The distinctive brachiopod Dicoelosia King 1850 is characterized by a strongly bilobed outline. To date, studies have concentrated on its functional morphology, taxonomy and evolution; little attention has been paid to its ontogeny. Here, we map population variation by principal component analysis for over 80 specimens distributed across five species of Dicoelosia. Using geometric morphometrics with landmarks for some 40 specimens, the ontogenic trends in D. sp. nov. are compared with those of Dicoelosia biloba. In addition, the ontogenic pathway in D. sp. nov. is investigated by morphing with control points, a new technique introduced here to palaeontology. Combining the results above, the ontogeny of the key character of the genus, emargination, is modelled. Within single populations, taxa may develop from broad weakly emarginate forms into those that are elongate and deeply emarginate. As the identification of the genus and its species depends on external morphological characters, the definition of ontogenetic trends in each species is essential for taxonomic discrimination. Substantial population variation exists in many of its species; however, the morphing technique provides a method of simulation, predicting the full range of ontogenetic variation in given populations.