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New dinosaur from China illuminates dino-bird link

Life reconstruction of Haplocheirus sollers.
[Image Portia Sloan]

Scientists have discovered a new member of a peculiar group of dinosaurs, the long-legged, stubby-armed alvarezsauroids. This one, found in China, is 63 million years older than other known members of this group, making it an important early member of the lineage that includes birds and their closest dinosaur relatives.

The alvarezsauroids are relatively small, bipedal dinosaurs, and until this new fossil discovery, all the known examples lived during the late Cretaceous Period. One of the most unusual characteristics of these Cretaceous alvarezsauroids was the single, massive claw on each hand that was probably used for digging.

In a study appearing in the 29 January issue of the journal Science, Jonah Choiniere of George Washington University and colleagues describe Haplocheirus sollers, an early alvarezsauroid from the Late Jurassic Period, which preceded the Cretaceous.

After analyzing the fossils and comparing them to those of other alvarezsauroids, the researchers concluded that the whole alvarezsauroid lineage reaches back further than we thought, near the "stem" of the group that includes both birds and their close theropod dinosaur relatives. (Many of the well-known two-legged dinosaurs, like T. rex, are theropods.)

Haplocheirus is also the largest known alvarezsauroid, suggesting that alvarezsauroid body sizes became smaller over time.

One of the most interesting things about Haplocheirus is that its hands have three fingers, the middle one much longer than the other two. Dr. Choiniere and colleagues think that, as alvarezsauroids evolved, these fingers probably fused into the giant claw that eventually became the dinosaurs' bizarre trademark.