In large groups of distantly related species—such as different families or orders of birds—the molecular clock is much "sloppier" than previously thought, Brown said.
This is because different lineages of birds can accumulate mutations at different rates, so applying a single rate to an entire family tree could lead to suspicious results.
The study is the first rigorous investigation into whether bad assumptions about birds' genetic data have led to the large difference—about 50 to 60 million years—between fossil- and genetic-based estimates, researchers say.
The most likely explanation is that these two sources of information deal with different stages of diversification.
Genetic data is used for the period when genes share a common ancestor, which could slightly predate the development of new species. Fossils, however, record only the products of evolution.
Patrick O'Connor, a paleontologist at Ohio University in Athens, was not involved with the study.
"It is important to stress that these data should not be viewed as better than fossils, but that the information we glean from both the fossil record and [living] animals represent complementary data sets—ones that can, and should, be integrated more so than is currently done," O'Connor said.
"This study should inspire paleontologists to continue looking for additional fossils of modern birds in Cretaceous-age strata," he added.
"It is important to note that there are already a few examples of fossil birds that support this view of avian evolution based on molecular evolution.
"In a sense this study corroborates some of these recent discoveries."
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