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Plotopteridae is an 
The second species to be named from rocks along the eastern Pacific Ocean was ''Tonsala hildegardae'' from the late Oligocene lower part of the Pysht Formation in Washington State. More fossils of ''T. hildegardae'' have since been described and included some of the first known examples of borings made by the marine bone-eating worm ''Osedax'' in bird bones.
The earliest known member of the family, ''Phocavis maritimus'' lived in the late Eocene, but most of the known species lived during Oligocene time, becoming 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.
extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
family of flightless seabirds with uncertain placement, generally considered as member of order Suliformes. They exhibited remarkable convergent evolution with the penguin
Penguins (order (biology), order List of Sphenisciformes by population, Sphenisciformes , family (biology), family Spheniscidae ) are a group of Water bird, aquatic flightless birds. They live almost exclusively in the Southern Hemisphere: on ...
s, particularly with the now extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
giant penguins. That they lived in the North Pacific, the other side of the world from the penguins, has led to them being described at times as the Northern Hemisphere's penguins, though they were not closely related. More recent studies have shown, however, that the shoulder-girdle, forelimb and sternum of plotopterids differ significantly from those of penguins, so comparisons in terms of function may not be entirely accurate. 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.
Their fossils have been found in California, Oregon, Washington, British Columbia, Hokkaido, Tōhoku, Chūbu, Kyushu
is the third-largest island of Japan's five main islands and the most southerly of the four largest islands ( i.e. excluding Okinawa). In the past, it has been known as , and . The historical regional name referred to Kyushu and its surroun ...
. They seem to have evolved on arctic islands during the mid-Eocene, spreading southwards with the formation of kelp forests They ranged in size from that of a large cormorant
Phalacrocoracidae is a family of approximately 40 species of aquatic birds commonly known as cormorants and shags. Several different classifications of the family have been proposed, but in 2021 the IOC adopted a consensus taxonomy of seven ge ...
(such as a Brandt's cormorant
Brandt's cormorant (''Urile penicillatus'') is a strictly marine bird of the cormorant family of seabirds that inhabits the Pacific coast of North America. It ranges, in the summer, from Alaska to the Gulf of California, but the population nort ...
), to very large size, with femur length two times longer than emperor penguin
The emperor penguin (''Aptenodytes forsteri'') is the tallest and heaviest of all living penguin species and is endemic to Antarctica. The male and female are similar in plumage and size, reaching in length and weighing from . Feathers of th ...
. They had shortened wings optimised for underwater wing-propelled pursuit diving (like penguins or the now extinct great auk
The great auk (''Pinguinus impennis'') is a species of flightless alcid that became extinct in the mid-19th century. It was the only modern species in the genus ''Pinguinus''. It is not closely related to the birds now known as penguins, wh ...
), and a body skeleton
A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
similar to that of the darter.
The second species to be named from rocks along the eastern Pacific Ocean was ''Tonsala hildegardae'' from the late Oligocene lower part of the Pysht Formation in Washington State. More fossils of ''T. hildegardae'' have since been described and included some of the first known examples of borings made by the marine bone-eating worm ''Osedax'' in bird bones.
The earliest known member of the family, ''Phocavis maritimus'' lived in the late Eocene, but most of the known species lived during Oligocene time, becoming 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.
References
* * * * * * * {{Taxonbar, from=Q145724 Extinct flightless birds Paleogene birds of North America Extinct animals of the United States Taxa named by Hildegarde Howard Prehistoric bird families