It contains a single species Polarornis gregorii, known from incomplete remains of one individual found on Seymour Island, Antarctica, in rocks which are dated to the Late Cretaceous (López de Bertodano Formation, about 66 Ma).

Gerald Mayr, for example, noted that Polarornis differed from loons in some important characteristics, and criticized Chatterjee's original description of the fossils for overstating the specimen's completeness.

[5] However, other Mesozoic bird specialists, including Storrs Olson and Sylvia Hope, have supported the classification of Polarornis as an early member of the loon lineage.

[6][7] Some recent studies seem to vindicate its status as a stem-loon;[8] alongside Neogaeornis and some unnamed Antarctic specimens, it seems to suggest a Gondwanan origin for this clade, possibly displaced northwards by early penguins.

[9] Polarornis was in all likelihood aquatic and fed on fish and large invertebrates, probably being an ecological equivalent of loons, grebes, or the Cretaceous Hesperornithes of the Northern Hemisphere.