Scientists say they have discovered two new dinosaur fossils in China, shedding light on evolution of insect-eating dinosaurs.
The dinosaurs are both alvarezsaurs, an enigmatic group of theropod (meat-eating) dinosaurs which have many similarities with birds.
"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, a professor at the Wits University.
However, alvarezsaurs did not originally eat insects, said researchers, including those from the Institute for Vertebrate Palaeontology and Palaeoanthropology in China.
The earliest members of the group had more typically meat-eating teeth and hands, useful for catching small prey.
Only later-evolving members reduced their teeth and evolved a hand with a huge, single claw capable, perhaps, of tearing open rotting logs and anthills.
"The new fossils have long arms, and so show that alvarezsaurs evolved short arms only later in their evolutionary history, in species with small body sizes," said Roger Benson, a professor at Oxford University in the UK.
"This is quite different to what happens in the classic example of tyrannosaurs, which have short arms and giant size," Benson said.
Bannykus and Xiyunykus are important because they show transitional steps in the process of alvarezsaurs adapting to new diets.
The specimens were discovered during collaborative international fieldwork in China.
Xiyunykus was discovered in 2005 in Xinjiang, northwestern China.
Bannykus was discovered a few years later, in 2009, in Inner Mongolia, north-central China.
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