Did birds evolve independently or from dinosaurs? The question has been settled by a recent discovery; the findings are published in the latest issue of Science. A ten-foot long, nearly complete fossil discovered in 160-million-year old mudstone beds in northwest China provides indisputable evidence that birds evolved from dinosaurs. The young adult fossil is clearly a transitional one between dinosaurs and birds. It belongs to a new genus of alvarezsauroid dinosaurs: Haplocheirus sollers (meaning “simple, skilful hand”). The discovery has pushed back the fossil record of alvarezsauroid by 63 million years. What is particularly significant is that the fossil unearthed is about 15 million years older than the earliest known bird — Archaeopteryx. To confirm that birds descended from dinosaurs, it was essential to have fossils of bird-like theropods in the early stages of the transition and predating the Archaeopteryx. The absence of such evidence led to a paradox in the time scale (temporal paradox) and made some scientists believe that birds developed independently. While some recent discoveries from the Jurassic Period have challenged the temporal paradox, they were not able to settle the question. They were not truly transitional fossils and shared many morphological characteristics with birds, and moreover the ages of these fossils are “poorly resolved,” as the paper notes. The discovery of Haplocheirus sollers has finally solved the temporal paradox.
Haplocheirus has all the morphological features to be called the transitional form and the earliest among the alvarezsauroid dinosaurs. The curved, serrated teeth and canine teeth indicate it was a carnivore. It is by far the largest alvarezsauroid ever found: big for a bird and small for a dinosaur. According to the authors of the Science paper, certain characteristics of its digits imply that the “hand was fully functional…and retained some grasping ability.” Compared with the Haplocheirus, the derived alvarezsauroid dinosaurs from the Jurassic Period have undergone several morphological modifications that place them later in the evolutionary lineage. There has been “extreme morphological convergence between birds and derived Alvarezsauroid,” the authors conclude. There has been a rich haul of dinosaur fossils from China and the latest discovery establishes that alvarezsauroidea originated in Asia rather than in South America as was originally thought. Looking for fossils in rocks belonging to the late Jurassic Period and the appropriate depositional environment will be the key to find other missing fossils between Haplocheirus and the derived alvarezsauroid.