Dinosaur Ancestors Of Birds Shrunk In Size To Survive

By Shweta Iyer on May 7, 2014 5:53 PM EDT

dinosaur
Dinosaurs that evolved into birds were able to maintain their lineage because shrinking their bodies may have helped them adapt to changing ecology through their evolution. (Photo: Photo courtesy of Shutterstock)

According to new research, dinosaurs that evolved into birds were able to maintain their lineage because shrinking their bodies may have helped them adapt to changing ecology through their evolution. The research was conducted by an international team and led by scientists at Oxford University and the Royal Ontario Museum. A report of the research is published in PLOS Biology, according to a press release Tuesday.

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The team estimated the body mass of 426 dinosaur species based on the thickness of their leg bones. Dinosaurs showed rapid rates of body size evolution shortly after they originated, around 220 million years ago owing to vegetation and location of the continents. However many species stopped evolving and only the evolutionary line leading to birds continued to change size at this rate, and continued to do so for 170 million years, producing new ecological diversity not seen in other dinosaurs.

Dr. Roger Benson of Oxford University's Department of Earth Sciences, who led the study says, "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."

The scientists believe that the ability of dinosaurs that predate birds, to transform from bulky creatures to small lithe ones was the key to their survival and evolution.

"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. "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."

The team examined rates of body size evolution on the entire family tree of dinosaurs, sampled throughout their first 160 million years on Earth. They found that in closely related dinosaurs that were all bulky evolution was slow, but related dinosaurs with huge variations in size evolved rapidly. "What we found was striking. Dinosaur body size evolved very rapidly in early forms, likely associated with the invasion of new ecological niches. In general, rates slowed down as these lineages continued to diversify," said Dr David Evans at the Royal Ontario Museum, who co-devised the project. "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."

Dinosaur ancestors of birds kept evolving radically with changes in body shape and size and addition or removal of appendages to adapt to the ever changing environment. Other species of dinosaurs could not replicate this successful evolution and hence couldn't adapt, leading to their extinction. The success of birds comes through constant evolution over millions of years as shown by fossil records.

Dr. Matthew Carrano of the Smithsonian Institution's National Museum of Natural History and a part of the research team says, "The fact that dinosaurs evolved to huge sizes is iconic. And yet we've understood very little about how size was related to their overall evolutionary history. This makes it clear that evolving different sizes was important to the success of dinosaurs."

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