Big Dinosaurs Had Small Genomes

March 8, 2007 — Tyrannosaurus rex may have been over 43 feet long, but the meat-loving predator and several other large dinosaurs had relatively little DNA in their cells, according to new research.

The study, published in this week's Nature, is the first to estimate dinosaur genome size. A genome is an organism's complete set of genetic information, which includes DNA and RNA.

Since prior research by the same team demonstrated that small genomes correlate with high metabolism, the findings suggest T. rex, Allosaurus, Deinonychus (aka "Terrible Claw") and certain other large carnivorous dinosaurs may have been very active, fast moving animals, as many popular films, such as "Jurassic Park," portray.

"There is a widely held idea that metabolic rate...may limit genome size, since maintaining a high metabolism would be most effective with smaller cell sizes," explained Andrew Shedlock, who worked on the study.

Shedlock, a research associate in the Department of Organismic and Evolutionary Biology at Harvard, and colleagues first studied birds and reptiles living today.

Their findings suggested that bone cell size in vertebrates is linked to genome size. The smaller the bone cells, they found, the smaller the genome.

The specific cells they studied are found in durable, protected pockets found in bone tissue. By measuring the size of those pockets in 31 extinct birds and dinosaurs, the scientists were able to estimate genome sizes in those species as well.

In addition to the findings about carnivorous dinosaurs, the researchers discovered certain plant-eating dinos, like Protoceratops and Psittacosaurus, also possessed genomes that were smaller than those of other dinosaurs and most other vertebrates — living or extinct.

The genetic "downsizing" appears to be due to fewer repetitive elements and less non-coding DNA.

"Aside from higher metabolic rates, smaller genomes have been correlated to longevity and faster rates of development," Edwards told Discovery News.

That suggests certain dinosaurs grew up fast, lived fast and yet might have been relatively long-lived.

It also indicates that small genomes, which today are associated with many living birds, first evolved between 230-250 million years ago within the Saurischian dinosaur lineage. This group of dinosaurs, which includes T. rex, possessed hips similar to those of today's reptiles.

Since the world's first birds did not emerge until 110 million years ago, the findings suggest small genomes were already around long before theropod dinosaurs evolved into birds.

Other characteristics typical of birds, such as feathers, pulmonary innovations, parental care and nesting, also developed in dinosaurs first, according to the research team.

Kevin Padian, professor of Integrative Biology and Curtator of the Museum of Paleontology at the University of California at Berkeley, told Discovery News that the techniques used by the Harvard team represent "an interesting approach."

"It's good if this data supports what we already know about dinosaur metabolism rates and other information," Padian said. "Just keep in mind that the new research extrapolates about genome size. No one has actually sequenced dinosaur DNA."

Corgan admitted that might never be possible, "but you never know," he added.