It stood ten feet high, weighed over eight hundred pounds, and had a beak that could tear the head off pretty much anything that irked it. Its family survived in formidable fashion for fifty million years, yet until a skeleton was discovered in Florida in 1963, we had no idea that it had ever existed.

The giant Diplodocus that dominates the entrance hall of the Natural History Museum in London and has delighted and informed generations of visitors is made of plaster — built in 1903 in Pittsburgh and presented to the museum by Andrew Carnegie. The entrance hall of the American Museum of Natural History in New York is dominated by an even grander tableau: a skeleton of a large Barosaurus defending her baby from attack by a darting and toothy Allosaurus. It is a wonderfully impressive display — the Barosaurus rises perhaps thirty feet toward the high ceiling — but also entirely fake. Every one of the several hundred bones in the display is a cast. Visit almost any large natural history museum in the world — in Paris, Vienna, Frankfurt, Buenos Aires, Mexico City — and what will greet you are antique models, not ancient bones.