[3] In 1979, Storrs L. Olson and Hasegawa Yoshikazu identified several fossilized specimens of Late Oligocene and Early Miocene birds found in the State of Washington and in Japan as members of the family Plotopteridae, but distinct enough from Plotopterum in their general anatomy to warrant their own genus.

The Washington fossils, collected by Douglas Emlong in Late Oligocene terrains of the Pysht Formation, in the north of the Olympic Peninsula, were formally described the next year, 1980, by Olson himself, as the type of the new genus and species Tonsala hildegardae.

Olson ascribed to the genus the holotype USNM 256518, an incomplete specimen comprising a fragmentary humerus, fragments of a distal wing, a patella and a pectoral girdle.

[7] The genus was heavily adapted towards swimming and diving; the wings were paddle-shaped, and the scapula had a thin and expanded blade, similar to that of penguins, to help the animal propel through water.

The structure of the wing, and notably the shape of the humerus and radius, shared more resemblances with early penguins and flightless auks like Pinguinus and Mancalla than with the related Pelecaniformes, in a case of convergent evolution.

[5] Another undescribed species referred by Mayr and Goedert to the genus as ?Tonsala sp., known from a single right coracoid from the Makah Formation, differs from T. hildegardae mostly by its lesser size.

[3] In 2004, Gerald Mayr tried to introduce an hypothesis on presumed phylogenetic relationships between the plotopterids and the modern penguins, on the basis of ecological and physiological similarities, as well as an attempt to explain the convergent apparition of Spheniscids in the South Pacific and Plotopterids in the North Pacific during the Eocene; in this hypothesis, Plotopteridae would have been the sister group to Spheniscidae, and their clade would have been the sister clade of modern Suliformes.

[12] This hypothesis has since been heavily criticized,[13] and several factors, such as recent progress in molecular DNA analysis in order to study the relationships between extant species, as well as the discovery of the well-preserved, early Spheniscidae Waimanu, that lacked the derived traits present in both modern penguins and plotopterids used to clade them by Mayr, have pushed him to disavow this theory; most researchers considering today that the evolution of wing-propelled diving in penguins, plotopterids and extinct flightless auks is an example of convergent evolution.

[8] In the Pysht Formation, remains of Tonsala hildegardae are found associated with those of primitive cetaceans like the stem-mysticete Borealodon and the Aetiocetidae Fucaia, shorebirds related to the genus Calidris, the Desmostylian Behemotops, and the plotopterids Klallamornis buchanani and K.

[10] Based on two fragmentary skeletons of plotopterids, including one of Tonsala, it has been demonstrated that the bone-eating detritivorous worm Osedax, today specialized in the consumption of Cetacean corpses, used to have a more diverse diet and to also be able to feed on the remains of large marine birds.

[6] The extinction of Tonsala, alongside that of all of the plotopterids from the Pacific Northwest during the Late Oligocene can be explained by a combination of factors, including the disappearance of the numerous offshore volcanic islands present along the Cascadian coast and presumably used for nidification, the warming of the oceans leading to a dwindling of the native food ressources, and the apparition in North Pacific marine environments of early pinnipeds like Enaliarctos, that may have competed with plotopterids for food, breeding sites, and preyed upon them.