How did bird-like meat-eating dinosaurs lost their fingers?

An international team of paleontologists have found how a meat-eating species of dinosaurs, that share many similarities with birds, lost their fingers.

The fossil specimens of "Xiyunykus pengi" and "Bannykus wulatensis", discovered in China, are an enigmatic group of theropods known as alvarezsaurs.

Their bodies were found to be slender, with a bird-like skull and many small teeth instead of the usual large, sharp cutting teeth of their meat-eating relatives.

"Alvarezsaurs are weird animals. With their strong, clawed hands and weak jaws, they appear to be the dinosaurian analogue to today's aardvarks and anteaters," said Jonah Choiniere, Associate Professor at the Wits University in South Africa.

However, "the new fossils have long arms, and show that alvarezsaurs evolved short arms only later in their evolutionary history, in species with small body sizes. This is quite different to what happens in the classic example of tyrannosaurs, which have short arms and giant size," said co-author Professor Roger Benson from Britain's Oxford University.

According to the researchers, the alvarezsaurs did not always look this way. Early members of the group had relatively long arms with strong-clawed hands and typical meat-eating teeth. Over time, they evolved into dinosaurs with mole-like arms and a single claw.

Bannykus and Xiyunykus are important because they show transitional steps in the process of alvarezsaurs adapting to new diets, the researchers said, in the paper published in the journal Current Biology.

The new Alvarezsaurian dinosaurs are among the most bizarre groups of theropods, with extremely short, robust forelimbs with a single functional claw and gracile, bird-like skulls and hind limbs.

These specialisations have led to a controversy about their phylogenetic placement, biogeographic history and ecological role in Mesozoic ecosystems, the team noted.

--IANS

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