Flights of Fancy in Avian Evolution
From mousebirds to terror birds, the class Aves has encompassed a remarkable diversity of species over the past 150 million years.
Returning to the Sea
Since the first terrestrial vertebrates evolved nearly 400 million years ago, multiple lineages returned to the oceans, giving rise to secondarily aquatic groups such as whales, seals, and sea turtles. Penguins are the best-known avian lineage to make this transition, but they are by no means the only one.
Penguins made the plunge just a few million years after the Cretaceous mass extinction, during the Paleocene. Some species like Kairuku grebneffi subsequently reached standing heights of about 1.3 meters—30 percent taller than the largest penguin living today, the Emperor penguin. Plotopterids, a poorly understood, extinct group of diving birds, followed penguins into the sea during the Eocene. Whereas penguins are almost exclusively from the Southern Hemisphere (with the exception of the Galápagos penguin), plotopterids lived along the coasts of Japan and the Pacific Northwest from 35 to 15 million years ago. Despite their penguin-like appearance, plotopterids are thought to have been more closely related to gannets and cormorants.
A third group, the Alcidae, also adapted to marine diving in the Neogene. Like penguins and plotopterids before them, two lineages of stout, web-footed alcid birds lost flight and evolved flipper-like wings during the Miocene and Pliocene. The Great Auk represented a less advanced step on this journey back to the sea, because these birds never evolved the more stiffened wing and miniaturized scale-like wing feathers seen in penguins. Tragically, this species was wiped out in the mid-19th century because of overhunting. Farther back in time, from the Miocene to the Pleistocene, a different lineage of alcids, the Lucas Auks, took the same adaptive route further, evolving shorter, more flipper-like wing bones. The Great Auk once ranged along the northern Atlantic Ocean; in contrast, Lucas Auks were exclusively Pacific Ocean inhabitants.
Together, these four lineages present a classic study in convergence, the evolution of similar features in independent lineages. Each of these groups solved the problem of moving underwater in the same way: by adopting a style of locomotion called wing-propelled diving. Convergence occurred readily because bird wings are adapted for flying through a fluid—air. But in these cases of convergence, the birds “fly” through seawater, a fluid that is almost 800 times as dense as air.