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FEATURE ARTICLE

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.

Daniel T. Ksepka

Reign of the Terror Birds

Click to Enlarge ImageToday, carnivorous mammals such as lions and wolves fill the apex predator role in most terrestrial ecosystems. However, over much of the Cenozoic Era, predatory birds struck fear into the mammals of South America.

Phorusrhacids, commonly known as terror birds, were a lineage of Click to Enlarge Imagegiant, flightless terrestrial birds that appeared shortly after the extinction of the nonavian dinosaurs. In a way, phorusrhacids represented a repeat of evolutionary history. Like their dinosaurian predecessors from the Cretaceous Period, they were huge-headed, bipedal, egg-laying killing machines. Add teeth, replace the tiny wings with tiny arms, and throw in a bony tail and you would essentially have a smaller version of Tyrannosaurus rex.

The largest bird skull ever discovered measures a whopping 70 centimeters long (shown above). This huge head belonged to the titanic Kelenken guillermoi, a species that lived during the middle of the Miocene (about 15 million years ago). Today, the only surviving relatives of terror birds are two species of seriema. These carnivorous birds reach heights of just 90 centimeters or so. Nevertheless, they terrorize rodents and small reptiles with their sharp claws and beaks.




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