Fossils are familiar to most of us as the hardened, three-dimensional remains of ancient plants and animals, whose tissues have been impregnated by, or replaced with, minerals such as calcite, iron, and silica, thus arresting their inevitable decay. Because the mineralization process results in a rock that’s usually harder than the material around it, especially in the case of petrification, fossils are durable snapshots of the past.

But the universe of fossils also includes the impressions of such organisms, like the outline of a leaf preserved on a layer of slate, or the footprint of an animal left in mud that then filled in with sandy sediment, thus capturing this trace of the animal’s fleeting time on earth.

Collectors of fossils have an extraordinary number of pieces to choose from. Invertebrate fossils range from sponges and corals to trilobites and ammonites, the later of which were wiped out some 66 million years ago when an asteroid about six miles wide and traveling 45,000 miles per hour plowed into what is now the Yucatan peninsula, vaporizing pretty much everything on the surface of North America in about two minutes and causing the extinction of about 75 percent of the rest of the planet’s life forms in the years that followed.

Vertebrate fossils include fish, whether it’s the delicate outline of a perch or a single menacing shark’s tooth, as well as the bones of birds, reptiles, amphibians, mammals, and, yes, dinosaurs, which joined the ammonites in the planet’s last great die-off, prior to the one we’re in right now.

Trace fossils are particularly interesting. In addition to prints, trace fossils includes borings and burrows. Chondrite fossils are often mistaken for plant fossils because they resemble the branching stems of a conifer or perhaps delicate roots. In fact, they are the remains of burrows created by roundworms. Another group of invertebrates that resemble plants are graptolites, which can look like leaves (retiolites) and fanlike ferns (rhabdinopora).

Crustacea are favorites because of their familiar shapes (crabs, shrimp), as are bivalves, mollusks, and insects—for some people, the shape of a fossilized dragonfly is not only visually appealing, but also seems a link between ancient and modern times.

Still, bones are the prize, especially those of diapsid reptiles such as the megalania, which lived in Australia during the Pleistocene epoch and was the larger (20-foot-long) forebear of the Komodo Dragon. More common are the teeth and other remains of equus, the ancient cousin to the contemporary horse.

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