Morphology and merom gradients in the Ordovician receptaculitid Fisherites reticulatus

The Middle to Late Ordovician receptaculitid Fisherites reticulatus (Owen 1844) is the most abundant and widespread receptaculitid known, and yet existing descriptions of its morphology do not reach the level of detail achieved in recent studies of other receptaculitid taxa. We present a redescription of the morphology of individual meroms and of the entire thallus, focusing on gradients in merom number and shape. We document positional information not with distance or angular measurements, but in terms of units of organization of the thallus itself, the number of merom sites from a reference pole. Although more labor intensive, this more accurately portrays thallus symmetry and yields a characterization of form that more directly reflects the developmental program of the organism.