Despite their heft, many dinosaurs had surprisingly tiny genomes

They might be giants, but many dinosaurs apparently had genomes no larger than those of a modern hummingbird.

So say scientists who've linked bone cell and genome size among living species and then used that new understanding to gauge the genome sizes of 31 species of extinct dinosaurs and birds, whose bone cells can be measured from fossilized bones.

The researchers, at Harvard University and the University of Reading, were led by Chris Organ and Scott V. Edwards at Harvard. They report their findings this week in the journal Nature.

"We see distinct differences between two major lineages of dinosaurs," says Organ, a postdoctoral fellow in organismic and evolutionary biology supported by the National Institutes of Health (NIH). "The theropods - carnivores such as Tyrannosaurus rex and Velociraptor - had very small genomes, in the range of modern birds. Ornithischians - which include Stegosaurus and Triceratops - had more moderately sized genomes, akin to those of living lizards and crocodilians. We aren't sure about the genomes of the long-necked sauropods yet."

Organ and Edwards say the clear-cut dichotomy in dinosaur genomes is likely due to different amounts of repetitive and non-coding DNA in the two groups' genetic material, a factor largely responsible for variation in genome size across animal species. They estimate that active repetitive DNA might have comprised an average 12 percent of the ornithischian genome but just 8.4 percent of theropod genetic constitution.

Organ and Edwards' Nature co-authors are Andrew M. Shedlock of Harvard's Museum of Comparative Zoology and Andrew Meade and Mark Pagel of the University of Reading. Their work was supported by the NIH, the National Science Foundation, and the Natural Environment Research Council.