No mass extinction! Dinosaurs shrank to become birds

Sixty five million years ago, the theory goes, a 6 mile long asteroid slammed into the earth and caused the extinction of all non-avian dinosaurs as part of a mass extinction event. Some 50% of all species living at the time – the hypothesis proclaims – vanished in this mass extinction event. It’s the stuff of catastrophe theories and movie scripts.

Smithsonian: Sixty-five million years ago the dinosaurs died out along with more than 50% of other life forms on the planet. This mass extinction is so dramatic that for many years it was used to mark the boundary between the Cretaceous Period, when the last dinosaurs lived, and the Tertiary Period, when no dinosaurs remained. This is called the Cretaceous/Tertiary (or K/T) boundary, and the associated extinction is often termed the K/T extinction event. …….  Most theories focused on climate change, perhaps brought on by volcanism, lowering sea level, and shifting continents. But hundreds of other theories were developed, some reasonable but others rather far-fetched (including decimation by visiting aliens, widespread dinosaur “wars”, and “paläoweltschmertz”­the idea that dinosaurs just got tired and went extinct). It was often popularly thought that the evolving mammals simply ate enough of the dinosaurs’ eggs to drive them to extinction.

But a new paper now suggests that dinosaurs actually shrank as they evolved over 50 million years to become the birds we know today. This still means that it was the dinosaurs vacating space on land which gave rise to the growth of mammal species, but it was not a one-time catastrophic event. No mass extinction then!

No fireworks apparently. Just a gradual, fairly mundane process where the large and cumbersome were deselected as they ran out of the ability to feed themselves and died off.  While the small and the nimble both needed less food and were more capable of getting it.

Michael S. Y. Lee, Andrea Cau, Darren Naish, Gareth J. Dyke. Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds. Science, 1 August 2014: Vol. 345 no. 6196 pp. 562-566 DOI:10.1126/science.1252243

University of Southampton Press ReleaseA new study involving scientists from the University of Southampton has revealed how massive, meat-eating, ground-dwelling dinosaurs evolved into agile flying birds: they just kept shrinking and shrinking, for over 50 million years.  

Today, in the journal Science, the researchers present a detailed family tree of dinosaurs and their bird descendants, which maps out this unlikely transformation.  They showed that the branch of theropod dinosaurs, which gave rise to modern birds, were the only dinosaurs that kept getting inexorably smaller.  

“These bird ancestors also evolved new adaptations, such as feathers, wishbones and wings, four times faster than other dinosaurs,” says co-author Darren Naish, Vertebrate Palaeontologist at the University of Southampton.  

“Birds evolved through a unique phase of sustained miniaturisation in dinosaurs,” says lead author Associate Professor Michael Lee, from the University of Adelaide’s School of Earth and Environmental Sciences and the South Australian Museum.  

“Being smaller and lighter in the land of giants, with rapidly evolving anatomical adaptations, provided these bird ancestors with new ecological opportunities, such as the ability to climb trees, glide and fly. Ultimately, this evolutionary flexibility helped birds survive the deadly meteorite impact which killed off all their dinosaurian cousins.”  

Co-author Gareth Dyke, Senior Lecturer in Vertebrate Palaeontology at the University of Southampton, adds: “The dinosaurs most closely related to birds are all small, and many of them – such as the aptly named Microraptor – had some ability to climb and glide.”  

The study examined over 1,500 anatomical traits of dinosaurs to reconstruct their family tree. The researchers used sophisticated mathematical modelling to trace evolving adaptions and changing body size over time and across dinosaur branches.  The study concluded that the branch of dinosaurs leading to birds was more evolutionary innovative than other dinosaur lineages. “Birds out-shrank and out-evolved their dinosaurian ancestors, surviving where their larger, less evolvable relatives could not,” says Associate Professor Lee.


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