The oral apparatus composition of the Early Carboniferous elictognathid conodont ‘ Siphonodella ’

Abstract

Conodont P1 elements of ‘Siphonodella’ are the most important guide fossils for the Tournaisian and topmost Famennian. Hypotheses on the origin and evolution of the elictognathid (‘Siphonodella’) clade are based exclusively on the morphology of one pair of elements in the 15 element apparatus, because of difficulties with its reconstruction. An unusually rich sample taken from the Kowala Quarry in the Holy Cross Mountains, dominated by the core elictognathid species ‘S.’ cooperi, enables corroboration of the interpretation by Sandberg et al. (1978) and falsification of some more recent hypotheses. The elements P1 and P2 of ‘S.’ cooperi show a relatively narrow population variability and do not change morphologically in the course of their ontogeny. In contrast, elements S and M profoundly transformed their pattern of denticulation and general shape during growth. Juveniles are relatively underived and rather easily homologized with elements of other polygnathid apparatuses, but adults are of bizarre morphology unlike any other conodonts. Such a pattern of ontogenetic transformation makes it likely that small P2 elements of relatively generalized morphology and mature Dinodus-type S elements associated with P1 elements of ‘S.’ praesulcata in a Kowala sample taken from the topmost Famennian nodular limestone bed, belong together to the same apparatus. No Devonian conodont apparatus is known that could be compared with the highly derived ‘Siphonodella’ as its possible ancestor. Apparently, the elictognathid lineage immigrated to the Rheic Ocean realm from an unknown source near the end of the Devonian.