We restudy the holotype specimen and all the alleged fossil material assigned to Cayaoa bruneti Tonni from the Early Miocene marine deposits of the Gaiman Formation, Patagonia, Argentina. Cayaoa bruneti is phylogenetically placed close to the Erismaturinae (=”Oxyurinae”) that contains the diving birds. It was considered a flightless foot-propelled diving duck and represents the earliest example of flight loss in Anatidae. We carried out a comparative and morphometric descriptive study of different species of anseriforms with special attention to divers, to evaluate which is the expected intraspecific size variation within anatids to make an assignment of materials with a greater degree of certainty. Our results allow us to state that one coracoid, five humeri, two carpometacarpi, 12 femora, eight tibiotarsi, and eight tarsometatarsi can be assigned to the species. Consequently, it was possible to make a more detailed description and amend the original diagnosis.