Taxonomy is the very first step of most biodi- versity studies, but how confident can we be in the taxa delineation? One may hypothesize that the more abundant the material, the more accurate the description of morpho- logical variability and hence the better the taxonomic delin- eation. Yet, as we shall see, in the case of numerous transitional forms, this hypothesis may prove wrong. Simi- larly to rarefaction curves that assess the degree of knowledge on taxonomic diversity through sampling effort, we aim to test the impact of sampling effort on species delineation by subsampling a given assemblage. To do so, we use an abun- dant and morphologically diverse conodont fossil assemblage from the Smithian of Oman. We first recognize four well established morphospecies but about 80% of the specimens are transitional forms. We quantify the diagnostic characters in a sample of 159 P1 elements using geometric morphometrics and assess, via gradually subsampling the assemblage, the number of morphometric groups (i.e. mor- phospecies) using ordination and clustering analyses. Four morphospecies were detected when less than 20% of the specimens were considered. The number of detected clusters dropped to two when including more than 30% of the speci- mens. Such influence of sampling effort on species delinea- tion highlights the complexity of taxonomic work, especially when transitional forms are more abundant than typical specimens. These results should encourage researchers to extensively illustrate, measure and quantitatively compare their material to better constrain the morphological variabil- ity and delineation of taxa.