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Heavy and big-footed, the dinosaurs made plenty of footprints. But only live animals leave footprints to be fossilised, note Martin Lockley and Adrian Hunt in Dinosaur Tracks and Other Fossil Footprints of the Western United States (Columbia, £25/$29.95, ISBN 0 2310 7926 5). Dinosaur tracks are common in western North America, found right up to the 65-million-year-old layers rich in the iridium that marks an asteroid impact. Younger rocks hold only bird and mammal tracks – evidence that dinosaurs vanished at the time of the impact.