At first glance, the oddly shaped splotches darkening a vibrant yellow piece of amber appear to be amorphous blobs. But upon closer examination, a shape emerges. There are two forelimbs. At the end of each limb are four smaller bones, forming a distinctly handlike shape. Within the largest dark spot, which has a rounded top, eye sockets become distinguishable. It’s a skull. Encased within the smooth chunk of amber is the body of a tiny young tropical frog. Scientists say the diminutive critter, measuring less than an inch long, lived about 99 million years ago before it became entombed in...