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 the fossil record.
The researchers, at Harvard University and the University of Reading, were led by Chris Organ and Scott V. Edwards, both 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. "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.
The work indicates that the small genomes typically associated with birds -- whose genetic composition is noticeably sparer than that of other vertebrates -- evolved in dinosaurs some 230 to 250 million years ago, rather than with the emergence of modern living birds just 110 million years ago. Organ and Edwards suggest after this shrinking, theropod genomes then stabilized in size for hundreds of millions of years, a process that continues in mo