The apparatus composition and architecture of Erismodus quadridactylus and the implications for element homology in prioniodinin conodonts

The apparatus composition and architecture of prioniodinin conodonts is poorly understood, largely because few prioniodinin taxa are represented by articulated ‘natural assemblages’, but also due to the highly variable gradational morphology of their constituent elements that makes apparatus reconstruction problematic. We describe a natural assemblage of Erismodus quadridactylus (Stauffer, 1935), a basal prioniodinin conodont, from the Sandbian (Late Ordovician) of North Dakota, USA, is described here. The assemblage demonstrates that the apparatus architecture of Erismodus is similar to those of Late Palaeozoic prioniodinins namely, Kladognathus Rexroad and Hibbardella Bassler, but also has similarities with ozarkodinin apparatuses. In addition, there is evidence to suggest that E. quadridactylus also shared topological similarities to ‘prioniodontid’ architecture, with respect to the position of its inferred P elements. The apparatus composition and architecture presented here indicate that, at least with respect to the M-S array, an ‘ozarkodinin-type’ bauplan is likely more widely representative across ozarkodinids. The assemblage demonstrates that element morphotypes traditionally considered to lie within the S array are M elements, whereas others traditionally interpreted as P elements are found in the S array. These observations are used as a basis for revising the prioniodinin apparatus bauplan and for refining concepts of element homology among other prioniodinin conodonts and their closest relatives.