Riley Black in Smithsonian Magazine:
The repeated evolution of huge birds is part of the dinosaurian legacy. Beaked birds were the only dinosaurs to have survived the asteroid-triggered mass extinction at the end of the Cretaceous. Avians like the six-foot-tall Palaeeudyptes that waddled across ancient Antarctica about 30 million years ago and Titanis, a towering carnivore that was the only terror bird to live in North America between 1.8 million and 5 million years ago, underscore that prodigious dinosaurs were not only relegated to the times of Stegosaurus and Triceratops. The conditions that allowed birds to evolve to large size over and over again have varied from case to case, however, and the process has left a lingering question. If birds possess the traits that opened the possibility of truly giant, multi-ton statures for non-avian dinosaurs, why have we not seen a bird the size of a T. rex?
More here.
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