Abstract
Sturgeons belong to the family Acipenseridae, the most species-rich extant family of Acipenseriformes, a basal actinopterygian group of key importance in assessing the early radiations of the actinopterygians. At the same time, acipenseriforms display unique specializations in the morphology of the snout and jaws which make them a valuable model for studying evolutionary novelties. However, despite a long history of research, the homologies of the snout and the mandibular arch of acipenseriforms remain uncertain preventing further studies on the evolutionary origin of their unique snout and jaw structure, and in particular, of the upper jaw symphysis, the key apomorphy of the group and the preoral snout. In the present study, a detailed description of the upper jaw morphology and development in sturgeons is provided in order to address its composition in terms of the common actinopterygian archetype. Based on the obtained results, the upper jaw of acipenseriforms is assumed to have lost the autopalatine portion, which most likely is represented by the separate cartilages supporting the tentacles. Also, the conventional interpretation of the sturgeon's maxilla as dermopalatine is rejected on the grounds of this bone structure and development. Paedomorphosis is proposed to be the most likely mechanism explaining the evolutionary origin of the upper jaw symphysis and supposed modifications of the snout in sturgeons.