The peculiar "one-finger Zen" dinosaur, why did he lose his finger again?

A map of the restoration of representative species of the Alvarez dragon class shows the interesting evolution of the forelimbs. Courtesy of Xu Xing and illustrated by Vikto Radermacher

■ Ding Jia, a reporter of this newspaper

Creatures from ancient times often broke through the human imagination. Of these eccentric animals, a dinosaur called Alvarezosaurus is probably the most bizarre of theropods— they have skulls and hind limbs similar to those of birds, but only a large, specialized claw on their extremely short but very strong forelimbs.

On August 24, an international team led by Xu Xing, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, published a latest research result in the journal Contemporary Biology. They discovered two new dinosaurs in China, Wulat Hemikosaurus and Pang's Western ClawedOsaurus, raising a human understanding of the complex process of how Alvarezians reduced and gradually lost their fingers.

The decrease in the number of fingers or toes has occurred many times in the evolution of tetrapods. The most famous example of this is the "Horse Fossil Sequence" in North America. This is a series of equine fossils from primitive taxa to progressive taxa that reveal the gradual reduction of limb fingers/toes to 1 in the evolution of horses.

So, how did the Alvarez dragons gradually lose their fingers?

"The Alvarez dragons are an incredible class of animals." Paper collaborator Dr. Joaniel of the University of the Witwatersrand in South Africa, said, "They are like aardvarks and anteaters in dinosaurs. ”

Until this discovery, however, the fossil record of the Alvarezosaurus had a gap of more than 90 million years between the most primitive and progressive taxa, especially in the Early Cretaceous, where there was no definitive fossil record worldwide.

A joint expedition led by Xu Xing, James Clarke, a professor at George Washington University, and Tan Lin, a professor at the Longhao Institute of Geology and Paleontology in Inner Mongolia, discovered The Western Clawed Dragon was discovered in the Junggar Basin of Xinjiang, China, in 2005, and in the northwestern region of the Inner Mongolia Autonomous Region of China in 2009.

These two new Alvarezosaurus fossils are produced precisely from this 90 million-year geological interval. They have some typical late Alvarezosaurus characteristics, such as a specialized enlarged first finger claw, a mechanically efficient forearm structure, and a thick humerus. However, the relative proportions of their forelimbs are closer to those of the primitive taxon.

Based on this, researchers have conducted morphological observations on the morphology of Haptaurus and Western Clawed Dragon, and gradually revealed the macro evolution of the forelimbs of Alvarezosaurus: from the early long "gripping" forelimbs, to the long forelimbs and specialized claws of The Half-Clawed Dragon and the Western Clawed Dragon, and then to the late highly specialized, shortened, and functional single-fingered forelimbs.

"This shift has been going on in a gradual way for almost 50 million years." Xu Xing said, "Maybe one day, the evolution of Alvarez dragons will become a classic macro-evolutionary example like the 'horse fossil sequence'." ”

Interestingly, the early Alvarezosaurus had typical carnivorous teeth and hands that were more conducive to grasping prey, while the late taxa evolved large single claws, which were likely used to dig up decaying wood and ant nests and eat ants or termites inside. Fossils of Diplodocus and Western Clawed Dragon show exactly how these dinosaurs gradually adapted to new foods.

In addition, these "short-armed" dinosaurs were very small, and some were even less than 1 meter long. This is markedly different from another famous dinosaur with short forelimbs, the massive Tyrannosaurus rex.

Clark said: "These dinosaurs show how organisms have changed their ecological niche over time, such as from carnivorous to insecticidal. ”

Related thesis information: doi:10.1016/j.cub.2018.07.057

China Science Daily (2018-08-28 4th Edition General)