Opis

Apparatus composed of only coniform elements mimicking those of the Ordovician protopanderodontids.

Interpretation of taxomic affinities of the Famennian ‘simple cones’ is a difficult task. The geologically youngest conodont being unquestionably a continuation of the early Paleozoic coniform conodonts is Belodella, the protopanderodontid which did not survive to the Famennian. The last occurrences of that peri-reefal species are latest Frasnian in age. The seemingly coniform conodont elements of the coeval icriodontids are mostly isolated denticles of elements with otherwise unmineralized processes. At least three lineages of the icriodontids persisted into the Famennian. Their coniform denticles are of virtually the same morphology as those of geologically older species, being associated with icrion-bearing P1 elements. Yet, ‘simple cones’ occur in abundance also in horizons and whole sections of the Famennian where icriodontid P1 elements are missing. Some of them are very similar to the Ordovician protopanderodontids or distacodontids. The Famennian Mitrellataxis is a homeomorph of the early Ordovician Clavohamulus. Elements indistinguishable from those of the Ordovician Drepanoistodus have been described as parts of the apparatus of Famennian Pelekysgnathus. In the Holy Cross Mountains such elements occur abundantly in some samples. No recurrent association with the P1 elements of Pelekysgnathus or Icriodus has been noticed, and wherever those are represented in a reasonable number, they are associated with flat and wide triangular elements like those from older strata.
It is tempting to assume that these are cases of further reduction in mineralization of the icriodontid oral apparatus. Perhaps in some cases this actually took place. Nevertheless, some other associations of coniform elements show a recurrent morphological differentiation closely resembling that in the early Palaeozoic protopanderodontid conodonts. Although not all element types represent truly discrete classes and morphological transitions are more common than in undoubted protopanderodontids, the similarity is striking and extends even to development of geniculation in some elements, mimicking the Ordovician distacodontids. A possibility of such an affinity cannot be excluded, as much of the Famennian conodont fauna is of cryptic origin. It emerged as an effect of extensive migration events and the present knowledge of the Silurian and Devonian simple cones is still very limited outside restricted regions of Europe.
The alternative, that this is a case of secondary introduction of regularity to earlier rather indifferent morphologic variability of the icriodontid denticles is supported by the relatively late emergence of these conodonts and the tremendous time gap separating them from the last indisputable protopanderodontids (end of the Ordovician). If these were secondarily simplified icriodontids, then all the platform series locations were either lost, or reduced to the cusp.
There are thus two possible solutions to the problem of the Famennian ‘simple-cones’: (1) these are denticles of elements from an icriodontid lineage with completely reduced P elements or (2) these are survivors of the Ordovician protopanderodontids, that is a Lazarus lineage not recognized previously from the whole Silurian and most of the Devonian. Until this issue is settled, the material of simple cones not belonging to Icriodus and Pelekysgnathus is here described provisionally in terms of coniform apparatus terminology.

Synonimy

Zasięg czasowy

Famennian

Literatura

Dzik, J. 2006. The Famennian "Golden Age" of conodonts and ammonoids in the Polish part of the Variscan sea. Palaeontologia Polonica 63, 1-359.