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Challenging the classical theory about the development of the animal kingdom, a new study says major categories of mammals and birds appeared long before the dinosaurs died out.

The dinosaurs vanished about 65 million years ago. By examining animal genes to compute how long certain creatures have been around, researchers estimated that the major orders of birds and mammals evolved 100 million years ago.An order is a category of animals. Rodents, for example, represent an order.

The work argues against the classical idea that the disappearance of the dinosaurs opened the door for the diversification of mammals and birds. Instead, the breaking up of ancient continents may have been the cause, the researchers suggested in Thursday's issue of the journal Nature.

The early date for the emergence of the major orders was a surprise because there's virtually no trace of such diversification in the fossil record until about 40 million years later, said lead author S. Blair Hedges of Pennsylvania State University.

The earlier creatures might have left few fossils because high sea levels restricted the amount of land available for them.