Taxonomy
They have uncertain placement, generally considered as member of order Suliformes which includes sulids such as gannets and boobies, frigatebirds, cormorants and darters. Plotopterids are regarded as closely related to Anhingidae (darters) and Phalacrocoracidae (cormorants). On the other hand, there is a theory that this group may have a common ancestor with penguins due to the similarity of forelimb and brain morphology. However, the endocast morphology of stem group Sphenisciformes differs from both Plotopteridae and modern penguins.
Plotopterids were not closely related to penguins annd instead exhibited remarkable convergent evolution with penguins and in particular with the now extinct giant penguins. They differ from modern penguins in that they lived in the Northern Pacific Ocean which is located on the other side of the world from where penguins lives. This has led Plotopterids to them being described at times as the Northern Hemisphere's penguins, More recent studies have shown that the shoulder-girdle, forelimb and sternum of plotopterids differ significantly from those of penguins. This means that comparisons in terms of function may not be entirely accurate.
Evolution
They seem to have evolved in the arctic islands during the mid-Eocene. From there, they would spread southwards with the formation of kelp forests. The earliest known member of the family, Phocavis maritimus lived during the late Eocene, but most of the known species lived during Oligocene. Plotopterids would become extinct in the early to mid-Miocene. That they became extinct at the same time as the giant penguins of the Southern Hemisphere, which also coincided with the radiation of the seals and dolphins, has led to speculation that the expansion of marine mammals was responsible for the extinction of the Plotopteridae, though this has not been formally tested.
