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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

The Eocene Explosion

Click to Enlarge ImageAccording to the fossil record, bird diversity expanded during the Eocene Epoch, which began 56 million years ago. The Eocene was characterized by a warm and mostly ice-free climate worldwide, with temperate and subtropical forests extending across much of the globe. Fossil-rich rock units, such as the Green River Formation of the western United States Click to Enlarge Imageand the Messel Formation of Germany, formed as sediments accumulated at the bottom of ancient lakes, often burying animal and plant remains that later fossilized. Many important groups of birds make their first appearance in these ancient lakebed treasure troves.

Click to Enlarge ImageEocene fossil skeletons reveal the new roles that birds began to play as they diversified into a plethora of new lineages. Over these Eocene lakes the earliest known frigate birds glided in search of fish to catch (or steal from other birds). Early members of the swift lineage sped across the sky, taking insects on the wing. In the trees, Click to Enlarge Imageprimitive members of the parrot lineage maneuvered through the branches using specialized grasping feet. Along muddy shorelines, some of the first curve-billed ibises probed for invertebrate prey. A radiation of nocturnal birds was also under way, with the first appearances of fossil relatives of the fruit-loving oilbird, the camouflaged insect-ambushing potoos, and the wide-beaked predatory frogmouths, whose modern descendants indeed consume frogs.

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