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.
People would never imagine the astonishing diversity of birds that once existed on Earth if they think only of the tiny songsters most commonly encountered around their homes and workplaces, such as cardinals, sparrows, and finches. These birds are a lovely part of our backyard life, but they represent a tiny sliver of the ecological roles that birds have explored over their 150-million-year history.
Over the course of avian evolution, specialized lineages of birds have played the parts of landscape-altering browsers, marine divers, nocturnal hunters, and even terrestrial apex predators. Many of the most unusual species are now extinct. Others diversified into the forms we see today. As prolific rock deposits in places like China, Wyoming, and Peru yield new discoveries, fossils are providing an increasingly rich source of insight into this evolutionary journey. More than 300 new fossil species of birds have been named since 2000, filling in the scientific picture of bird evolution.
The earliest known bird, Archaeopteryx lithographica, lived 150 million years ago during the Jurassic Period. Dinosaurs such as the armored Stegosaurus armatus and gigantic long-necked Brachiosaurus altithorax roamed the landscape, and plants like conifers, ginkos, and cycads dominated the flora. Archaeopteryx did not have the skies to itself, but instead lived alongside flying reptiles called pterosaurs.
During the Jurassic, the continents were in the process of breaking apart. Africa, South America, Australia, and Antarctica remained united into the supercontinent Gondwana. Much of Europe was inundated with shallow seas at this time, and the eleven known Archaeopteryx skeletons found their final resting places on the bottom of a lagoon in an area that is now part of present-day Germany.
Over the following 84 million years, a large number of primitive bird species, equipped with the evolutionary baggage of teeth, hand claws, and long bony tails, appeared and lived alongside their dinosaurian relatives. Then, 66 million years ago, an asteroid impact triggered a mass extinction that wiped out the nonavian dinosaurs, the flying pterosaurs, marine reptiles like plesiosaurs, and many other groups. Some birds survived, but many species went extinct, including all the toothed forms.
In the aftermath of this mass extinction, “modern” birds—those belonging to the radiation that includes over 10,000 living species—took the stage. Out of the chaos of extinction emerged survivors that diverged and adapted to fill the ample niches opened up by the enormous loss of species. This period of diversification following the Cretaceous-Paleogene extinction gave rise to groups both familiar and strange, including the tree-loving parrots and mousebirds, aquatic penguins, giant soaring pseudotoothed birds, and land-dwelling terror birds.