An essential purchase--Gillette (The Search for Seismosaurus, 1994) has written another involving book about a topic of guaranteed interest to children. The book opens in 1947, when paleontologists dug up tons of tightly packed bones in the canyons near Ghost Ranch, N.M.; since then they have studied and puzzled over the cause of the sudden extinction of the speedy, dog-sized dinosaurs called Coelophysis, who roamed New Mexico 225 million years ago. Gillette demonstrates the scientific method, postulating a hypothesis and then investigating it against the evidence found in rocks and fossils. Did a sticky pit like La Brea trap the little dinosaurs? Was it a volcanic eruption? Asteroids? Poisoned water? She tests each hypothesis, presents the most likely explanation, given current evidence, and concludes that the answer is not yet final: ""Scientists are always ready to change their ideas to fit what they learn."" The paintings by Henderson are satisfying, full-color renderings of the dinosaurs, their habitat, and the catastrophe envisioned in each hypothesis. Photos of excavators and bones provide additional intrigue.