Dinosaurs took on many different shapes and sizes, but paleontologists are learning their success had nothing to do with their iconically massive bodies. If anything, their evolutionary success would appear to have come from their longstanding ability to keep shrinking. Just ask the birds.
Shortly after the dinosaurs made their appearance some 220 million years ago, they rapidly evolved and spread out into many different forms. This mad dash to invade all viable ecological niches is what evolutionary biologists call adaptive radiation. Soon after this initial phase, however, dinosaur evolution — at least as far as body size evolution goes — slowed down appreciably. Indeed, biologists have observed that rate of body size evolution slows down as lineages continue to diversify.
Evolution is for the Birds
Well, except for one particular line of dinosaurs — one that's still around today, in the form of birds. According to an international team led by scientists at Oxford University and the Royal Ontario Museum, continually shrinking morphologies and persistent high rates of evolution may have helped the group that became birds to continue exploiting new ecological niches over the course of their evolutionary history. And this is what still allows them to be hugely successful even today.
To reach this conclusion, the scientists estimated the body masses of 426 dinosaur species, based on the thickness of their leg bones. They then examined the rates of body size evolution on dinosaurs that existed during the first 160 million years of their existence. Results showed that dinosaur body size evolved very rapidly in the early days — and that this high rate of evolution persisted only for the feathered maniraptoran dinosaur lineage that led to birds.
Lead researcher Roger Benson of Oxford University's Department of Earth Sciences had this to say in a prepared statement:
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. We found exceptional body mass variation in the dinosaur line leading to birds, especially in the feathered dinosaurs called maniraptorans. These include Jurassic Park's Velociraptor, birds, and a huge range of other forms, weighing anything from 15 grams to 3 tonnes, and eating meat, plants, and more omnivorous diets.
Maintaining Evolutionary Potential
Small body size, says Benson, was the key to maintaining evolutionary potential in birds.
Image: Julius Csotonyi
His research suggests that the evolutionary line that lead to birds — arguably the most successful line given that all the others are now extinct — kept experimenting with different, often radically smaller, body sizes. This enabled new bodily morphologies and adaptations to arise more readily than among the more larger dinosaurs. The other dinosaur groups that failed to do this got trapped in narrow ecological niches that didn't allow for this kind of diversity and flexibility — and they ultimately went extinct.
Read the entire study at PLoS One: "Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage."