Have birds descended from dinosaurs?

The 72,000,000-year-old bones of a baby dinosaur have given scientists added proof that birds may be the direct descendants of dinosaurs. The presence of growth plates--discs of cartilage that occur near the ends of long bones and enable bones to grow in length-in dinosaur fossils not only helps cement the link between modern birds and dinosaurs, but is a further indication that these reptiles which once were the Earth's dominant life form were warm-blooded and could grow to giant proportions in a relatively short time, according to Claudia Barreto, a University of Wisconsin-Madison scientist,

The growth plates--never before seen in dinosaur fossils--were discovered in the well-preserved bones of young maiasaurs found in the nesting sites of the Two Medicine Formation in northwestern Montana. The bones of the bipedal, duck-billed dinosaur were excavated by paleontologist John R. Horner of the Museum of the Rockies at Montana State University.

Growth plate cartilage provides a cellular scaffolding for new bone, adding to the length of bone shafts. In birds and mammals, growth plates differ at both the cellular level and in their over-all structure. "Bird growth plates have a unique shape which reflects their method of rapid bone growth," Barreto points out. "What we see in the dinosaur bone is very similar to what we see in birds. It is plausible to assume that these are the same things."

While they also are found in modern mammals and reptiles, the dinosaur growth plates are markedly different and most closely resemble those of birds. In fast-growing mammals and birds growth plates occur relatively...