for I. dixonensis will play an important role in discussions of receptaculitid morphogenesis and orientation.