Though he doesn't say how "the animals' bones turn to stone," Freedman explains at the outset how "scientists learn about prehistoric animals by digging up fossils." He does less well with the idea that some prehistoric animals who "lived on" have "changed greatly as time passed." (This can be a stumper for kids unless it's explained how the species, not the individuals, changed.) But this is about those "living fossils" that have not changed--beginning with starfish, horseshoe crabs, cockroaches, and others which, a chart makes clear, long predated the dinosaurs. Again, there are problems with levels of understanding: coelacanths, we learn, give birth to live young--interesting, but only when you know that other fish don't. However, Freedman's descriptions of the animals are generally on target, their unique and interesting features are brought out without frivolous dramatics, and the detail is well integrated into the overall "living fossil" concept. And, as usual with Freedman's books, the text is considerably sharpened by an excellent selection of photos, showing both the fossils of prehistoric specimens and their contemporary counterparts.