Deciphering the morphological variation and its ontogenetic dynamics in the Late Devonian conodont Icriodus alternatus

Identification of relevant taxonomic and evolutionary units is a recurrent issue in the fossil record, all more for ancient fossils devoid modern equivalents such as conodonts. Extensive morphological variation has often led to description numerous species, subspecies or morphotypes, which may correspond end-member morphologies reached through ontogeny. The platform elements Late Devonian conodont species Icriodus alternatus were characterized by rows denticles coming into occlusion between opposite elements; each element grew incremental addition lamellae successive triads during During late Frasnian early Famennian, important within this three subspecies. An extensive sample Famennian was quantified using 2D biometric measurements denticle counts on pictures, showing that mainly differed their size range but not general morphology. A 3D morphometric analysis further performed subsample characterize shape ontogenetically older part elements. ontogeny, valleys tended be filled, asymmetry inner outer side increased. These ontogenetic trends are responsible formerly described Ic. alt. mawsonae helmsi . Slight discrepancies temporal ranges achieved variations response environmental changes. Disparity along ontogeny seems follow an “hourglass model” suggesting shift from relatively loose developmental constraints pattern growth modulated functional occlusion.