Evolution Evidence Found Between Dinosaurs And Birds

A newly discovered dinosaur provides a fossil snapshot of the reptiles’ evolution into birds, and neatly fills a troublesome transitional gap.

Living 159 million years ago in what is now Western China, Limusaurus inextricabilis was a small, herbivorous member of the theropod family. The coelurosaur branch of that family survives today, in highly modified form, as birds.

But while bird wings appear to have developed from the middle three digits of a five-digit hand, theropod forelimbs have just three digits, leaving a double-digit gap in the evolutionary record. Limusaurus inextricabilis, described in a paper published Wednesday in Nature, appears to fill that gap.

It has four digits. The first is shrunken, while the second is enlarged, as if compensating for the dimunition of the first. And though this transitional creature didn’t yet have the feather-like structures found in later proto-bird dinosaurs, it did have a toothless upper and lower jaw — in other words, a beak.

• Limusaurus inextricabilis had features which indicate a half-way point in the transition from dinosaurs to birds.
• The creature is a small plant-eating dinosaur with a toothless beak, stunted arms, and unusual hands.
• Limusaurus inextricabilis had features which indicate a half-way point in the transition from dinosaurs to birds.
• Dinosaurs technically vanished from the face of the Earth at the end of the Cretaceous period 65 million years ago.
• But most experts believe they live on disguised as birds.
• There is plenty of evidence that modern birds are descended from dinosaurs.
• Two-legged theropod dinosaurs, such as Tyrannosaurus rex, had bird-like feet, a bird-like pelvis, and a "wishbone" typical of birds.
• Like birds, they also had hollow bones that may have contained air sacs used in breathing. Some theropods are also believed to have sported primitive feathers.
• Limusaurus lived around 160 million years ago and was one of the most primitive members of a theropod group known as ceratosaurs.
• Its skeleton was unearthed from a dinosaur "mass grave" in the Gobi desert of north-west China.
• Besides its bird-like hands, Limusaurus had toothless beak-like jaws and short arms lacking sharp claws.
• It also had gizzard stones similar to those used by birds to help them grind up tough plant foods.
• The evidence points to Limusaurus being vegetarian, unlike most other theropods.