Known material is limited to a single humerus, Early Miocene in age, found in the Monte León Formation near Puerto San Julián in Santa Cruz Province, Argentina.

Florentino Ameghino in 1905 described some penguin bones which he thought to be specifically distinct as Isotremornis nordenskjöldi: a tarsometatarsus, a humerus, and a part of a femur.

George Gaylord Simpson (1946) and Pierce Brodkorb (1963) argued about whether the bones could all be considered syntypes or whether only the wrongly assigned tarsometatarsus was designated as the holotype.

Thus, another name had to be given - and added to the already long and confusing list of valid and invalid fossil penguin taxa - to the distinct humerus of the new species Ameghino had thought he described.

Acosta Hospitaleche (2005) considered the humerus to be assignable to Paraptenodytes robustus; Bertelli et al. (2006) disagree, but believe that it belongs into a different genus.