DINOSAURS EVOLVED TO BIRDS

Dinosaurs evolved to Birds

Dinosaurs evolved to Birds

Introduction

Most paleontologists believe that a group of dinosaurs evolved into today's birds, but in trying to understand that gradual process they've been bedeviled by some details. Yesterday, researchers announced that they may have solved one of those small conundrums. A fossilized dinosaur found in China appears to settle the matter of exactly how the bones in dinosaurs' feet evolved into the digits hidden in bird wings.

Analysis

The story (almost certainly apocryphal, but they're the best kind) goes something like this: Thomas Huxley was eating quail one evening while ruminating on a palaeontological puzzle poised by a strange bone back in the lab. He knew it was the lower leg bone (tibia) of a meat-eating dinosaur but smeared across the bottom of it was an unidentified extra bone (Xing et al. 2009). He happened to suck the flesh off the bottom of the quail leg and there, smeared across the bottom of the quail tibia, was the same enigmatic bone. Dealing with a more complete bird leg, Huxley realised that the osseous stranger was the anklebone (astragalus). More importantly, Huxley concluded that the form of the astragalus in both the dinosaur and the bird were so similar that they must be closely related (Wolff et al, 2009).

Huxley's dino-bird theory fell into disfavour when a new theory postulated that birds evolved from some pre-dinosaurian reptile. In 1916 the Danish medical doctor Gerhard Heilmann published The Origin Of Birds, in which he commented on the many similarities between the skeletons of meat-eating dinosaurs (theropods) and birds (Sereno et al. 2008) . But Heilmann also noted that theropods lacked collarbones (clavicles) which fuse together to become the wishbone (furcula) in birds. Heilmann argued that such a feature could not be lost and then re-evolved at a later date, so theropods couldn't be the ancestors of birds. Thus the theropods were banished from the bird's family tree for the next fifty years (Over, 1995).

Then, in the late 1960's, John Ostrom from Yale University noted 22 features in the skeletons of meat-eating dinosaurs that were also found in birds and nowhere else. This reset the thinking on bird ancestry. Subsequent work has found up to 85 characters that tie theropods and birds together. Although some of these characters may be of dubious significance, so many characteristics shared between theropods and birds is a pretty convincing argument in favour of the relationship. But what of Heilmann's missing collarbones? It turns out that theropods not only had clavicles but that they were fused together into a furcula. Unfortunately for Heilmann, the fossil evidence was somewhat sparse in his day and the few theropod furculae that had been found were misidentified, usually as belly ribs (Ostrom, 1976).

Objections to dinobirds

Two vocal opponents of the "Birds Are Dinosaurs" theory are Alan Feduccia, of the University of North Carolina and Larry Martin, from the University of Kansas. They contend that birds evolved from some unknown reptile from a time before the dinosaurs ...