The New Dinosaur Hall
1The natural history museum downtown has opened a new hall of skeletons. At one end stand casts of two fossil animals: Velociraptor, a turkey-sized meat-eating dinosaur about 75 million years old, and Archaeopteryx, a crow-sized animal about 150 million years old with feathered wings. Beside them are skeletons of three living animals: a pigeon, an American alligator and a green iguana. Ezekiel Nakamura, an eighth grader, is writing a report on the hall, and museum educator Halina Brzezinski is helping him compare the skeletons.
2Ms. Brzezinski pointed out five features. A wishbone is a V-shaped bone in the chest, formed from two collarbones joined in the middle. Air-filled bones have hollow spaces inside that connect to the lungs, which makes them light for their size. The hand is the end of the front limb, which can have separate clawed fingers or fingers joined into one piece. Teeth grow in the jaws, while a beak has none. Feathers rarely fossilize, but in many birds the big wing feathers anchor to a row of small bumps on a forearm bone, and a Velociraptor fossil has the same row of bumps.
3She also showed Ezekiel the tails. Velociraptor, Archaeopteryx, the alligator and the iguana all have long tails with many separate bones. The fossil of Archaeopteryx even shows a row of feathers along each side of its long tail. The pigeon's tail feathers sit on a short tail that ends in a small knob of joined bones.
4Not every feature shows up in a skeleton. Ms. Brzezinski explained that alligators and birds both have a heart with four chambers and a muscular gizzard that grinds food. Most lizards, including the iguana, have a heart with three chambers and no gizzard.
5Scientists use patterns like these to work out relationships. Organisms that share many detailed features of anatomy most likely share a more recent common ancestor than organisms that share fewer features. Scientists also watch for features that two animals share only because they do the same job. Bats and birds both fly, for example, but a bat's wing is skin stretched between long finger bones, with no feathers at all, and bats have no wishbone.