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'Frankenstein dinosaur' enigma solved

Posted August 16

The so-called Frankenstein dinosaur is the missing link connecting plant-eating dinosaurs such as the Stegosaurus to a group including carnivorous ones such as the Tyrannosaurus Rex, scientists have suggested.

The vegetarian dinosaur walked the earth around 150 million years ago and is officially known as the Chilesaurus. It initially confused scientists because of its strange physical characteristics, apparently drawn from two groups of dinosaurs that were thought to be separate. Originally discovered in South America, the Chilesaurus has the head of a carnivore but flat teeth suited to grinding plants.

"Chilesaurus almost looks like it was stitched together from different animals, which is why it baffled everybody," said Matthew Baron, a Ph.D. student in the University of Cambridge's department of earth sciences and joint first author of a new study on the dinosaur published Wednesday.

The researchers, from Cambridge and London's Natural History Museum, analyzed more than 450 anatomical characteristics of early dinosaurs to find the right place for the Chilesaurus in the dinosaur family tree.

At an earlier point, scientists had thought Chilesaurus belonged to the Theropoda, the 'lizard-hipped' group of dinosaurs that includes Tyrannosaurus. But the latest study, published in the Royal Society's Biology Letters journal, suggests that the dinosaur is in fact a very early member of a group called the Ornithischia, a "bird-hipped" group that includes dinosaurs such as the Stegosaurus and Triceratops.

The Chilesaurus shares the inverted hip structure of the Ornithischia group, which allowed for complex digestive systems. In turn, this digestive system led to larger plant-eaters evolving. But the Chilesaurus lacked the distinctive beak that other dinosaurs classified as Ornithischia have for eating, making it a likely transitional dinosaur that fills the gap between the two different groups.

"Before this, there were no transitional specimens -- we didn't know what order these characteristics evolved in," said Baron in a statement. "This shows that in bird-hipped dinosaurs, the gut evolved first, and the jaws evolved later -- it fills the gap quite nicely."

Baron suggested that the Chilesaurus' existence supports the theory that there was a split in the dinosaur family tree.

"The two branches took different evolutionary directions. This seems to have happened because of a change in diet for Chilesaurus. It seems it became more advantageous for some of the meat-eating dinosaurs to start eating plants, possibly even out of necessity."

"Chilesaurus is one of the most puzzling and intriguing dinosaurs ever discovered," said co-author Paul Barrett of the Natural History Museum in a statement. "Its weird mix of features places it in a key position in dinosaur evolution and helps to show how some of the really big splits between the major groups might have come about."

Earlier this year, in the journal Nature, the same scientists had suggested that the entire dinosaur family tree should be rearranged because it is possible both major groups of dinosaurs are descended from one common ancestor. Their suggestion could overturn over a century of theory about the evolution of dinosaurs.

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