New observations on the holotype of the Maastrichtian aristonectine elasmosaurid Aristonectes parvidens Cabrera from Chubut Province, Argentina, reveals features that were not previously described while resulting in new interpretations. Quantitative comparison with non-aristonectine elasmosaurids demonstrates that the increase in the number of alveoli in the premaxilla, maxilla and mandible is not a consequence of skull size increase. Instead, decrease in alveolar size and interalveolar space, compared with that observed in non-aristonectine elasmosaurids, is at least as important. Increase in skull length compared with non-aristonectine elasmosaurids is not as marked as typically considered: skull length is equivalent to the summatory of length of the first 8–10 cervical vertebrae in non-aristonectines and the first 10–12 in aristonectines. The cervical region is characterized by a rapid increase of the relative broadness of the vertebral centra, which indicates a reduction in lateral mobility of the neck. Increase in alveoli number is achieved as a result of a number of changes that seem to indicate the importance of the biological role of the length of the alveolar row, this is probably related to changes in prey size and capture strategy such as trading for smaller fishes or invertebrates and/or a upgrading from ambushing one individual prey to simultaneously ambushing multiple prey individuals.