Evolutionary roots of the conodonts with increased number of elements in the apparatus

Abstract

ABSTRACTFour kinds of robust elements have been recognised inAmorphognathus quinquiradiatusMoskalenko, 1977 (inKanyginet al. 1977) from the early Late Ordovician of Siberia, indicating that at least 17 elements were present in the apparatus, one of them similar to the P1element of the Early SilurianDistomodus. The new generic nameMoskalenkodusis proposed for these conodonts with a pterospathodontid-like S series element morphology. This implies that the relatedDistomodus,PterospathodusandGamachignathuslineages had a long cryptic evolutionary history, probably ranging back to the early Ordovician, when they split from the lineage ofIcriodella, having a duplicated M location in common. The balognathidPromissum, with a 19-element apparatus, may have shared ancestry withIcriodellain Ordovician high latitudes, withSagittodontina,Lenodus,TrapezognathusandPhragmodusas possible connecting links. The pattern of the unbalanced contribution ofBaltonioduselement types to samples suggests that duplication of M and P2series elements may have been an early event in the evolution of balognathids. The proposed scenario implies a profound transformation of the mouth region in the evolution of conodonts. The probable original state was a chaetognath-like arrangement of coniform elements; all paired and of relatively uniform morphology. This was modified at the origin of protopanderodontids by the introduction of a medial S0element, which resulted in the separation of the exposed unit of M and S series elements from the P series elements hidden in the throat. A rotation of the S series elements to an almost horizontal position in early prioniodontids may have promoted duplication of the M element pair. InGamachignathus,IcriodellaandPterospathoduslineages, these elements are differentiated morphologically. Subsequent anteriorward bending of the P element series caused duplication of the balognathid P2element pair, but they remained undifferentiated, even in the otherwise elaboratePromissum. The whole clade of conodonts with supernumerary element pairs in the apparatus has its roots in high latitudes of the Ordovician.