Reduction in Body Size Helped Dinosaurs Survive as Birds

Dinosaurs avoided going extinct by getting smaller and evolving into birds, a new study from Oxford has found.

Birds are among the most successful terrestrial vertebrates today. They live in almost all continents and their ability to fly has helped them invade several ecological niches.

The new study, conducted by Oxford University researchers, found that a lineage of dinosaurs experimented with different body size and evolved into birds.

"Dinosaurs aren't extinct; there are about 10,000 species alive today in the form of birds. We wanted to understand the evolutionary links between this exceptional living group, and their Mesozoic relatives, including well-known extinct species like T. rex, Triceratops, and Stegosaurus," said Dr Roger Benson of Oxford University's Department of Earth Sciences, who led the study.

Dinosaurs are known for their impressive size; the sauropods for example, weighed around 60 tons, about eight times more than the African elephant. Previous research has suggested that their bird-like lungs and egg-laying ability helped them get so big.

The latest research shows that some dinosaurs drastically reduced their size. The experiments with body shape design over millions of years paid-off and the lineage still survives on earth as birds.

"But it's the sustained high rates of evolution in the feathered maniraptoran dinosaur lineage that led to birds - the second great evolutionary radiation of dinosaurs," Dr David Evans at the Royal Ontario Museum, who co-authored the study.

Weighing Scales for Dinosaurs

Researchers at Royal Ontario Museum collaborated with Oxford researchers. For the study, the team estimated body mass of 426 dinosaur species. The team found that dinosaurs' body size changed at a rapid rate around 220 million years ago, but slowed down once they established themselves as top vertebrates. However, there was one group that kept on changing its size for 170 million years. This line of dinosaurs later became birds.

The team "weighed" dinosaurs using the thickness of the creature's leg bones.

"How do you weigh a dinosaur? You can do it by measuring the thickness of its leg bones, like the femur. This is quite reliable," said Dr Nicolás Campione, of the Uppsala University, a member of the team, according to a news release. "This shows that the biggest dinosaur Argentinosaurus, at 90 tonnes, was 6 million times the weight of the smallest Mesozoic dinosaur, a sparrow-sized bird called Qiliania, weighing 15 grams. Clearly, the dinosaur body plan was extremely versatile."

According to the team, feathered dinosaurs called maniraptorans showed the greatest variation in body size. Velociraptors, birds and several other creatures had a rapid rate of body size change; they weighed anywhere between 15 grams to 3 tons and ate plants and animals.

Researchers used dinosaurs' family tree to see how body size affected their growth and survival. They found a trend that governed a dinosaur species growth; the species would grow rapidly after origin, but slow down as they diversified. The group that became birds continued showing variation in body size with diversification.

The study is published in the journal PLOS One.