Another Dinosaur-to-Bird Transitional Fossil

Science | Mon, Jun 22, 2009 at 5:28:50 pm PDT

Here’s some important science news from last week that slipped through LGF’s cracks: the discovery of a four-fingered theropod that fills in another step in the evolution of dinosaurs into birds.

Image credit: James Clark

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.