The apparatus architecture and function of Promissum pulchrum Kovács-Endrödy (Conodonta, Upper Ordovician) and the prioniodontid plan

Abstract

More than 100 complete apparatuses of the conodont Promissum pulchrum have been collected as natural assemblages on bedding surfaces of the Soom Shale Member, Cedarberg Formation, South Africa. Recurrent patterns of flattening shown by the assemblages reflect the orientation of the conodont head on the sediment surface prior to decay and collapse of the soft tissues; we have used these patterns to model the three-dimensional architecture of the apparatus of Promissum . The apparatus is bilaterally symmetrical and comprises 19 elements: two Pa, two Pb, two Pc, two Pd, two M and nine S elements. The Pa, Pb and Pc elements are horizontally aligned as opposed pairs, with the Pd pair positioned below the Pb elements; the S elements form an oblique array below the P elements and a pair of M elements is at the anterior. Functional interpretation of the apparatus is difficult but it probably operated as an integrated set of movable mouthparts, the S element array grasping prey to be crushed by the P elements. The Promissum animal was probably macrophagous, feeding on soft-bodied members of the Soom Shale fauna. The apparatus of Promissum is more complex than that known or reconstructed for any other conodont taxon. Interpretation of evolutionary pathways in Ordovician conodonts with denticulate elements must await evaluation of other prioniodontid taxa in the light of the Promissum template.