It is interesting to compare the dentition and arms of these three dinosaurs. Allosaurus, or a T. rex-like dinosaur, had sharp pointy teeth. The Alvarezsaur, Haplocheirus, had many shorter teeth, and the most bird-like dinosaur of the group, Shuvuuia, had reduced peg-like dentition. Also note the three claws of roughly equal size of the Allosaurus, the thick short thumb and two skinny fingers of Haplocheirus, and the completely reduced single digit of Shuvuuia.
Click on image for full size
Image Courtesy of Zina Deretsky, National Science Foundation

Dinosaur Discovery Helps Solve Piece of Evolutionary Puzzle
News story originally written on January 28, 2010

After going on an expedition to the Gobi desert of China, scientists have solved the puzzle of how one group of dinosaurs came to look like birds even though they are not ancestors of birds. They were studying the family Alvarezsauridae, which is a bizarre group of bird-like dinosaurs with a large claw on the hand and very short, powerful arms. The scientists found fossils of these dinosaurs dating back 63 million years.

Jonah Choiniere, a scientist from George Washington University, named the newly discovered species of dinosaur Haplocheirus sollers (which means simple, skillful hand). He said that the fossil of this type of dinosaur shows how the bizarre hands of alvarezsaurs evolved from earlier predatory dinosaurs. Choiniere said that this fossil also confirms that Alvarezsauridae evolved in the Late Jurassic time period.

Even though the skeletons of Alvarezsauridae have many similarities to birds, they actually evolved parallel to birds and did not descend from them.

"The primary goal of our expeditions was to find evidence of the theropod dinosaurs closest to birds, and the discovery of Haplocheirus is one of our major discoveries, said James Clark, one of the lead scientists on the expedition. "This spectacular skeleton shows how the strange arms of Mononykus and other alvarezsaurs evolved from a more typically theropod grasping hand."

Theropod dinosaurs include meat-eaters like Tyrannosaurus rex but also modern birds. Alvarezsaurs are one of several groups of theropods closely related to birds, including well-known species like Velociraptor.

Last modified March 1, 2010 by Becca Hatheway.

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