structural:expansion
Fossil record

Nearly ten prehistoric species have been named to date in the genus Gavia, and about as many undescribed ones await further study. The genus is known from the Early Miocene onwards, and the oldest members are rather small (some are smaller than the red-throated loon). Throughout the late Neogene, the genus by and large follows Cope's Rule (that population lineages tend to increase in body size over evolutionary time).

List of fossil Gavia species

List of fossil Gavia specimens

"Gavia" portisi from the Late Pliocene of Orciano Pisano, Italy, is known from a cervical vertebra that may or may not have been from a loon. If so, it was from a bird slightly smaller than the common loon. Older authors were quite sure the bone was indeed from a Gavia and even considered G. concinna a possibly junior synonym of it. This is now regarded as rather unlikely due to the quite distinct range and age. The Early Pliocene Gavia skull from Empoli (Italy) was referred to G. concinna, and thus could conceivably have been of "G." portisi if that was indeed a loon. The holotype vertebra may now be lost, which would make "G." portisi a nomen dubium.