Birds' ancestors had small genomes too.

Today's birds have the smallest genomes among amniotes, the animal group that includes mammals, reptiles, and birds. A new study suggests that the dinosaur group that gave rise to birds had small genomes as well.

Scientists have noted that in living species, there's a correlation between the size of an animal's genome and the size of its cells, says Chris L. Organ, an evolutionary biologist at Harvard University. So, he and his colleagues used the size of certain bone cells, discernible from the cavities preserved in ancient fossils, to estimate the genome size of 31 species of dinosaurs and extinct birds.

Among ornithischian dinosaurs, which weren't closely related to birds or their ancestors, genome sizes averaged about 2.49 billion base pairs of DNA, the researchers estimate.

However, all but one of the inferred genome sizes for extinct birds and theropods--the group of dinosaurs most closely related to birds--had between 970 million and 2.16 billion base pairs of DNA, the known range of genome sizes for modern birds. The team's analysis, reported in the March 8 Nature, suggests that the small-genome trend on this branch of the dinosaur family tree began at least 230 million years ago.--S.P.