As far as paleontology is concerned, it is still unclear what ended the dinosaurs' reign on Earth. Some scientists say a huge asteroid that struck in Mexico wiped them out. Other experts say their population has long been in a decline even before the posited asteroid impact.

Unbeknownst to many though, dinosaurs are not completely extinct. In fact, we still see some of them today but not in their gargantuan forms anymore that put the "dino" in the word dinosaur. They still roam the world as the modern-day birds.

After they began appearing some 220 million years ago, dinosaurs had actually gone ahead of the evolution race and shrunk in size at a surprisingly rapid and steady rate, a group of researchers at the Oxford University discovered.

The process is natural and evolutionary biologists call it adaptive radiation, wherein organisms quickly evolve into new forms to adapt to the changing environment. This brings to mind Charles Darwin's observation with the wild finches, and how adaptive radiation diversified their beak shapes, that conceived the famous Theory of Natural Selection.

However, the evolution in the physical form of dinosaurs slowed down as their line branched out further, except for the evolutionary lineage leading to birds, which continued to undergo changes for the next 170 million years.

The reduction in size may have been the chief factor in the dinosaurs' long-enduring lineage that is now found in birds, the team suggested. The rest of the dinosaurs, meanwhile, failed to adapt and got themselves stuck in narrow ecological niches, hence their extinction.

"Dinosaurs aren't extinct. There are about 10,000 species alive today in the form of birds," said lead author of the study Roger Benson, M.D. of Oxford University's Department of Earth Sciences. "We wanted to understand the evolutionary links between this exceptional living group and their Mesozoic relatives, including well-known extinct species like T. (Tyrannosaurus) Rex, Triceratops, and Stegosaurus."

To find out, the Oxford researchers, together with the Royal Ontario Museum, calculated the body mass of 426 dinosaur species by the thickness of their leg bones such as the femur.

Afterwards, they compared the measurements of the entire family tree of dinosaurs dating as far back as 160 million years. If closer relatives show similarities in sizes, their evolution was slow. On the other hand, if the sizes varied significantly, then it may be fast, the findings revealed.

Benson said their team found an "exceptional body mass variation" with the evolutionary lineage leading to birds, especially the three-fingered maniraptorans, including birds and the non-avian dinosaurs such as the mid-sized Velociraptor. These species weighed from 15 grams to three tonnes, and they are mostly in an omnivorous diet.

The study debuted in the journal PLOS Biology.

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