Dinosaur Alert! — Carolyn Keene

NANCY DISCOVERS DINOSAUR FOOTPRINTS IN HER OWN BACKYARD!
The most annoying boys in Nancy's class offer her a deal. If she can prove that a giant footprint they've found on the school grounds was really made by a dinosaur, they'll stop teasing her and her friends -- forever!
Always ready to take on a case, Nancy agrees, though she thinks the boys are wrong. Dinosaurs never roamed River Heights -- or did they? But then she gets a mean note: "Don't go where you don't belong!" When Nancy finds huge footprints in her own backyard, she's hunting everywhere for answers. It's a monster of a mystery, and she doesn't have a zillion years to solve it!